Specification: Ballerina GraphQL Library

Owners: @shafreenAnfar @DimuthuMadushan @ThisaruGuruge @MohamedSabthar
Reviewers: @shafreenAnfar @ThisaruGuruge @DimuthuMadushan @ldclakmal
Created: 2022/01/06
Updated: 2023/09/14
Edition: Swan Lake
GraphQL Specification: October 2021

Introduction

This is the specification for the GraphQL package of the Ballerina language, which provides GraphQL server functionalities to produce GraphQL APIs and GraphQL client functionalities to communicate with GraphQL APIs.

The GraphQL library specification has evolved and may continue to evolve in the future. The released versions of the specification can be found under the relevant GitHub tag.

If you have any feedback or suggestions about the library, start a discussion via a GitHub issue or in the Discord server. Based on the outcome of the discussion, the specification and implementation can be updated. Community feedback is always welcome. Any accepted proposal, which affects the specification is stored under /docs/proposals. Proposals under discussion can be found with the label type/proposal on GitHub.

The conforming implementation of the specification is released and included in the distribution. Any deviation from the specification is considered a bug.

Contents

  1. Overview
  2. Components
  3. Schema Generation
  4. Types
  5. Directives
  6. Errors
  7. Annotations
  8. Security
  9. Tools
  10. Advanced Features

1. Overview

The Ballerina language provides first-class support for writing network-oriented programs. The GraphQL package uses these language constructs and creates the programming model to produce/consume GraphQL APIs.

The GraphQL package is designed to work with GraphQL specification. There are two main approaches when writing GraphQL APIs. The schema-first approach and the code-first approach. The Ballerina GraphQL package uses the code-first first approach to write GraphQL APIs (which means no GraphQL schema is required to create a GraphQL service), while it also supports the schema-first approach through Ballerina GraphQL CLI tool.

In addition to functional requirements, this library deals with none functional requirements such as security. Each requirement is discussed in detail in the coming sections.

2. Components

This section describes the components of the Ballerina GraphQL package. To use the Ballerina GraphQL package, a user must import the Ballerina GraphQL package first.

Example: Importing the GraphQL Package
import ballerina/graphql;

2.1 Listener

2.1.1 HTTP Listener

Since the GraphQL spec does not mandate an underlying client-server protocol, a GraphQL implementation can use any protocol underneath. The Ballerina GraphQL package, like most of the other implementations, uses HTTP as the protocol. The Ballerina GraphQL listener is using an HTTP listener to listen to incoming GraphQL requests through HTTP.

2.1.2 WebSocket Listener

If the schema contains the Subscription type (as described in Subscription Type), The GraphQL listener will establish a new WebSocket listener to listen to incoming subscription requests.

In Ballerina, WebSocket is used as the communication protocol for GraphQL subscriptions as it is capable of dispatching data continuously while maintaining a persistent connection. Ballerina GraphQL utilizes the graphql-transport-ws (graphql-ws) WebSocket sub-protocol in subscriptions. If a WebSocket connection is established with the graphql-transport-ws sub-protocol, all subscription responses will be formatted according to the standard message structure outlined in the specification.

A standard response includes JSON fields for type, id, and payload. The type field specifies the message type of the response. The id field is used to uniquely identify the client. The payload field includes the GraphQL response returned from the GraphQL engine. If a subscription request is sent without the graphql-transport-ws sub-protocol then the WebSocket handshake fails with an error.

Note: The Ballerina GraphQL subscription includes a default implementation for sending ping messages over a graphql-ws connection, and periodically checking for pong messages in response. By default, these messages are sent and checked every 15 seconds. If a pong message is not received within this time frame, the service will close the WebSocket connection.

A Ballerina GraphQL listener can be declared as described below, honoring the Ballerina generic listener declaration.

2.1.3 Initializing the Listener Using Port Number

If a GraphQL listener requires to be listening to a port number, that port number must be provided as the first parameter of the listener constructor.

Example: Initializing the Listener Using Port Number
listener graphql:Listener graphqlListener = new (9090);

2.1.4 Initializing the Listener using an HTTP Listener

If a GraphQL listener requires to listen to the same port as an existing http:Listener object, that http:Listener object must be provided as the first parameter of the listener constructor.

Example: Initializing the Listener using an HTTP Listener
listener http:Listener httpListener = new (9090);
listener graphql:Listener graphqlListener = new (httpListener);

Note: The client does not need to explicitly create a websocket:Listener for subscriptions. If there is a subscription resolver in a service, the graphql:Listener will initiate a websocket:Listener over the same port used in the http:Listener.

2.1.5 Listener Configuration

Since the GraphQL listener uses the http:Listener and the websocket:Listener as the underlying listeners, some additional configurations can be passed to these listeners, when creating a graphql:Listener. These configurations are defined in the graphql:ListenerConfiguration record.

Example: Listener Configuration
listener graphql:Listener graphqlListener = new (9090, timeout = 10);

Note: If the GraphQL service includes subscription operations, the httpVersion of the graphql:ListenerConfiguration must be either "1.0" or "1.1". Otherwise, this will cause a runtime error when attaching the service to the listener.

2.2 Service

The service represents the GraphQL schema in the Ballerina GraphQL package. When a service is attached to a GraphQL listener, it is considered a GraphQL service. When a service is identified as a GraphQL service, it will be used to Generate the Schema. Attaching the same service to multiple listeners is not allowed, and will cause a compilation error.

Example: Service
service /graphql on new graphql:Listener(9090) {

}

In the above example, a GraphQL service is attached to a GraphQL listener. This is syntactic sugar to declare a service and attach it to a GraphQL listener.

2.2.1 Service Type

The following distinct service type provided by the Ballerina GraphQL package can be used by the users. It can be referred to as graphql:Service. Since the language support is yet to be implemented for the service typing, service validation is done using the Ballerina GraphQL compiler plugin.

public type Service distinct service object {

};

2.2.2 Service Base Path

The base path is used to discover the GraphQL service to dispatch the requests. identifiers and string literals can be used as the base path, and it should be started with /. The base path is optional and if not provided, will be defaulted to /. If the base path contains any special characters, they should be escaped or defined as string literals.

Example: Base Path
service hello\-graphql on new graphql:Listener(9090) {

}

2.2.3 Service Declaration

The service declaration is syntactic sugar for creating a service. This is the mostly-used approach for creating a service.

Example: Service Declaration
service graphql:Service /graphql on new graphql:Listener(9090) {

}

2.2.4 Service Object Declaration

A service can be instantiated using the service object. This approach provides full control of the service life cycle to the user. The listener life cycle methods can be used to handle this.

Example: Service Object Declaration
graphql:Service graphqlService = service object {
    resource function get greeting() returns string {
        return "Hello, world!";
    }
}

public function main() returns error? {
    graphql:Listener graphqlListener = check new (9090);
    check graphqlListener.attach(graphqlService, "graphql");
    check graphqlListener.'start();
    runtime:registerListener(graphqlListener);
}

Note: The service object declaration is only supported when the service object is defined in global scope. If the service object is defined anywhere else, the schema generation will fail. This is due to a known current limitation in the Ballerina language.

2.2.5 Service Configuration

The graphql:ServiceConfiguration annotation can be used to provide additional configurations to the GraphQL service. These configurations are described in the Service Configuration section.

2.3 Parser

The Ballerina GraphQL parser is responsible for parsing the incoming GraphQL documents. This will parse each document and then report any errors. If the document is valid, it will return a syntax tree.

Note: The Ballerina GraphQL parser is implemented as a separate module and is not exposed outside the Ballerina GraphQL package.

2.4 Engine

The GraphQL engine acts as the main processing unit in the Ballerina GraphQL package. It connects all the other components in the Ballerina GraphQL service together.

When a request is received by the GraphQL Listener, it dispatches the request to the GraphQL engine, where it extracts the document from the request, then passes it to the parser. Then the parser will parse the document and return an error (if there is any) or the syntax tree to the engine. Then the engine will validate the document against the generated schema, and then if the document is valid, the engine will execute the document.

2.5 Client

The GraphQL client can be used to connect to a GraphQL service and retrieve data. This client currently supports the Query and Mutation operations. The Ballerina GraphQL client uses HTTP as the underlying protocol to communicate with the GraphQL service.

2.5.1 Initializing the Client

The graphql:Client init method requires a valid URL and optional configuration to initialize the client.

graphql:Client graphqlClient = check new (“http://localhost:9090/graphql”, {timeout: 10});

2.5.2 Executing Operations

The graphql client provides execute API to execute graphql query and mutation operations. The execute method of graphql:Client takes a GraphQL document as the required argument and sends a request to the specified backend URL seeking a response. Further, the execute method could take the following optional arguments.

  • variables - A map containing the GraphQL variables. All the variables that may be required by the graphql document can be set via this variables argument.
  • operationName - The GraphQL operation name. If the document has more than one operation, then each operation must have a name. A single GraphQL request can only execute one operation; the operation name must be set if the document has more than one operation. Otherwise, the GraphQL server responds with an error.
  • headers - A map containing headers that may be required by the graphql server to execute each operation.

The method definition of the execute API is given below.

remote isolated function execute(string document, map<anydata>? variables = (), string? operationName = (),
    map<string|string[]>? headers = (), typedesc<GenericResponseWithErrors|record {}|json> targetType = <>)
    returns targetType|ClientError ;

2.5.3 Client Data Binding

When sending a GraphQL request to a GraphQL server using the Ballerina GraphQL client, the response can be data-bound. That means the user can define the expected shape of the GraphQL response by defining a type. The data type defined by the user should be a subtype of graphql:GenericResponseWithErrors|record{}|json. Otherwise, the data binding fails with an error.

Note: It is recommended to use the graphql:GenericResponseWithErrors or any subtype of it when retrieving a response using the graphql:Client using the execute method.

When defining the expected type, nullable fields should be defined as a union of the field type and nil (()). A Payload Binding Error can occur otherwise.

Example: Handle Response with Data Binding

This example shows how to data-bind the response to a pre-defined type. Note how the age field is defined as a nullable field.

type ProfileResponseWithErrors record {|
    *graphql:GenericResponseWithErrors;
    record {|Profile profile;|} data;
|};

type Profile record {|
    string name;
    int? age;
|};

public function main() returns error? {
    graphql:Client graphqlClient = check new ("localhost:9090/graphql");
    string document = "{ profile(id: 100) {name age} }";
    ProfileResponseWithErrors response = check graphqlClient->execute(document);
    string name = response.data.profile.name;
    io:println(name);
}

The execute method can return errors when retrieving a response from a GraphQL API. For information about handling errors, check the section Client Error Handling

2.5.4 Client Configuration

The graphql:Client uses http:Client as its underlying implementation; this http:Client can be configured by providing the graphql:ClientConfiguration as an optional parameter via the graphql:Client init method.

3. Schema Generation

The GraphQL schema is generated by analyzing the Ballerina service attached to the GraphQL listener. The Ballerina GraphQL package will walk through the service and the types related to the service to generate the complete GraphQL schema.

When an incompatible type is used inside a GraphQL service, a compilation error will be thrown.

3.1 Root Types

Root types are a special set of types in a GraphQL schema. These types are associated with an operation, which can be done on the GraphQL scheme. There are three root types.

  • Query
  • Mutation
  • Subscription

3.1.1 The Query Type

The Query type is the main root type in a GraphQL schema. It is used to query the schema. The Query must be defined for a GraphQL schema to be valid. In Ballerina, the service itself is the schema, and each resource method with the get accessor inside a GraphQL service is mapped to a field in the root Query type.

Example: Adding a Field to the Query Type
service on new graphql:Listener(9090) {
    resource function get greeting() returns string {
        return "Hello, World!";
    }
}

Note: Since the Query type must be defined in a GraphQL schema, a Ballerina GraphQL service must have at least one resource method with the get accessor. Otherwise, the service will cause a compilation error.

3.1.2 The Mutation Type

The Mutation type in a GraphQL schema is used to mutate the data. In Ballerina, each remote method inside the GraphQL service is mapped to a field in the root Mutation type. If no remote method is defined in the service, the generated schema will not have a Mutation type.

Example: Adding a Field to the Mutation Type
service on new graphql:Listener(9090) {
    remote function setName(string name) returns string {
        //...
    }
}

As per the GraphQL specification, the Mutation type is expected to perform side effects on the underlying data system. Therefore, the mutation operations should be executed serially. This is ensured in the Ballerina GraphQL package. Each remote method invocation in a request is done serially, unlike the resource method invocations, which are executed in parallel.

3.1.3 The Subscription Type

The Subscription type in a GraphQL schema is used to continuously fetch data from a GraphQL service. In Ballerina, each resource method with the subscribe accessor inside a GraphQL service is mapped to a field in the root Subscription type. If the resource method has the subscribe accessor, it must return a stream. Otherwise, the compilation error will occur.

Example: Adding a Field to the Subscription Type
service on new graphql:Listener(9090) {
    resource function subscribe greetings() returns stream<stream> {
        return ["Hello", "Hi", "Hello World!"].toStream();
    }
}

3.2 Wrapping Types

Wrapping types are used to wrap the named types in GraphQL. A wrapping type has an underlying named type. There are two wrapping types defined in the GraphQL schema.

3.2.1 NON_NULL Type

NON_NULL type is a wrapper type to denote that the resulting value will never be null. Ballerina types do not implicitly allow nil. Therefore, each type is inherently a NON_NULL type until specified explicitly otherwise. If a type is meant to be a nullable value, it should be unionized with nil.

Note: nil (represented by ()) is the Ballerina's version of null.

In the following example, the type of the name field is String!. This means the String type is wrapped by the NON_NULL type.

Example: NON_NULL Type
service on new graphql:Listener(9090) {
    resource function get name returns string {
        return "Walter White";
    }
}

To make it a nullable type, it should be unionized with ?. The following example shows the field name of the type String. This means the name field can have a null value.

Example: Nullable Type
service on new graphql:Listener(9090) {
    resource function get name returns string? {
        return "Walter White";
    }
}

Note: ? is syntactic sugar for |().

3.2.2 LIST Type

The list type represents a list of values of another type. Therefore, LIST is considered a wrapping type. In Ballerina, a LIST type is defined using an array. The following represents a field called names of the type of LIST of String! type.

Example: LIST Type
service on new graphql:Listener(9090) {
    resource function get names() returns string[] {
        return ["Walter White", "Jesse Pinkman"];
    }
}

3.3 Resource Methods

Resource methods are a special kind of method in Ballerina. In the Ballerina GraphQL package, resource methods are used to define GraphQL object fields. The resource methods in a GraphQL service are validated at the compile-time.

3.3.1 Resource Accessor

The only allowed accessors in Ballerina GraphQL resource are, get and subscribe. Any other accessor usage will result in a compilation error.

Example: Resource Accessor
resource function get greeting() returns string {
    // ...
}
Counter Example: Resource Accessor
resource function post greeting() returns string {
    // ...
}

3.3.2 Resource Name

As the resource methods are mapped to a field of a GraphQL Object type, the resource name represents the name of the corresponding field.

Example: Resource Name
resource function get greeting() returns string {
    // ...
}

In the above example, the resource represents a field named greeting of type String!. Check Types Section for more information on types and fields.

3.3.3 Hierarchical Resource Path

GraphQL represents the data as a hierarchical structure. Ballerina resources provide different ways to define this structure. Hierarchical resource paths are one approach, which is also the simplest way.

The path of a resource can be defined hierarchically so that the schema generation can generate the types using the hierarchical path. When a service has resources with hierarchical resource paths, the first path segment and each intermediate path segment of a resource represent an Object type field. The GraphQL type represented by the return type of the resource method is assigned to the field represented by the leaf-level (final) path segment. Each intermediate type has the same name as the path segment. Therefore, the field name and the type name are the same for the intermediate path segments.

Example: Hierarchical Resource Path
service graphql:Service on new graphql:Listener(9090) {
    resource function get profile/address/number() returns int {
        return 308;
    }

    resource function get profile/address/street() returns string {
        return "Negra Arroyo Lane";
    }

    resource function get profile/address/city() returns string {
        return "Albuquerque";
    }

    resource function get profile/name() returns string {
        return "Walter White";
    }

    resource function get profile/age() returns int {
        return 52;
    }
}

In the above example shows how to use hierarchical resource paths to create a hierarchical data model. When the schema is generated using this service, the root Query operation has a single field, profile, as it is the only path segment at the top level. The type of this field is also profile, which is an Object type. This object type has three fields: address, name, and age. The type of the address field is also Object as it is an intermediate path segment (i.e. has child path segments). The name of this object type is address. It has three fields: the number (type Int!), the street (type String!), and the city (type String!). The name field is of type String!, and the age field is of type Int!. Check Types Section for more information on types and fields.

3.4 Remote Methods

The remote methods are used to define the fields of the Mutation type in a GraphQL schema. Remote methods are validated at the compile-time.

Note: The resource and remote methods are called resolvers in GraphQL terminology. Therefore, in this spec, sometimes the term resolver is used to refer resource and remote methods.

3.4.1 Remote Method Name

The name of the remote method is the name of the corresponding GraphQL field in the Mutation type.

3.5 Documentation

A GraphQL schema can have documentation for the types, fields, enums, schema, etc.

In Ballerina, the Ballerina doc comments can be used to add documentation for the generated schema. Each comment belonging to a field, argument, or enum will be applied to the particular GraphQL schema member.

Example: Documentation
# Service to query people database.
service on new graphql:Listener(9090) {

    # Returns a profile with the given ID.
    #
    # + id - The ID of the profile
    # + return - The profile with the given ID
    resource function get profile(int id) returns Profile {
        // ...
    }
}

# Represents a profile.
#
# + id - The ID of the profile
# + name - The name of the profile
# + age - The age of the profile
public type Profile record {|
    int id;
    string name;
    int age;
|};

This will generate the documentation for all the fields of the Query type including the field descriptions of the Profile type.

Note: When a field or an argument name contains Unicode characters or any other escape characters, they are unescaped when generating the schema.

Example: Escaping Characters
service on new graphql:Listener(9090) {
    resource function get 'type(string 'version) returns string {
        return "";
    }

    resource function get name(string \u{0076}ersion) returns string {
        return "";
    }
}

The above code will generate the following schema:

type Query {
    type(version: String!): String!
    name(version: String!): String!
}

4. Types

GraphQL type system is represented using a hierarchical structure. Type is the fundamental unit of any GraphQL schema.

4.1 Scalars

Scalar types represent primitive leaf values in the GraphQL type system. The following built-in types are supported in the Ballerina GraphQL package. Scalar values are represented by the primitive types in Ballerina.

4.1.1 Int

The Int type is represented using the int type in Ballerina.

4.1.2 Float

The Float type is represented using the float type in Ballerina.

Note: When used as an input value type, both integer and float values are accepted as valid inputs.

4.1.3 String

The String type is represented using the string type in Ballerina. It can represent Unicode values.

4.1.4 Boolean

The Boolean type is represented using the boolean type in Ballerina.

4.1.5 ID

The ID scalar type represents a unique identifier, often used to re-fetch an object or as the key for a cache. The ID type is serialized in the same way as a String; however, it is not intended to be human-readable.

In Ballerina, the ID type is represented using the @graphql:ID annotation. The following Ballerina types are supported as ID types.

  • int
  • string
  • float
  • decimal
  • uuid:Uuid (From the ballerina/uuid module)

When the @graphql:ID annotation is used, the generated schema will show the field type as ID, regardless of the actual type of the field.

Note: If the @graphql:ID annotation is used for a field, the values of those fields will always be serialized as strings.

Note: Applying a @graphql:ID annotation to an array indicates it as a list of ID elements.

Example: ID Scalar Type
service on new graphql:Listener(9090) {

    resource function get profileById(@graphql:ID int id) returns Profile {
        // ...
    }

    resource function get profileByIds(@graphql:ID int[] ids) returns Profile[] {
        // ...
    }

    resource function get studentByStringId(@graphql:ID string id) returns Student {
        // ...
    }

    resource function get studentByUuidId(@graphql:ID uuid:Uuid id) returns Student {
        // ...
    }
}

public type Profile record {|
    @graphql:ID int id;
    string name;
    int age;
|};

public distinct service class Student {
    final string id;
    final string name;

    function init(string id, string name) {
        self.id = id;
        self.name = name;
    }

    resource function get id() returns @graphql:ID string {
        return self.id;
    }

    resource function get name() returns string {
        return self.name;
    }
}

Apart from the above types, the decimal type can also be used inside a GraphQL service, which will create the Decimal scalar type in the corresponding GraphQL schema.

4.2 Objects

Objects represent the intermediate levels of the type hierarchy. Objects can have a list of named fields, each of which has a specific type.

In Ballerina, a GraphQL object type can be represented using either a service type or a record type.

4.2.1 Record Type as Object

A Ballerina record type can be used as an Object type in GraphQL. Each record field is mapped to a field in the GraphQL object and the type of the record field will be mapped to the type of the corresponding GraphQL field.

Note: A GraphQL object must have at least one field. Therefore, an empty record type cannot be used as an object type in GraphQL, and using an empty record type will result in a compilation error.

Example: Record Type as Object
service on new graphql:Listener(9090) {
    resource function get profile() returns Profile {
        return {name: "Walter White", age: 52};
    }
}

type Profile record {|
    string name;
    int age;
|};

Note: Even though anonymous record types are supported in Ballerina, they cannot be used as types in a GraphQL schema. This is because a type in a GraphQL schema must have a name. Therefore, if an anonymous record is used in a GraphQL service, it will result in a compilation error.

Note: When a record field is defined as an optional field, the Ballerina GraphQL engine identifies it as a nullable field. Even though it is supported, it is recommended to use Ballerina nilable types instead of optional fields to define nullable fields in GraphQL.

Note: Alias types of record types are not allowed to be used as object types in a GraphQL schema. If there is a need to utilize fields from an existing type repeatedly, ballerina type inclusion can be used.

Hint: Open records are supported in GraphQL services, but they do not make sense in the context of GraphQL since a GraphQL type cannot have dynamic fields. Therefore, it is recommended to use closed records in GraphQL services unless it is absolutely needed.

4.2.2 Service Type as Object

A Ballerina service type can be used as an Object type in GraphQL. Similar to the Query type, each resource method inside a service type represents a field of the object.

Since GraphQL only allows mutations at the top level and the remote methods are used to represent the Mutation type, any service type used to represent a GraphQL Object cannot have remote methods inside them.

Note: As per the GraphQL spec, only the root type can have Mutations. Therefore, defining remote methods inside subsequent object types does not make any sense.

The resource methods in these service types can have input parameters. These input parameters are mapped to arguments in the corresponding field.

Note: The service types representing an Object can be either distinct or non-distinct type. But if a service type is used as a member of a Union type, they must be distinct service classes.

Example: Service Type as Object
service on new graphql:Listener(9090) {
    resource function get profile() returns Profile {
        return new ("Walter White", 52);
    }
}

service class Profile {
    private final string name;
    private final int age;

    function init(string name, int age) {
        self.name = name;
        self.age = age;
    }

    resource function get name() returns string {
        return self.name;
    }

    resource function get age() returns int {
        return self.age;
    }
}

Note: Although both the record and service type can be used to represent the Object type, using record type as Object has limitations. For example, a field represented as a record field can not have an input argument, as opposed to a field represented using a resource method in a service class.

4.3 Unions

GraphQL Union type represents an object that could be one of a list of possible GraphQL Object types but provides no guarantee for common fields in the member types. Ballerina has first-class support for union types. The Ballerina GraphQL package uses this feature to define Union types in a schema.

In Ballerina, only distinct service classes are supported as union-type members. The reason behind this is the difference between GraphQL and the Ballerina type systems. The Ballerina is a structurally-typed language whereas GraphQL is a nominally-typed language. The distinct types of Ballerina have similar behavior to nominal types. If one or more member type in a union type is not distinct service class, a compilation error will occur.

Example: Union Types

In the following example, two distinct service types are defined first, Teacher and Student. Then a Union type is defined using Ballerina syntax for defining union types. The resource method in the GraphQL service is returning the union type.

service on new graphql:Listener(9090) {
    resource function get profile() returns Profile {
        return new Teacher("Walter White", "Chemistry");
    }
}

distinct service class Teacher {
    private final string name;
    private final string subject;

    function init(string name, string subject) {
        self.name = name;
        self.subject = subject;
    }

    resource function get name() returns string {
        return self.name;
    }

    resource function get subject() returns string {
        return self.subject;
    }
}

distinct service class Student {
    private final string name;
    private final float gpa;

    function init(string name, int gpa) {
        self.name = name;
        self.gpa = gpa;
    }

    resource function get name() returns string {
        return self.name;
    }

    resource function get gpa() returns float {
        return self.gpa;
    }
}

type Profile Teacher|Student; // Defining the union type

4.4 Enums

In GraphQL, the Enum type represents leaf values in the GraphQL schema, similar to the Scalar types. But the Enum types describe the set of possible values. In Ballerina, the enum type is used to define the Enum type in GraphQL.

Example: Enums
service on new graphql:Listener(9090) {
    resource function get direction() returns Direction {
        return NORTH;
    }
}

enum Direction {
    NORTH,
    EAST,
    SOUTH,
    WEST
}

Note: While it is possible to assign a string value to an enum member in the Ballerina language, the GraphQL specification does not support this behaviour. Therefore, it's not recommended to use string values for enum members when defining an enum type for use in a graphql:Service. Using string values for enum members may result in an invalid generated schema.

4.5 Input Types

In GraphQL, a field can have zero or more input arguments. These arguments can be either a Scalar type, an Enum type, or an INPUT_OBJECT type.

4.5.1 Input Union Types

An input type can be a Ballerina union type, if and only if the union consists of one of the supported types and the other member type is nil. A union with nil means the input type is nullable. Any other union type will be resulting in a compilation error.

Example: Input Union Types
service on new graphql:Listener(9090) {
    resource function get greet(string? name) returns string {
        if name is string {
            return string `Hello, ${name}`;
        }
        return "Hello, world!";
    }
}
Counter Example: Invalid Input Union Types
service on new graphql:Listener(9090) {
    resource function get greeting(string|error name) returns string { // Results in a compilation error
        return "Hello, World!"
    }
}

4.5.2 Input Objects

Although Scalar and enum types can be used as input and output types without a limitation, an object type can not be used as an input type and an output type at the same time. Therefore, separate kinds of objects are used to define input objects.

In Ballerina, a record type can be used as an input object. When a record type is used as the type of the input argument of a resource or remote method in a GraphQL service (or in a resource method in a service type returned from the GraphQL service), it is mapped to an INPUT_OBJECT type in GraphQL.

Note: A GraphQL input object must have at least one field. Therefore, an empty record type cannot be used as an input object type in GraphQL, and using an empty record type will result in a compilation error.

Note: Since GraphQL schema can not use the same type as an input and an output type when a record type is used as an input and an output, a compilation error will be thrown.

Note: Alias types of record types are not allowed to be used as input object types in a GraphQL schema. If there is a need to utilize fields from an existing type repeatedly, ballerina type inclusion can be used.

Example: Input Objects
service on new graphql:Listener(9090) {
    resource function get author(Book book) returns string {
        return book.author;
    }
}

type Book record {|
    string title;
    string author;
|};

4.5.3 Default Values

The input arguments of a GraphQL field can have default values. In Ballerina, this is allowed by providing default values to input parameters of a resource or remote method that represents a GraphQL field. When a resource or remote method input parameter has a default value, it will be added to the generated GraphQL schema. Then, the input parameter can be omitted in the GraphQL document, even if the input type is NON_NULL.

Note: To generate a schema with a valid default value, remember to use either a literal value, a list constructor expression, or a mapping constructor expression for the default parameters or input object fields. The generated schema will use an empty string if the default value is not one of the mentioned types of expressions. Avoid using other types of expressions, such as variable assignment, as they may result in an invalid schema.

Example: Default Values
resource function get greeting(string name = "Stranger") returns string {
    return "Hello, " + name;
}

4.6 Interfaces

In GraphQL, an interface can be used to define a set of common fields for objects. Then the Object types can implement the interface with the common fields and optionally, additional fields.

In Ballerina, distinct service objects can be used to define GraphQL interfaces. The other distinct service classes can be used to implement the interface. All the service classes that are implementing the interface must provide the implementation for all resource methods declared in the interface, and they can define additional resource methods.

Non-distinct service objects and service classes can not be used to define or implement GraphQL interfaces.

Note: In order to be recognized as GraphQL objects or interfaces, the Ballerina service objects and service classes must be defined within the same module as the GraphQL service.

Example: Interfaces
public type Profile distinct service object {
    isolated resource function get name() returns string;
};

# Represents a Student as a class.
public isolated distinct service class Student {
    *Profile;
    final string name;
    final int id;

    isolated function init(string name, int id) {
        self.name = name;
        self.id = id;
    }

    isolated resource function get name() returns string {
        return self.name;
    }

    isolated resource function get id() returns int {
        return self.id;
    }
}

# Represents a Teacher as a class.
public isolated distinct service class Teacher {
    *Profile;
    final string name;
    final string subject;

    isolated function init(string name, string subject) {
        self.name = name;
        self.subject = subject;
    }

    isolated resource function get name() returns string {
        return self.name;
    }

    isolated resource function get subject() returns string {
        return self.subject;
    }
}

In the above example, the Profile object is an interface. The Student and Teacher classes are Object types that implement the Profile interface.

Counter Example: Invalid Interfaces
public type Profile service object {
    resource function get name() returns string;
};

public isolated service class Student {
    *Profile;
    final string name;
    final int id;

    isolated function init(string name, int id) {
        self.name = name;
        self.id = id;
    }

    isolated resource function get name() returns string {
        return self.name;
    }

    isolated resource function get id() returns int {
        return self.id;
    }
}

The above example results in a compilation error because the Profile object is not a distinct object, and the Student class is not a distinct class.

4.6.1 Interfaces Implementing Interfaces

In GraphQL, an interface can implement another interface. The implementing interface must define each field that is specified by the implemented interface. Interface definitions must not contain cyclic references nor implement themselves.

In Ballerina, distinct service objects can be included in other distinct service objects to achieve interface-implementing interface functionality. Including these interface objects, to itself or cyclically within other interface objects results in a compilation error.

An Object type that implements an interface must implement all the fields from that interface and its parent interfaces.

Example: Interfaces Implementing Interfaces
service on new graphql:Listener(9000) {
    resource function get node() returns Node {
        return new Image("001", "https://ballerina.io/images/ballerina-logo-white.svg", "logo");
    }
}

public type Node distinct service object {
    resource function get id() returns string;
};

public type Resource distinct service object {
    *Node;
    resource function get url() returns string;
};

public isolated distinct service class Image {
    *Resource;

    final string id;
    final string url;
    final string thumbnail;

    isolated function init(string id, string url, string thumbnail) {
        self.id = id;
        self.url = url;
        self.thumbnail = thumbnail;
    }

    isolated resource function get id() returns string {
        return self.id;
    }

    isolated resource function get url() returns string {
        return self.url;
    }

    isolated resource function get thumbnail() returns string {
        return self.thumbnail;
    }
}

In the above example, the Node and Resource objects are interfaces. The Resource interface implements the Node interface. The Image class is an Object type that implements the Resource interface. Since the Image object implements the Resource interface and the Resource interface implements the Node interface, the Image object must implement the fields from both interfaces.

5. Directives

Ballerina GraphQL services support three default directives.

5.1 @skip

The @skip directive is used to skip a field execution depending on a given condition. It can be used on a field, fragment spread, or inline fragment. The directive expects exactly one argument if, which is of type Boolean!.

The field is skipped if the value of the if argument is true.

Example: @skip

In the following query, the name field will not be queried if the variable skipName is true.

query getProfile ($skipName: Boolean!) {
    profile(id: 1) {
        name @skip(if: $skipName)
        age
    }
}

5.2 @include

The @include directive is used to include a field execution depending on a given condition. It can be used on a field, fragment spread, or inline fragment. The directive expects exactly one argument if, which is of type Boolean!.

The field is included if the value of the if argument is true.

Example: @include

In the following query, the name field will be queried only if the variable includeName is true.

query getProfile ($includeName: Boolean!) {
    profile(id: 1) {
        name @include(if: $includeName)
        age
    }
}

Note: Neither the @skip nor the @include has precedence over the other. In the case that both the @skip and @include directives are provided on the same field or fragment, it will be queried only if the @skip condition is false and the @include condition is true. Stated conversely, the field or fragment will not be queried if either the @skip condition is true or the @include condition is false.

5.3 Deprecated

The @deprecated directive is used to indicate a deprecated field on a type or a deprecated enum value. Deprecation can use a deprecation reason as a string, which is formatted using Markdown syntax.

The @deprecated directive has one argument, reason, which is of type String.

The Ballerina GraphQL package uses the Ballerina's in-built @deprecated annotation to deprecate a field (resource/remote methods or record fields) or an enum value. The deprecation reason can be provided as a part of the doc comment of the particular schema member.

Example: Using the @deprecated Annotation

The following code shows how to mark a field and an enum value as deprecated with the deprecation reason.

import ballerina/graphql;

service on new graphql:Listener(9090) {

    # Greets back with a customized greeting with the provided name.
    # + name - The name of the person to greet
    # + return - The customized greeting message
    # # Deprecated
    # The `hello` field is deprecated. Use the `greeting` field instead of this.
    @deprecated
    resource function get hello(string name) returns string {
        return "Hello, " + name;
    }

    # Return the name of the member
    # + return - The full name of the member
    resource function get name() returns Name {
        return {first: "John", last: ""};
    }

    # Returns the current admission status of the pub.
    # + return - The current admission status of the pub
    resource function get status() returns Status {
        return OPEN;
    }
}

# Represents the different admission statuses of the pub.
public enum Status {
    # Open for everyone
    OPEN,
    # Pub is closed
    CLOSED,
    # Only the members are allowed
    MEMBERS_ONLY,
    # Only the VIPs are allowed
    VIP,
    # A private party is being held, only invitees are allowed
    # # Deprecated
    # Private parties are no longer supported
    @deprecated
    PRIVATE_PARTY
}

# Represents the name of the member.
type Name record {|
    # The first name
    string first;
    # The last name
    # # Deprecated
    # This field is deprecated
    @deprecated
    string last;
|};

In the above service, the generated schema will indicate that the hello field of the Query type, the PRIVATE_PARTY value of the Status enum type and the last field of the Name type are deprecated, with the reasons provided in the doc comments. (The reason will be the line after the # # Deprecated line.)

6. Errors

6.1 Error Detail Record

The graphql:ErrorDetail is used to describe an error (according to the GraphQL specification) occurred in a GraphQL response. It contains the following fields.

6.1.1 Message

The message contains the error message from the particular error.

6.1.2 Locations

The locations field contains the locations of the GraphQL document associated with the error. There can be cases where more than one location can cause the error, therefore, this field is an array of locations. There are also cases where a location can not be associated with an error, therefore, this field is optional.

6.1.3 Path

The path field is an array of Int and String, that points to a particular path of the document tree associated with the error. This field will have a value only when a particular error has occurred at the execution phase. Therefore, this field is optional.

6.1.4 Extensions

The extensions field is an optional field containing a map with string keys. This can be used to send additional metadata related to the error.

6.2 Service Error Handling

A GraphQL service can return errors so that the client can handle them. The Ballerina errors types are used to return errors to the GraphQL client. The Ballerina GraphQL service will convert the Ballerina errors to GraphQL errors and send them to the client.

6.2.1 Returning Errors

A Ballerina resource or remote method representing a GraphQL object field can return an error. When an error is returned, it will be added to the errors field in the GraphQL response. The shape of the error object is described in the Error Detail Record section.

When an error is returned from a GraphQL resolver, the error message is added as the message of the error, and the location of the document that caused the error will be added to the locations field. The path of the error (from the root of the document) will be added as the path in the error response. Currently, the Ballerina GraphQL service will not add any metadata in the extensions field.

If a resolver execution results in an error, the stacktrace of the error will be logged to the stderr of the server.

Example: Returning Errors
service on new graphql:Listener(9090) {
    resource function get greeting(string name) returns string|error {
        if name == "" {
            return error("Invalid name provided");
        }
        return string`Hello ${name}`;
    }
}

The above example shows how to return an error from a Ballerina GraphQL resource method.

The following document can be used to query the above GraphQL service.

{
    greeting(name: "")
}

The result of the above document is the following.

{
    "errors": [
        {
            "message": "Invalid name provided",
            "locations": [
                {
                    "line": 2,
                    "column": 4
                }
            ],
            "path": [
                "greeting"
            ]
        }
    ],
    "data": null
}

6.2.2 Returning Errors and Nil Values

If a resource or remote method representing a field of a GraphQL object returns an error, the corresponding field value under the data field will be null in the response, in addition to adding an entry in the errors field. In this case, if the field type is NON_NULL, the null value will be propagated to the upper levels until the null value is allowed. This might cause the whole data field to become null.

To avoid this, a method can optionally include nil type (denoted by () in Ballerina). When the method return type includes nil, the response is allowed to add null as the field value, thus it stops propagating the null value to upper levels.

Note: Making a field nullable should be a conscious decision made by the developer. This usually comes to the point on what is considered an error. In some use cases, partial data is considered valid. For example, when retrieving profile, the receiving the name field and not receiving the age field can be considered valid based on the use case. Similarly, receiving the age field, but not receiving the name field can be considered an error. In that case, the age field can be nullable, while the name field can be NON_NULL type.

Example: Returning Errors with Nil Values
service on new graphql:Listener(9090) {

    resource function get profile(int id) returns Profile {
        return new ("Walter White", 50);
    }
}

service class Profile {
    resource function get name() returns string|error {
        // Implementation
    }

    resource function get age() returns int|error? {
        // Implementation
    }
}

Above example shows how the name field of the Profile object can return an error. But the return type does not include the nil type. Therefore, if this field returns an error, first, the name field will become null. Since the name field is NON_NULL, the value is propagated to the upper level, making the profile field null in the data field of the response. But the profile field is also wrapped with the NON_NULL type as the profile resource method does not include nil as the return type. Hence, the null value will be propagated further, making the whole data field null.

Similarly, the age field of the Profile object can return an error too. But it has nil as one of the possible return types (denoted by ?: a syntactic sugar for |()). In this case, if an error is returned from the method, the age field of the Profile object will become null. Since it does allow null values (i.e. the type is not wrapped by NON_NULL type), the null value will not be propagated further. In such cases, the response can contain the error as well as the part of the data field.

Example: Sample Response for Error Returned from a NON_NULL field
{
    "errors": [
        {
            "message":"Error occurred while retrieving name",
            "locations": [
                {
                    "line": 1,
                    "column": 20
                }
            ],
            "path": ["profile", "name"]
        }
    ],
    "data":null
}
Example: Sample Response for Error Returned from a Nullable field
{
    "errors": [
        {
            "message":"Error occurred while retrieving age",
            "locations": [
                {
                    "line":1,
                    "column":25
                }
            ],
            "path": ["profile", "age"]
        }
    ],
    "data": {
        "profile": {
            "name":"Walter White",
            "age":null
        }
    }
}

6.2.3 The graphql:__addError Function

The graphql:__addError function can be used to add a custom error to the response. This function accepts two parameters.

  1. graphql:Context - The context of the GraphQL request.
  2. graphql:ErrorDetail - The details of the error.

Note: Using the graphql:__addError function is not recommended until it is absolutely necessary. It is recommended to return an error from the resource/remote method instead.

Example: Using the graphql:__addError Function
service on new graphql:Listener(9090) {
    resource function get greeting(graphql:Context context, graphql:Field 'field, string name) returns string? {
        if name == "" {
            graphql:ErrorDetail errorDetail = {
                message: "Invalid name provided",
                locations: ['field.getLocation()],
                path: 'field.getPath(),
                extensions: {
                    code: "INVALID_NAME"
                }
            };
            graphql:__addError(context, errorDetail);
            return;
        }
        return string `Hello, ${name}!`;
    }
}

6.3 Client Error Handling

The response returned from the execute method of the GraphQL client can include errors. This section describes how to handle those errors on the Ballerina client side. All the errors that occurred during a GraphQL client operation are categorized as graphql:ClientError error type. All the other errors are subtypes of this error type.

Example: Handle Client Error
graphql:Client graphqlClient = check new ("localhost:9090/graphql");
string document = "{ profile { name } }";
ProfileResponse|graphql:ClientError response = graphqlClient->execute(document);

if response is graphql:ClientError {
    // Handle error
}

The above example shows how to capture the graphql:ClientError. This way all the errors that are returned are treated the same.

6.3.1 Request Error

There can be errors occurred during sending and validating a GraphQL request. These errors are categorized under the graphql:RequestError error type, which is a subtype of the graphql:ClientError.

Example: Handle Request Error

Above example shows how the graphql:RequestErrors are handled separately.

6.3.1.1 HTTP Error

When a network error is occurred while sending a request to the GraphQL API, those errors are captured by graphql:HttpError error type. These errors can be related to any HTTP level error including invalid URL, network-level failures, timeouts, etc. When the HTTP response has a body, it can be accessed by the body field of the details of the error.

Example: Handle HTTP Error
graphql:Client graphqlClient = check new ("localhost:9090/graphql");
string document = "{ profile { name } }";
ProfileResponse|graphql:ClientError response = graphqlClient->execute(document);

if response is graphql:HttpError {
    // Get response body
    anydata body = response.detail().body;
    // Handle error
}

The above example shows how the graphql:HttpErrors are handled separately.

6.3.1.2 Invalid Document Error

When the document sent to the GraphQL API is invalid, a validation error will be returned. These errors are captured by graphql:InvalidDocumentError error type. The ErrorDetail records are added to the errors field of the error detail so the details of the error can be retrieved.

Example: Handle Invalid Document Error
graphql:Client graphqlClient = check new ("localhost:9090/graphql");
string document = "{ profile { name } }";
ProfileResponse|graphql:ClientError response = graphqlClient->execute(document);

if response is graphql:InvalidDocumentError {
    // Get error details
    graphql:ErrorDetail[]? errors = response.detail().errors;
    // Handle error
}

The above example shows how the graphql:InvalidDocumentErrors are handled separately.

6.3.2 Payload Binding Error

As described in the section Client Data Binding, the response can be data-bound. When the data binding fails, a graphql:PayloadBindingError will be returned. This error can occur due to a mismatch between the shape of the expected type and the actual response from the GraphQL API. The ErrorDetail records are added to the errors field of the error detail so the details of the error can be retrieved.

Example: Handle Payload Binding Error
graphql:Client graphqlClient = check new ("localhost:9090/graphql");
string document = "{ profile { name } }";
ProfileResponse|graphql:ClientError response = graphqlClient->execute(document);

if response is graphql:PayloadBindingError {
    // Get error details
    graphql:ErrorDetail[]? errors = response.detail().errors;
    // Handle error
}

The above example shows how the graphql:PayloadBindingErrors are handled separately.

Example: GraphQL Client Error Handling

The following example demonstrates graphql:Client error handling and shows how to obtain GraphQL-specific errors returned by the graphql server.

type ProfileResponse record {|
    *graphql:GenericResponseWithErrors;
    record {|Profile profile;|} data;
|};

type Profile record {|
    string name;
    int age;
|};

public function main() returns error? {
    do {
        graphql:Client graphqlClient = check new ("localhost:9090/graphql");
        string document = "{ profile { name } }";
        ProfileResponse response = check graphqlClient->execute(document);
        io:println(response);
    } on fail graphql:ClientError err {
        handleErrors(err);
    }
}

function handleErrors(graphql:ClientError clientError) {
    if clientError is graphql:PayloadBindingError {
        // Get error details
        graphql:ErrorDetail[]? errors = clientError.detail().errors;
        // Handle Payload Binding Error
    } else if clientError is graphql:InvalidDocumentError {
        // Get error details
        graphql:ErrorDetail[]? errors = clientError.detail().errors;
        // Handle error
    } else if clientError is graphql:HttpError {
        // Get response body
        anydata body = response.detail().body;
        // Handle error
    }
}

7. Annotations

7.1 Service Configuration

The configurations stated in the graphql:ServiceConfig, are used to change the behavior of a particular GraphQL service. These configurations are applied to the service.

This annotation consists of the following fields.

7.1.1 Max Query Depth

The maxQueryDepth field is used to provide a limit on the depth of an incoming request. When this is set, every incoming request is validated by checking the depth of the query. This includes the depths of the spread fragments. If a particular GraphQL document exceeds the maximum query depth, the request is invalidated and the server will respond with an error.

Example: Setting Max Query Depth
@graphql:ServiceConfig {
    maxQueryDepth: 3
}
service on new graphql:Listener(9090) {

}

In the above example, when a document has a depth of more than 3, the request will be failed.

Example: Invalid Document with Exceeding Max Query Depth
{
    profile {
        friend {
            friend {
                name # Depth is 4
            }
        }
    }
}

This will result in the following response.

{
  "error": {
    "errors": [
      {
        "message": "Query has depth of 4, which exceeds max depth of 3",
        "locations": [
          {
            "line": 1,
            "column": 1
          }
        ]
      }
    ]
  }
}

7.1.2 Auth Configurations

The auth field is used to provide configurations related to authentication and authorization for the GraphQL API. The Security section will explain this configuration in detail.

7.1.3 Context Initializer Function

The contextInit field is used to provide a method to initialize the graphql:Context object. It is called per each request to create a graphql:Context object.

The context initializer function can return an error if the validation is failed. In such cases, the request will not proceed, and an error will be returned immediately.

Following is the function template for the contextInit function.

isolated function (http:RequestContext requestContext, http:Request request) returns graphql:Context|error {}

When contextInit is not provided, a default function will be set as the value of the field. The default function definition is below.

isolated function initDefaultContext(http:RequestContext requestContext, http:Request request) returns Context|error {
    return new;
}

The contextInit function can be provided inline, or as a function pointer.

Example: Provide Context Initializer Function Inline
@graphql:ServiceConfig {
    contextInit: isolated function(http:RequestContext requestContext, http:Request request) returns graphql:Context|error {
        // ...
    }
}
Example: Provide Context Initializer Function as a Function Pointer
isolated function initContext(http:RequestContext requestContext, http:Request request) returns graphql:Context|error {
    // ...
}

@graphql:ServiceConfig {
    contextInit: initContext
}
service on new graphql:Listener(9090) {
    // ...
}

Note: The init function has http:RequestContext and http:Request objects as inputs. These objects are passed into the function when a request is received. The HTTP headers and the request context can be used to perform additional validations to a request before proceeding to the GraphQL validations. This can be useful to validate the HTTP request before performing the GraphQL operations. The Imperative Approach in Security section will discuss this in detail.

7.1.4 CORS Configurations

The cors field is used to configure CORS configurations for the GraphQL service.

Example: CORS Configurations
@graphql:ServiceConfig {
    cors: {
        allowOrigins: ["http://www.wso2.com", "http://www.ballerina.io"],
        allowCredentials: false,
        allowHeaders: ["CORELATION_ID"],
        exposeHeaders: ["X-CUSTOM-HEADER"],
        maxAge: 84900
    }
}
service on new graphql:Listener(9090) {
    // ...
}

7.1.5 GraphiQL Configurations

The graphiql field is used to provide the GraphiQL client configuration to enable the GraphiQL client for a given GraphQL service.

Example: GraphiQL Configurations
@graphql:ServiceConfig {
    graphiql: {
        enabled: true,
        path: "/ballerina/graphiql",
        printUrl: false
    }
}
service on new graphql:Listener(9090) {
    // ...
}
7.1.5.1 The enabled Field

The field enabled accepts a boolean that denotes whether the client is enabled or not. By default, it has been set to false.

7.1.5.2 The path Field

The optional field path accepts a valid string for the GraphiQL service. If the path is not given in the configuration, /graphiql is set as the default path.

7.1.5.3 The printUrl Field

The optional field printUrl accepts a boolean that denotes whether the GraphiQL url is printed to stdout or not. By default, it has been set to true.

7.1.6 Service Interceptors

The interceptors field is used to provide the service interceptors.

Example: Single Service Interceptor
@graphql:ServiceConfig {
    interceptors: new Interceptor1()
}
service on new graphql:Listener(9090) {
    // ...
}
Example: Array of Service Interceptors
@graphql:ServiceConfig {
    interceptors: [new Interceptor1(), new Interceptor2()]
}
service on new graphql:Listener(9090) {
    // ...
}

7.1.7 Introspection Configurations

The introspection field is used to enable or disable the GraphQL introspection query support. If the introspection query support is disabled, the GraphQL service won't allow the execution of the __schema and the __type introspection queries. However, the __typename introspection will work even if the introspection query support is disabled. By default, introspection is enabled for Ballerina GraphQL services.

Example: Disable Introspection Query Support
@graphql:ServiceConfig {
    introspection: false
}
service on new graphql:Listener(9090) {
    // ...
}

Note: It is recommended to disable introspection in production environments until it is required.

7.1.8 Constraint Configurations

The validation field is used to enable or disable the validation of constraints defined on GraphQL input types. If constraint validation support is enabled, the GraphQL service verifies all constraints set on the GraphQL inputs when executing the resolver. By default, constraint validation is enabled for Ballerina GraphQL services.

Example: Disable Constraint Validation Support
@graphql:ServiceConfig {
    validation: false
}
service on new graphql:Listener(9090) {
    // ...
}

7.2 Resource Configuration

The configurations stated in the graphql:ResourceConfig, are used to change the behavior of a particular GraphQL resolver. These configurations are applied to the resolver functions.

This annotation consists of the following field.

7.2.1 Field Interceptors

The interceptors field is used to provide the field interceptors.

Example: Single Field Interceptor
service on new graphql:Listener(9090) {

    @graphql:ResourceConfig {
        interceptors: new Interceptor1()
    }
    resource function get name(int id) returns string {
      // ...
   }
}
Example: Array of Field Interceptors
service on new graphql:Listener(9090) {

    @graphql:ResourceConfig {
        interceptors: [new Interceptor1(), new Interceptor2()]
    }
    resource function get name(int id) returns string {
      // ...
   }
}

7.2.2 Prefetch Method Name Configuration

The prefetchMethodName field is used override the default prefetch method name. To know more about the prefetch method, refer to the Define the Corresponding prefetch Method section.

Example: Override Prefetch Method Name
service on new graphql:Listener(9090) {

    function loadBooks(graphql:Context ctx) {
        // ...
    }

    @graphql:ResourceConfig {
        prefetchMethodName: "loadBooks"
    }
    resource function get books(graphql:Context ctx) returns Book[] {
      // ...
   }
}

7.3 Interceptor Configuration

The configurations stated in the graphql:InterceptorConfig, are used to change the behavior of a particular GraphQL interceptor.

7.3.1 Scope Configuration

The field global is used to configure the scope of the interceptor. If the global field is set as true, the interceptor will be applied to each field and subfield of the service. If the flag is set as false, the interceptor will be applied top level fields of the Query, Mutation and Subscription types, but not to the subfields of them. By default, the global flag is set as true.

Note: The scope configuration is applied only to the GraphQL service interceptors.

Example: Scope Configuration
@graphql:InterceptorConfig {
    global:false
}
readonly service class LogInterceptor {
   *graphql:Interceptor;

    isolated remote function execute(graphql:Context context, graphql:Field 'field) returns anydata|error {
        // ...
    }
}

7.4 ID Annotation

The graphql:ID annotation is used to define the GraphQL ID scalar type. This annotation can be used to annotate a field of a GraphQL object type or a GraphQL input type.

8. Security

8.1 Service Authentication and Authorization

There are two ways to enable authentication and authorization in Ballerina GraphQL service.

  1. Declarative approach
  2. Imperative approach

8.1.1 Declarative Approach

This is also known as the configuration-driven approach, which is used for simple use cases, where users have to provide a set of configurations and do not need to be worried more about how authentication and authorization work. The user does not have full control over the configuration-driven approach.

The service configurations are used to define the authentication and authorization configurations. Users can configure the configurations needed for different authentication schemes and configurations needed for authorizations of each authentication scheme. The configurations can be provided at the service level. The auth handler creation and request authentication/authorization is handled internally without user intervention. The requests that succeeded in authentication and/or authorization phases according to the configurations will be passed to the business logic layer.

8.1.1.1 Basic Authentication - File User Store

A GraphQL service can be secured using Basic Authentication with File User Store and optionally by enforcing authorization.

When configured, it validates the Authorization header in the HTTP request that contains the GraphQL document. This reads the data from a TOML file, that stores the usernames and passwords for authentication and the scopes for authorization.

Example: Declarative Basic Authentication with File User Store
@graphql:ServiceConfig {
    auth: [
        {
            fileUserStoreConfig: {},
            scopes: "admin"
        }
    ]
}
service on new graphql:Listener(9090) {
    resource function get greeting() returns string {
        return "Hello, World!";
    }
}

The Config.toml file below will be used to define the users.

[[ballerina.auth.users]]
username="alice"
password="alice@123"
scopes=["developer"]


[[ballerina.auth.users]]
username="bob"
password="bob@123"
scopes=["developer", "admin"]
8.1.1.2 Basic Authentication - LDAP User Store

A GraphQL service can be secured using Basic Authentication with LDAP User Store and optionally by enforcing authorization.

When configured, it validates the Authorization header in the HTTP request that contains the GraphQL document. This reads the data from the configured LDAP, which stores the usernames and passwords for authentication and the scopes for authorization.

Example: Declarative Basic Authentication with LDAP User Store
@graphql:ServiceConfig {
    auth: [
        {
            ldapUserStoreConfig: {
                domainName: "bcssl.lk",
                connectionUrl: "ldap://localhost:389",
                connectionName: "cn=admin,dc=bcssl,dc=lk",
                connectionPassword: "bcssl123",
                userSearchBase: "ou=Users,dc=bcssl,dc=lk",
                userEntryObjectClass: "inetOrgPerson",
                userNameAttribute: "uid",
                userNameSearchFilter: "(&(objectClass=inetOrgPerson)(uid=?))",
                userNameListFilter: "(objectClass=inetOrgPerson)",
                groupSearchBase: ["ou=Groups,dc=bcssl,dc=lk"],
                groupEntryObjectClass: "groupOfNames",
                groupNameAttribute: "cn",
                groupNameSearchFilter: "(&(objectClass=groupOfNames)(cn=?))",
                groupNameListFilter: "(objectClass=groupOfNames)",
                membershipAttribute: "member",
                userRolesCacheEnabled: true,
                connectionPoolingEnabled: false,
                connectionTimeout: 5,
                readTimeout: 60
            },
            scopes: "admin"
        }
    ]
}
service /graphql on securedEP {
    resource function get greeting() returns string {
        return "Hello, World!";
    }
}
8.1.1.3 JWT Authentication

A GraphQL service can be secured using JWT Authentication and by enforcing authorization optionally.

When configured, it validates the JWT sent in the Authorization header in the HTTP request that contains the GraphQL document.

Example: Declarative JWT Authentication
@graphql:ServiceConfig {
    auth: [
        {
            jwtValidatorConfig: {
                issuer: "wso2",
                audience: "ballerina",
                signatureConfig: {
                    certFile: "path/to/public.crt"
                },
                scopeKey: "scp"
            },
            scopes: "admin"
        }
    ]
}
service /graphql on securedEP {
    resource function get greeting() returns string {
        return "Hello, World!";
    }
}
8.1.1.4 OAuth2

A GraphQL service can be secured using OAuth2 and by enforcing authorization optionally.

When configured, it validates the OAuth2 token sent in the Authorization header in the HTTP request that contains the GraphQL document. This calls the configured OAuth2 introspection endpoint to validate.

Example: Declarative Approach to Secure Service Using OAuth2
@graphql:ServiceConfig {
    auth: [
        {
            oauth2IntrospectionConfig: {
                url: "https://localhost:9445/oauth2/introspect",
                tokenTypeHint: "access_token",
                scopeKey: "scp",
                clientConfig: {
                    customHeaders: {"Authorization": "Basic YWRtaW46YWRtaW4="},
                    secureSocket: {
                        cert: "path/to/public.crt"
                    }
                }
            },
            scopes: "admin"
        }
    ]
}
service /graphql on securedEP {
    resource function get greeting() returns string {
        return "Hello, World!";
    }
}

8.1.2 Imperative Approach

This is also known as the code-driven approach, which is used for advanced use cases, where users need to be worried more about how authentication and authorization work and need to have further customizations. The user has full control of the code-driven approach. The handler creation and authentication/authorization calls are made by the user at the business logic layer.

The graphql:Context object and the contextInit method can be used to achieve this.

8.1.2.1 Basic Authentication - File User Store

A file user store can be used to validate the Authorization header in the HTTP request that contains the GraphQL document.

Example: Imperative Basic Authentication with File User Store
final http:ListenerFileUserStoreBasicAuthHandler handler = new;

isolated function contextInit(http:RequestContext reqCtx, http:Request request) returns graphql:Context|error {
    string authorization = check request.getHeader("Authorization");
    graphql:Context context = new;
    context.set("Authorization", authorization);
    return context;
}

readonly service class AuthInterceptor {
    *graphql:Interceptor;

    isolated remote function execute(graphql:Context context, graphql:Field 'field) returns anydata|error {
        value:Cloneable|isolated object {} authorization = check context.get("Authorization");
        if authorization !is string {
            return error("Failed to authorize");
        }
        auth:UserDetails|http:Unauthorized authn = handler.authenticate(authorization);
        if authn is http:Unauthorized {
            return error("Unauthorized");
        }
        http:Forbidden? authz = handler.authorize(authn, "admin");
        if authz is http:Forbidden {
            return error("Forbidden");
        }
        return context.resolve('field);
    }
}

@graphql:ServiceConfig {
    contextInit: contextInit,
    interceptors: new AuthInterceptor()
}
service on new graphql:Listener(9090) {

    resource function get greeting(graphql:Context context) returns string {
        return "welcome";
    }
}

The Config.toml file below will be used to define the users.

[[ballerina.auth.users]]
username="alice"
password="alice@123"
scopes=["developer"]


[[ballerina.auth.users]]
username="bob"
password="bob@123"
scopes=["developer", "admin"]
8.1.2.2 Basic Authentication - LDAP User Store

An LDAP user store can be used to validate the Authorization header in the HTTP request that contains the GraphQL document.

Example: Imperative Basic Authentication with LDAP User Store
graphql:LdapUserStoreConfig config = {
    domainName: "bcssl.lk",
    connectionUrl: "ldap://localhost:389",
    connectionName: "cn=admin,dc=bcssl,dc=lk",
    connectionPassword: "bcssl123",
    userSearchBase: "ou=Users,dc=bcssl,dc=lk",
    userEntryObjectClass: "inetOrgPerson",
    userNameAttribute: "uid",
    userNameSearchFilter: "(&(objectClass=inetOrgPerson)(uid=?))",
    userNameListFilter: "(objectClass=inetOrgPerson)",
    groupSearchBase: ["ou=Groups,dc=bcssl,dc=lk"],
    groupEntryObjectClass: "groupOfNames",
    groupNameAttribute: "cn",
    groupNameSearchFilter: "(&(objectClass=groupOfNames)(cn=?))",
    groupNameListFilter: "(objectClass=groupOfNames)",
    membershipAttribute: "member",
    userRolesCacheEnabled: true,
    connectionPoolingEnabled: false,
    connectionTimeout: 5,
    readTimeout: 60
};
final http:ListenerLdapUserStoreBasicAuthHandler handler = new (config);

isolated function contextInit(http:RequestContext reqCtx, http:Request request) returns graphql:Context|error {
    string authorization = check request.getHeader("Authorization");
    graphql:Context context = new;
    context.set("Authorization", authorization);
    return context;
}

readonly service class AuthInterceptor {
    *graphql:Interceptor;

    isolated remote function execute(graphql:Context context, graphql:Field 'field) returns anydata|error {
        value:Cloneable|isolated object {} authorization = check context.get("Authorization");
        if authorization !is string {
            return error("Failed to authorize");
        }
        auth:UserDetails|http:Unauthorized authn = handler->authenticate(authorization);
        if authn is http:Unauthorized {
            return error("Unauthorized");
        }
        http:Forbidden? authz = handler->authorize(authn, "admin");
        if authz is http:Forbidden {
            return error("Forbidden");
        }
        return context.resolve('field);
    }
}

@graphql:ServiceConfig {
    contextInit: contextInit,
    interceptors: new AuthInterceptor()
}
service on new graphql:Listener(9090) {

    resource function get greeting(graphql:Context context) returns string {
        return "welcome";
    }
}
8.1.2.3 JWT Authentication

A JWT configuration can be used to validate the Authorization header in the HTTP request that contains the GraphQL document.

Example: Imperative JWT Authentication
graphql:JwtValidatorConfig config = {
    issuer: "ballerina",
    audience: ["wso2"],
    signatureConfig: {
        jwksConfig: {
            url: "https://localhost:8080/jwks"
        }
    }
};
final http:ListenerJwtAuthHandler handler = new (config);

isolated function contextInit(http:RequestContext reqCtx, http:Request request) returns graphql:Context|error {
    string authorization = check request.getHeader("Authorization");
    graphql:Context context = new;
    context.set("Authorization", authorization);
    return context;
}

readonly service class AuthInterceptor {
    *graphql:Interceptor;

    isolated remote function execute(graphql:Context context, graphql:Field 'field) returns anydata|error {
        value:Cloneable|isolated object {} authorization = check context.get("Authorization");
        if authorization !is string {
            return error("Failed to authorize");
        }
        jwt:Payload|http:Unauthorized authn = handler.authenticate(authorization);
        if authn is http:Unauthorized {
            return error("Unauthorized");
        }
        if authn is jwt:Payload {
            http:Forbidden? authz = handler.authorize(authn, "admin");
            if authz is http:Forbidden {
                return error("Forbidden");
            }
        }
        return context.resolve('field);
    }
}

@graphql:ServiceConfig {
    contextInit: contextInit,
    interceptors: new AuthInterceptor()
}
service on new graphql:Listener(9090) {

    resource function get greeting(graphql:Context context) returns string {
        return "welcome";
    }
}
8.1.2.4 OAuth2

An OAuth2 introspection endpoint can be used to validate the Authorization header in the HTTP request that contains the GraphQL document.

Example: Imperative Approach to Secure Service Using OAuth2
graphql:OAuth2IntrospectionConfig config = {
    url: "https://localhost:8080/oauth2/introspect",
    tokenTypeHint: "access_token"
};
final http:ListenerOAuth2Handler handler = new (config);

isolated function contextInit(http:RequestContext reqCtx, http:Request request) returns graphql:Context|error {
    string authorization = check request.getHeader("Authorization");
    graphql:Context context = new;
    context.set("Authorization", authorization);
    return context;
}

readonly service class AuthInterceptor {
    *graphql:Interceptor;

    isolated remote function execute(graphql:Context context, graphql:Field 'field) returns anydata|error {
         value:Cloneable|isolated object {} authorization = check context.get("Authorization");
        if authorization !is string {
            return error("Failed to authorize");
        }
        oauth2:IntrospectionResponse|http:Unauthorized|http:Forbidden auth = handler->authorize(authorization, "admin");
        if auth is http:Unauthorized {
            return error("Unauthorized");
        }
        if auth is http:Forbidden {
            return error("Forbidden");
        }
        return context.resolve('field);
    }
}

@graphql:ServiceConfig {
    contextInit: contextInit,
    interceptors: new AuthInterceptor()
}
service on new graphql:Listener(9090) {

    resource function get greeting(graphql:Context context) returns string {
        return "welcome";
    }
}

8.2 Client Authentication and Authorization

Authentication and authorization in Ballerina GraphQL clients can be enabled using the declarative approach.

8.2.1 Basic Authentication

Ballerina GraphQL clients enable basic authentication with credentials by setting the graphql:CredentialsConfig configurations in the client. The requests from the client are automatically enriched with the Authorization: Basic <token> header when passing the graphql:CredentialsConfig for the auth configuration of the client.

Example: Client Using Declarative Basic Authentication
public function main() returns error? {
    graphql:Client graphqlClient = check new ("localhost:9090/graphql",
        auth = {
            username: "bob",
            password: "bob@123"
        }
    );
    string document = "{ one: profile(id: 100) {name} }";
    ProfileResponseWithErrors response = check graphqlClient->execute(document);
    // ...
}
8.2.2 Bearer Token Authentication

Ballerina GraphQL clients enable authentication using bearer tokens by setting the graphql:BearerTokenConfig configurations in the client. The requests from the client are automatically enriched with the Authorization: Bearer <token> header when passing the graphql:BearerTokenConfig for the auth configuration of the client.

Example: Client Using Declarative Bearer Token Authentication
public function main() returns error? {
    graphql:Client graphqlClient = check new ("localhost:9090/graphql",
        auth = {
            token: "56ede317-4511-44b4-8579-a08f094ee8c5"
        }
    );
    string document = "{ one: profile(id: 100) {name} }";
    ProfileResponseWithErrors response = check graphqlClient->execute(document);
    // ...
}
8.2.3 Self-Signed JWT Authentication

Ballerina GraphQL clients enable authentication using JWTs by setting the graphql:JwtIssuerConfig configurations in the client. The requests from the client are automatically enriched with the Authorization: Bearer <token> header when passing the graphql:JwtIssuerConfig for the auth configuration of the client.

Example: Client Using Declarative Self Signed JWT Authentication
public function main() returns error? {
    graphql:Client graphqlClient = check new ("localhost:9090/graphql",
        auth = {
            username: "ballerina",
            issuer: "wso2",
            audience: ["ballerina", "ballerina.org", "ballerina.io"],
            keyId: "5a0b754-895f-4279-8843-b745e11a57e9",
            jwtId: "JlbmMiOiJBMTI4Q0JDLUhTMjU2In",
            customClaims: {scp: "admin"},
            expTime: 3600,
            signatureConfig: {
                config: {
                    keyFile: "path/to/private.key"
                }
            }
        }
    );
    string document = "{ one: profile(id: 100) {name} }";
    ProfileResponseWithErrors response = check graphqlClient->execute(document);
    // ...
}
8.2.4 OAuth2

Ballerina GraphQL clients enable authentication using OAuth2 by setting the graphql:OAuth2GrantConfig configurations in the client. The requests from the client are automatically enriched with the Authorization: Bearer <token> header when passing the graphql:OAuth2GrantConfig for the auth configuration of the client. OAuth2 can configure in following 4 ways

8.2.4.1 Client Credentials Grant Type
graphql:Client graphqlClient = check new ("localhost:9090/graphql",
    auth = {
        tokenUrl: "localhost:9445/oauth2/token",
        clientId: "FlfJYKBD2c925h4lkycqNZlC2l4a",
        clientSecret: "PJz0UhTJMrHOo68QQNpvnqAY_3Aa",
        scopes: "admin",
        clientConfig: {
            secureSocket: {
                cert: "path/to/public.crt"
            }
        }
    }
);
8.2.4.2 Password Grant Type
graphql:Client graphqlClient = check new ("localhost:9090/graphql",
    auth = {
        tokenUrl: "https://localhost:9445/oauth2/token",
        username: "admin",
        password: "admin",
        clientId: "FlfJYKBD2c925h4lkycqNZlC2l4a",
        clientSecret: "PJz0UhTJMrHOo68QQNpvnqAY_3Aa",
        scopes: "admin",
        refreshConfig: {
            refreshUrl: "https://localhost:9445/oauth2/token",
            scopes: "hello",
            clientConfig: {
                secureSocket: {
                    cert: "path/to/public.crt"
                }
            }
        },
        clientConfig: {
            secureSocket: {
                cert: "path/to/public.crt"
            }
        }
    }
);
8.2.4.3 Refresh Token Grant Type
graphql:Client graphqlClient = check new ("localhost:9090/graphql",
    auth = {
        refreshUrl: "https://localhost:9445/oauth2/token",
        refreshToken: "24f19603-8565-4b5f-a036-88a945e1f272",
        clientId: "FlfJYKBD2c925h4lkycqNZlC2l4a",
        clientSecret: "PJz0UhTJMrHOo68QQNpvnqAY_3Aa",
        scopes: "admin",
        clientConfig: {
            secureSocket: {
                cert: "path/to/public.crt"
            }
        }
    }
);
8.2.4.4 JWT Bearer Grant Type
graphql:Client graphqlClient = check new ("localhost:9090/graphql",
    auth = {
        tokenUrl: "https://localhost:9445/oauth2/token",
        assertion: "eyJhbGciOiJFUzI1NiIsImtpZCI6Ij[...omitted for brevity...]",
        clientId: "FlfJYKBD2c925h4lkycqNZlC2l4a",
        clientSecret: "PJz0UhTJMrHOo68QQNpvnqAY_3Aa",
        scopes: "admin",
        clientConfig: {
            secureSocket: {
                cert: "path/to/public.crt"
            }
        }
    }
);

8.3 SSL/TLS and Mutual SSL

The Ballerina GraphQL listeners/clients can be used to communicate via a secured connection. This section defines the specifications for creating Ballerina GraphQL listeners and clients to communicate via a secured connection.

8.3.1 Listener

8.3.1.1 SSL/TLS

The GraphQL listener can be secured to communicate via HTTPS using SSL/TLS. The graphql:ListenerSecureSocket can be used to configure the listener to expose an HTTPS connection.

Alternatively, an HTTP listener configured to connect with an HTTPS client can also be used to create the GraphQL listener to expose an HTTPS connection.

Example: SSL/TLS Configuration of the GraphQL Listener
listener graphql:Listener securedGraphqlListener = new(9090,
    secureSocket = {
        key: {
            certFile: "/path/to/public.crt",
            keyFile: "/path/to/private.key"
        }
    }
);

service on securedGraphqlListener {
    resource function get greeting() returns string {
        return "Hello, World!";
    }
}
Example: GraphQL Listener Using an SSL/TLS Configured HTTP Listener
listener http:Listener securedHttpListener = new(9090,
    secureSocket = {
        key: {
            certFile: "/path/to/public.crt",
            keyFile: "/path/to/private.key"
        }
    }
);
listener graphql:Listener securedGraphqlListener = new (securedHttpListener);

service on securedGraphqlListener {
    resource function get greeting() returns string {
        return "Hello, World!";
    }
}

8.3.1.2 Mutual SSL

The GraphQL listener supports mutual SSL, which is a certificate-based authentication process in which two parties (the client and the server) authenticate each other by verifying the digital certificates.

The graphql:ListenerSecureSocket configuration can be used to configure mutual SSL for a GraphQL listener.

Alternatively, an HTTP listener configured to connect with a client with mutual SSL can also be used to create the GraphQL listener to expose an HTTPS connection.

Example: Mutual SSL Configuration of the GraphQL Listener
listener graphql:Listener securedGraphqlListener = new(9090,
    secureSocket = {
        key: {
            certFile: "/path/to/public.crt",
            keyFile: "/path/to/private.key"
        },
        mutualSsl: {
            verifyClient: http:REQUIRE,
            cert: "/path/to/public.crt"
        },
        protocol: {
            name: http:TLS,
            versions: ["TLSv1.2", "TLSv1.1"]
        },
        ciphers: ["TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA"]
    }
);

service on securedGraphqlListener {
    resource function get greeting() returns string {
        return "Hello, World!";
    }
}
Example: GraphQL Listener Using a Mutual SSL Configured HTTP Listener
listener http:Listener securedHttpListener = new(9090,
    secureSocket = {
        key: {
            certFile: "/path/to/public.crt",
            keyFile: "/path/to/private.key"
        },
        mutualSsl: {
            verifyClient: http:REQUIRE,
            cert: "/path/to/public.crt"
        },
        protocol: {
            name: http:TLS,
            versions: ["TLSv1.2", "TLSv1.1"]
        },
        ciphers: ["TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA"]
    }
);
listener graphql:Listener securedGraphqlListener = new (securedHttpListener);

service on securedGraphqlListener {
    resource function get greeting() returns string {
        return "Hello, World!";
    }
}

8.3.2 Client

8.3.2.1 SSL/TLS

A GraphQL client can communicate with a secured GraphQL service via SSL/TLS. The graphql:ClientSecureSocket configuration can be used to provide configurations related to SSL/TLS.

Example: GraphQL Client Using SSL/TLS
public function main() returns error? {
    graphql:Client graphqlClient = check new ("localhost:9090/graphql",
       secureSocket = {
            cert: "path/to/public.crt"
        }
    );
    string document = "{ one: profile(id: 100) {name} }";
    ProfileResponseWithErrors response = check graphqlClient->execute(document);
    // ...
}

8.3.2.2 Mutual SSL

A GraphQL client can communicate with a secured GraphQL service using mutual SSL. Mutual SSL can be enabled by providing the graphql:ClientSecureSocket value for the auth configuration of the client along with providing the client certificate and key files via the key configuration of the graphql:ClientSecureSocket.

Example: GraphQL Client Using Mutual SSL
public function main() returns error? {
    graphql:Client graphqlClient = check new ("localhost:9090/graphql",
       secureSocket = {
            key: {
                certFile: "path/to/public.crt",
                keyFile: "path/to/private.key"
            },
            cert: "path/to/public.crt",
            protocol: {
                name: http:TLS
            },
            ciphers: ["TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA"]
        }
    );
    string document = "{ one: profile(id: 100) {name} }";
    ProfileResponseWithErrors response = check graphqlClient->execute(document);
    // ...
}

9. Tools

9.1 GraphiQL client

The Ballerina GraphQL package provides an integrated GraphiQL client tool which is provided by the GraphQL Foundation. The client is implemented using CDN assets, and it provides a Graphical User Interface to execute the GraphQL queries. To enable the GraphiQL client, configuration should be provided as mentioned in the GraphiQL configuration section.

If the configurations are provided correctly, the GraphiQL client tool will be served at the given path when the service starts. The client can be accessed via a web browser.

Example: Enable GraphiQL Client
@graphql:ServiceConfig {
    graphiql: {
        enabled: true,
        path: "/ballerina/graphiql"
    }
}
service on new graphql:Listener(9090) {
    resource function get greeting() returns string {
        return "Hello, World!";
    }
}

Note: The GraphiQL client is used as a tool to help developers when writing a GraphQL service, and It is recommended not to enable it in production environments.

10. Advanced Features

10.1 Context Object

The graphql:Context object is used to pass meta-information of a request among the graphql resolvers. It will be created per each request.

Attributes can be stored in the graphql:Context object using key-value pairs and can be retrieved back when needed.

10.1.1 Context Methods

The graphql:Context object provides a set of methods for handling attributes.

10.1.1.1 Set Attribute in Context

To set an attribute in the graphql:Context object, the set() method can be used. It requires two parameters.

  • key: The key of the attribute. This key can be used to retrieve the attribute back when needed. The key must be a string.
  • value: The value of the attribute. The type of this parameter is value:Cloneable|isolated object {}. This means the values can be any immutable type, readonly value, or an isolated object.
Example: Set Attribute in Context
graphql:Context context = new;

context.set("key", "value");

Note: If the provided key already exists in the context, the value will be replaced.

10.1.1.2 Get Attribute from Context

To get an attribute from the graphql:Context object, the get() method can be used. It requires one parameter.

  • key: This is the key of the attribute that needs to be retrieved.

If the key does not exist in the context, the get method will return a graphql:Error.

Example: Get Context Attribute
value:Cloneable|isolated object {}|graphql:Error attribute = context.get("key");
10.1.1.3 Remove Attribute from Context

To remove an attribute from the graphql:Context object, the remove method can be used. It requires one parameter.

  • key: This is the key of the attribute that needs to be removed.

If the key does not exist in the context, the remove method will return a graphql:Error.

Example: Remove Context Attribute
graphql:Error? result = context.remove("key");

Note: Even though the functionalities are provided to update/remove attributes in the context, it is discouraged to do such operations. The reason is that destructive modifications may cause issues in parallel executions of the Query operations.

10.1.1.4 Register DataLoader in Context

To register a DataLoader in the graphql:Context object, you can use the registerDataLoader() method, which requires two parameters.

  • key: The key used to identify a specific DataLoader instance. This key can later be used to retrieve the DataLoader instance when needed. The key must be a string.
  • dataloader: The DataLoader instance.
Example: Register DataLoader in Context
graphql:Context context = new;

context.registerDataLoader("authorLoader", new dataloader:DefaultDataLoader(authorBatchFunction));
10.1.1.5 Get DataLoader from Context

To obtain a DataLoader from the graphql:Context object, you can use the getDataLoader() method, which takes one parameter.

  • key: This is the key of the DataLoader instance that needs to be retrieved.

If the specified key does not exist in the context, the getDataLoader() method will raise a panic.

Example: Get DataLoader from Context
dataloader:DataLoader authorLoader = context.getDataLoader("authorLoader");

10.1.2 Accessing the Context Object

The graphql:Context can be accessed inside any resolver. When needed, the graphql:Context should be added as a parameter of the resource or remote method representing a GraphQL field.

Note: It is recommended to add the graphql:Context as the first parameter as a convention, to increase the readability of the code.

Example: Accessing the Context
service on new graphql:Listener(9090) {
    resource function get profile(graphql:Context context) returns Profile|error {
        value:Cloneable|isolated object {} attribute = check context.get("key");
        // ...
    }
}

type Profile record {|
    string name;
    int age;
|};

Note: The parameter graphql:Context should be used only when it is required to use the context.

Example: Accessing the Context from an Object

The following example shows how to access the context from an Object. When a Ballerina service type is used as an Object type in GraphQL, the resource methods in the service can also access the context when needed.

service on new graphql:Listener(9090) {
    resource function get profile() returns Profile {
    }
}

service class Profile {
    private final string name;
    private final int age;

    function init(string name, int age) {
        self.name = name;
        self.age = age;
    }

    resource function get name() returns string {
        return self.name;
    }

    // Access the context inside a GraphQL object
    resource function get age(graphql:Context context) returns int {
        value:Cloneable|isolated object {} attribute = check context.get("key");
        // ...
        return self.age;
    }
}

10.1.3 Resolving Field Value

To resolve the value of a field, the resolve() method can be used. This requires the graphql:Field object which is related to the particular field that is going to be resolved. If the resolver has interceptors attached, the interceptors will be executed until there are no more interceptors left. If there are no interceptors left, the actual resolver will be executed.

public isolated function resolve(graphql:Field ‘field) returns anydata;

10.2 Field Object

The graphql:Field object is used to access meta-information of a field in a GraphQL document. This object can be used to retrieve information about a particular field of a GraphQL document, from a Ballerina GraphQL resource or remote method.

10.2.1 Field Object Methods

10.2.1.1 Get Field Name

This method returns the name of the currently executing field. It returns a string.

Example: Get Field Name
service on new graphql:Listener(9090) {
    resource function get profile(graphql:Field 'field) returns Profile {
        string name = 'field.getName();
    }
}
10.2.1.2 Get Field Alias

This method returns the alias of the currently executing field as a string. If the field does not have an alias, it returns the field name.

Example: Get Field Alias
service on new graphql:Listener(9090) {
    resource function get profile(graphql:Field 'field) returns Profile {
        srting alias = 'field.getAlias();
    }
}
10.2.1.3 Get Field Path

This method returns the path of the currently executing field as an array of int and string. The path is an array of indices and keys that can be used to traverse the response JSON to reach the current field.

Example: Get Field Path
service on new graphql:Listener(9090) {
    resource function get profile(graphql:Field 'field) returns Profile {
        (int|string)[] path = 'field.getPath();
    }
}
10.2.1.4 Get Subfield Names

This method returns the names of the subfields of the currently executing field as an array of string. If the field does not have any subfields, it returns an empty array.

Example: Get Subfield Names
service on new graphql:Listener(9090) {
    resource function get profile(graphql:Field 'field) returns Profile {
        string[] subfieldNames = 'field.getSubfieldNames();
    }
}
10.2.1.5 Get Field Type

This method returns the type of the currently executing field as a graphql:__Type record. The graphql:__Type record can be used to retrieve information about the type of the currently executing field.

Example: Get Field Type
service on new graphql:Listener(9090) {
    resource function get profile(graphql:Field 'field) returns Profile {
        graphql:__Type type = 'field.getType();
        // ...
    }
}
10.2.1.6 Get Subfields

This method returns the subfields of the currently executing field as an array of graphql:Field objects. It will return nil if the field is not an INTERFACE or an OBJECT, i.e. if it does not have subfields.

Example: Get Subfields

service on new graphql:Listener(9090) {
    resource function get profile(graphql:Field 'field) returns Profile {
        graphql:Field[] subfields = 'field.getSubfields();
    }
}
10.2.1.7 Get Field Location

This method returns the location of the field in the GraphQL document as a graphql:Location record. It includes the line and the column number of the field in the GraphQL document.

Example: Get Field Location
service on new graphql:Listener(9090) {
    resource function get profile(graphql:Field 'field) returns Profile {
        graphql:Location location = 'field.getLocation();
    }
}

10.2.2 Accessing the Field Object

The graphql:Field can be accessed inside any resolver. When needed, the graphql:Field should be added as a parameter of the resource or remote method representing a GraphQL field.

Note: It is recommended to add the graphql:Field as the first parameter, or the second parameter (if the graphql:Context is present as the first parameter) as a convention, to increase the readability of the code.

Example: Accessing the Field
service on new graphql:Listener(9090) {
    resource function get profile(graphql:Field 'field) returns Profile|error {
        string fieldName = 'field.getName();
        // ...
    }
}

type Profile record {|
    string name;
    int age;
|};
Example: Accessing the Field when Context is Present
service on new graphql:Listener(9090) {
    resource function get profile(graphql:Context context, graphql:Field 'field) returns Profile|error {
        string fieldName = 'field.getName();
        // ...
    }
}

type Profile record {|
    string name;
    int age;
|};
Example: Accessing the Field from an Object
service on new graphql:Listener(9090) {
    resource function get profile() returns Profile {
        return new ("Walter White", 50);
    }
}

service class Profile {
    private final string name;
    private final int age;

    function init(string name, int age) {
        self.name = name;
        self.age = age;
    }

    resource function get name() returns string {
        return self.name;
    }

    // Access the field inside a GraphQL object
    resource function get age(graphql:Field 'field) returns int {
        string fieldName = 'field.getName();
        // ...
        return self.age;
    }
}

10.3 Interceptors

The GraphQL interceptors can be used to execute a custom code before and after a resolver gets invoked.

10.3.1 Interceptor Service Object

The interceptor service object is defined in the Ballerina GraphQL package. It includes a single remote method named execute that accepts Context and Field as the parameters. The return type of the method is a union of anydata and error.

public type Interceptor distinct service object {
    isolated remote function execute(Context context, Field 'field) returns anydata|error;
};

10.3.2 Writing an Interceptor

Interceptors can be defined as a readonly service class that infers the Interceptor object provided by the GraphQL package. A user-specific name can be used as the service class name.

readonly service class InterceptorName {
   *graphql:Interceptor;

    isolated remote function execute(graphql:Context context, graphql:Field 'field) returns anydata|error {
        // Do some work
        var output = context.resolve('field);
        // Do some work
    }
}

The Interceptor service class should have the implementation of the execute() remote method that infers from the interceptor service object. The code needed to be included in the interceptor should be kept inside the execute() method. Interceptors can not have any other resource/remote methods inside the interceptor. However, the users can define the general methods inside the interceptors.

10.3.3 Execution

When it comes to interceptor execution, it follows the onion principle. Each interceptor adds a layer before and after the actual resolver invocation. Therefore, the order of the interceptor array in the configuration will be important. In an Interceptor execute() method, all the code lines that are placed before the context.resolve() will be executed before the resolver execution, and the code lines placed after the context.resolve() will be executed after the resolver execution. The context.resolve() method invokes the next interceptor.

Note: The inserting order of the interceptors into the array, will be the execution order of Interceptors. The interceptors are applied to each event in response stream of subscription resolvers.

10.3.3.1 Service Interceptors

The service interceptors are applied to all the resolvers in the GraphQL service. A GraphQL service accepts a single service interceptor or an array of service interceptors, and it should be inserted as mentioned in the Service Interceptor section. The scope of the interceptor can be configured as defined in the scope configuration section.

Example: GraphQL Service Interceptor
import ballerina/graphql;
import ballerina/log;

readonly service class ServiceInterceptor {
    *graphql:Interceptor;
    isolated remote function execute(graphql:Context context, graphql:Field 'field) returns anydata|error {
        log:printInfo(string `Service Interceptor execution!`);
        var output = context.resolve('field);
        log:printInfo("Connection closed!");
        return output;
    }
}

@graphql:ServiceConfig {
    interceptors: [new ServiceInterceptor()]
}
service /graphql on new graphql:Listener(9000) {
    resource function get name(int id) returns string {
        log:printInfo("Resolver: name");
        return "Ballerina";
    }
}

Following is the output of the server when a request is processed:

1. Service Interceptor execution!
3. Resolver: name
5. Connection closed!
10.3.3.2 Field Interceptors

The field interceptors are applied to a specific resolver in the GraphQL service. A GraphQL resolver accepts a single field interceptor or an array of field interceptors, and it should be inserted as mentioned in the Field Interceptor section.

Example: GraphQL Field Interceptor
import ballerina/graphql;
import ballerina/log;

readonly service class FieldInterceptor {
    *graphql:Interceptor;
    isolated remote function execute(graphql:Context context, graphql:Field 'field) returns anydata|error {
        log:printInfo(string `Field Interceptor execution!`);
        var output = context.resolve('field);
        log:printInfo("Connection closed!");
        return output;
    }
}

service /graphql on new graphql:Listener(9000) {
    @graphql:ResourceConfig {
        interceptors: new FieldInterceptor()
    }
    resource function get name(int id) returns string {
        log:printInfo("Resolver: name");
        return "Ballerina";
    }
}

Following is the output of the server when a request is processed:

1. Field Interceptor execution!
3. Resolver: name
5. Connection closed!

10.4 File Upload

This section describes how file uploading in Ballerina GraphQL works.

10.4.1 File Upload Protocol

The Ballerina GraphQL package uses the GraphQL Multipart Request Specification for file uploads.

Following are the required, ordered fields that must be present in a multipart request to upload a file to a Ballerina GraphQL API that supports file uploads.

10.4.1.1 The operations Field

This field contains the JSON-encoded body of standard GraphQL POST requests where all the variable values storing files must be null.

10.4.1.2 The map Field

This field contains the JSON-encoded map of the path(s) of where the file(s) occurred in the GraphQL document provided in the operations field. Each variable must be mapped to a unique key.

10.4.1.3 File Fields

The path for each file that needs to be uploaded should be provided separately. The file path should be mapped to the corresponding key in the map field.

Example: Single File Upload Request
curl localhost:9090/graphql \
    -F operations='{ "query": "mutation($file: Upload!) { fileUpload(file: $file) { link } }", "variables": { "file": null } }' \
    -F map='{ "0": ["variables.file"] }' \
    -F 0=@file1.png
Example: Multiple File Upload Request
curl localhost:9090/graphql \
    -F operations='{ "query": "mutation($file: [Upload!]) { filesUpload(file: $file) { link } }", "variables": { "file": [null, null] } }' \
    -F map='{ "0": ["variables.file.0"], "1": ["variables.file.1"]}' \
    -F 0=@file1.png
    -F 1=@file2.png
10.4.2 The graphql:Upload Type

The graphql:Upload type is used to provide information on uploaded file. The graphql:Upload type is a record type with the following fields.

10.4.2.1 The fileName Field

This field contains the name of the file that is being uploaded. The type of field is `string``.

10.4.2.2 The mimeType Field

This field contains the mime type of the file being uploaded. The type of field is string.

10.4.2.3 The encoding Field

This field contains the encoding used to serialize the file. The type of field is string.

10.4.2.4 The byteStream Field

This field contains the serialized byte stream for the uploaded file. The type of the field is stream<byte[], io:Error?>.

10.4.3 File Upload Service

A Ballerina GraphQL service can have remote methods that lets clients upload files. To upload a file, the graphql:Upload type can be used as an input of the remote method.

10.4.3.1 File Upload Resolver

Uploading a file is considered a Mutation operation in GraphQL. Therefore, only remote methods can be used for uploading files.

Example: File Upload Resolver
service on new graphql:Listener(9090) {
    remote function fileUpload(graphql:Upload fileUpload) returns boolean {
        string fileName = fileUpload.fileName;
        string mimeType = fileUpload.mimeType;
        string encoding = fileUpload.encoding;
        stream<byte[], io:Error?> byteStream = fileUpload.byteStream;

        // ...
    }
}
Example: Multiple File Upload Resolver
service on new graphql:Listener(9090) {
    remote function fileUpload(graphql:Upload[] fileUploads) returns boolean {
        foreach graphql:Upload fileUpload in fileUploads {
            string fileName = fileUpload.fileName;
            string mimeType = fileUpload.mimeType;
            string encoding = fileUpload.encoding;
            stream<byte[], io:Error?> byteStream = fileUpload.byteStream;

            // ...
        }
    }
}

10.5. Federation

10.5.1 Federated Subgraph

The Ballerina GraphQL module provides the capability to expose a graphql:Service as a federation2 subgraph. To convert a Ballerina GraphQL service into a federation2 subgraph, the graphql.subgraph submodule must be imported.

import ballerina/graphql.subgraph;

Note: The current implementation of the subgraph only supports dynamic schema composition through introspection.

10.5.1.1 The @subgraph:Subgraph Annotation

To make a Ballerina GraphQL service a federation2 subgraph, it should be annotated with @subgraph:Subgraph. This annotation adds all the subgraph schema additions to the GraphQL schema as described in the subgraph specification. Moreover, this annotation automatically adds resolvers for the _entities and _service root Query fields.

Example: Federated Subgraph
@subgraph:Subgraph
service on new graphql:Listener(9090) {
    // ...
}
10.5.1.2 The @subgraph:Entity Annotation

In a federated graph, an entity is an object type that can resolve its fields across multiple subgraphs. Each subgraph can contribute different fields to the entity and is responsible for resolving only the fields that it contributes. The @subgraph:Entity designates an object type as an entity in Ballerina. The following type definition describes the shape of the @subgraph:Entity annotation.

# Describes the shape of the `subgraph:Entity` annotation
# + key - GraphQL fields and subfields that contribute to the entity's primary key/keys
# + resolveReference - Function pointer to resolve the entity
public type FederatedEntity record {|
    string|string[] key;
    ReferenceResolver? resolveReference;
|};

# The annotation to designate a GraphQL object type as a federated entity.
public annotation FederatedEntity Entity on class, type;

To fully define an entity within a Ballerina GraphQL subgraph, you must:

  1. Assign the @subgraph:Entity annotation to an object type.
  2. Define the key field of the annotation to be the fields and subfields that contribute to the entity's primary key/keys.
  3. Define the resolveReference field of the annotation to be a function pointer to resolve the entity. If this field is set to nil, it indicates to the graph router that this subgraph does not define a reference resolver for this entity. For more details, see ReferenceResolver.

Note: In order to be recognized as subgraph entities, GraphQL object types marked with @subgraph:Entity must be defined within the same Ballerina module as the GraphQL service.

Example: Federated Entity Definition and Corresponding GraphQL Schema
ExampleBallerina definitionGraphQL schema
Simple key 
@subgraph:Entity {
    key: "id",
    resolveReference: resolveProduct
}
type Product record {
    string id;
    string name;
    int price;
};


type Product @key(fields: "id") {
    id: String!
    name: String!
    price: Int!
}
Multiple keys 
@subgraph:Entity {
    key: ["id", "sku"],
    resolveReference: resolveProduct
}
type Product record {
    string id;
    string sku;
    string name;
    int price;
};


type Product @key(fields: "id") @key(fields: "sku") {
    id: String!
    sku: String!
    name: String!
    price: Int!
}
Compound key 
@subgraph:Entity {
    key: "id organization { id }",
    resolveReference: resolveUser
}
type User record {
    string id;
    Organization organization;
}


type User @key(fields: "id organization { id }") {
    id: String!
    organization: Organization!
}
Non resolvable 
@subgraph:Entity {
    key: "id"
    resolveReference: ()
}
type Product record {
  id: String!
}


type Product @key(fields: "id", resolvable: false) {
    id: ID!
}
10.5.1.3 The subgraph:ReferenceResolver Function Type

Reference resolver is a function that resolves an entity of a specific type using its primary key. When the router requires a particular entity to be resolved, it invokes the corresponding entity's reference resolver. Following is the type definition of a reference resolver defined in graphql.subgraph module.

public type ReferenceResolver isolated function (subgraph:Representation representation)
returns map<any>|service object {}|error?;

Here, subgraph:Representation is a type definition of the entity representation outlined in the federation specification, which includes the GraphQL __typename field of the entity being resolved and its primary key.

Example: A Product Entity Defined with Its Resolver
isolated function resolveProduct(subgraph:Representation representation) returns Product|error? {
    string id = check representation["id"].ensureType(); // obtain the primary key of the entity
    return findProduct(id);
}

@subgraph:Entity {
    key: "id", // primary key of the entity
    resolveReference: resolveProduct
}
type Product record {
    string id;
    string name;
    int price;
};

Note: If the reference resolver returns an entity of a different type than the entity being resolved, a runtime error will be returned to the router. For example, if the resolver returns a User for a Product entity, a runtime error will occur.

10.6 DataLoader

The Ballerina GraphQL module allows efficient batching of data retrieval from datasources and enables caching of fetched data using the graphql.dataloader submodule.

10.6.1 DataLoader API

The graphql.dataloader submodule provides the DataLoader object, which is used to batch and cache data requests from a data source. The DataLoader object type has the following public methods/APIs.

10.6.1.1 The add method

This method takes an anydata parameter called key, which is used to identify the data to be loaded. This method collects and stores the key to dispatch a batch operation at a later time. It does not return any values. The following is the method definition of this method.

public isolated function add(anydata key);
10.6.1.2 The get method

This method takes a key parameter and retrieves the result for the provided key. It performs data binding by examining the type of the assigned variable. In case of failure to retrieve the result or perform data binding, the method returns an error. The following is the method definition of this method.

public isolated function get(anydata key, typedesc<anydata> 'type = <>) returns 'type|error;
10.6.1.3 The dispatch method

This method does not take any parameters and does not return any values. This method is invoked by the GraphQL Engine to dispatch a user-defined batch load function for all the collected keys. For more information about the batch load function, refer to the The BatchLoadFunction section. The following is the method definition of the dispatch method.

public isolated function dispatch();
10.6.1.4 The clearAll method

This method does not take any parameters and does not return any values. The purpose of this method is to clear all the collected keys and cached values from the DataLoader cache. The following is the method definition of the this method.

public isolated function clearAll();

10.6.2 The DefaultDataLoader

The DefaultDataLoader is a built-in implementation of the DataLoader object available via the graphql.dataloader submodule. Users can use this implementation to batch and cache data loading operations.

10.6.2.1 The init Method

The init method of the DefaultDataLoader object takes a function pointer of type BatchLoadFunction. The following is the method definition of the init method.

public isolated function init(BatchLoadFunction loadFunction);
Example: Initializing a DefaultDataLoader
dataloader:DefaultDataLoader bookLoader = new (batchBooksForAuthors);
10.6.2.1.1 The BatchLoadFunction

The batch load function is responsible for retrieving data based on an array of keys and returning an array of corresponding results or an error if the operation fails. The following is the type definition of the batch load function.

public type BatchLoadFunction isolated function (readonly anydata[] keys) returns anydata[]|error;

When implementing a batch load function, it is important to ensure that the function returns an array of results that match the length of the keys array provided as input. If the lengths do not match, the DataLoader will return an error when the get method is called.

Example: Writing a Batch Load Function
isolated function batchBooksForAuthors(readonly & anydata[] ids) returns Book[][]|error {
    final readonly & int[] authorIds = <readonly & int[]>ids;
    // Logic to retrieve books from the data source for the given author ids
    // Book[][] books = ...
    return books;
};

10.6.3 Engaging DataLoaders

To engage a DataLoader with a GraphQL service, follow the steps discussed in the below sections.

10.6.3.1 Import graphql.dataloader Submodule

In order to engage the dataloader with a GraphQL service, the graphql.dataloader submodule must be imported. This submodule provides the DataLoader object, which is used to batch and cache data loading operations.

Example: Importing graphql.dataloader Submodule
import graphql.dataloader;
10.6.3.2 Register DataLoaders to Context via ContextInit Function

Users should register the DataLoader objects via the graphql:ContextInit function. The DataLoader objects are meant to be used per request. Therefore, the graphql:ContextInit function is the ideal place to register the DataLoader objects. By registering the DataLoader objects to the graphql:Context object, these objects become accessible to all resolver functions of the GraphQL service.

Example: Registering DataLoaders to Context via ContextInit Function
@graphql:ServiceConfig {
    contextInit: isolated function (http:RequestContext requestContext, http:Request request) returns graphql:Context {
        graphql:Context context = new;
        context.registerDataLoader("bookLoader", new dataloader:DefaultDataLoader(batchBooks));
        return context;
    }
}
service on new graphql:Listener(9090) {
    // ...
}
10.6.3.3 Define the Corresponding prefetch Method

To engage the DataLoader with a GraphQL field (let's assume the field name is foo), define a corresponding prefetch method named preFoo in the service, where Foo represents the Pascal-cased name of the GraphQL field. The preFoo method can include some or all of the parameters from the GraphQL field and must include the graphql:Context parameter. Adding the parameters of the GraphQL foo field to the preFoo method is optional. However, if these parameters are added, the GraphQL Engine will make the same parameter values of the GraphQL field available to the preFoo method.

The GraphQL Engine guarantees the execution of the preFoo method prior to the foo method. By default, the GraphQL engine searches for a method named preFoo in the service class before executing the foo method. If the method name is different, the user can override the prefetch method name using the prefetchMethodName configuration of the @graphql:ResourceConfig annotation.

The user is responsible for implementing the logic to collect the keys of the data to be loaded into the DataLoader in the preFoo method. Subsequently, the user can implement the logic to retrieve the data from the DataLoader within the foo method.

Note: If there are multiple resource/remote methods with the same name in a GraphQL service, such as bar, and there exists a prefetch method named preBar, the GraphQL Engine will execute the preBar method prior to executing each bar method. Users can modify this behavior by utilizing the prefetchMethodName configuration within the @graphql:ResourceConfig annotation.

Example: Defining the Corresponding prefetch Method
distinct service class Author {
    function preBooks(graphql:Context ctx) {
        // ...
    }

    resource function get books(graphql:Context ctx) returns Book[] {
        // ...
    }
}
Example: Overriding the Defalut prefetch Method Name
distinct service class Author {
    function addBooks(graphql:Context ctx) {
        // ...
    }

    @graphql:ResourceConfig {
        prefetchMethodName: "addBooks"
    }
    resource function get books(graphql:Context ctx) returns Book[] {
        // ...
    }
}

Bringing everything together, the subsequent examples demonstrates how to engage a DataLoader with a GraphQL service.

Example: Utilizing a DataLoader in a GraphQL Service
import ballerina/graphql;
import ballerina/graphql.dataloader;
import ballerina/http;

@graphql:ServiceConfig {
    contextInit: isolated function (http:RequestContext requestContext, http:Request request) returns graphql:Context {
        graphql:Context context = new;
        context.registerDataLoader("bookLoader", new dataloader:DefaultDataLoader(batchBooksForAuthors));
        return context;
    }
}
service on new graphql:Listener(9090) {
    resource function get authors() returns Author[] {
        return getAllAuthors();
    }
}

distinct service class Author {
    private final int authorId;

    function init(int authorId) {
       self.authorId = authorId;
    }

    resource function get preBooks(graphql:Context ctx) {
        dataloader:DataLoader bookLoader = ctx.getDataLoader("bookLoader");
        // Load author id to the DataLoader
        bookLoader.add(self.authorId);
    }

    resource function get books(graphql:Context ctx) returns Book[] {
        dataloader:DataLoader bookLoader = ctx.getDataLoader("bookLoader");
        // Obtain the books from the DataLoader by passing the author id
        Book[] books = bookLoader.get(self.authorId);
        return books;
    }
}

isolated function batchBooksForAuthors(readonly & anydata[] ids) returns Book[][]|error {
    final readonly & int[] authorIds = <readonly & int[]>ids;
    // Logic to retrieve books from the data source for the given author ids
    // Book[][] books = ...
    return books;
};

In the given example, both the books resource function and the preBooks function receive the graphql:Context parameter, which grants access to the DataLoader objects. By using the ctx.getDataLoader("bookLoader") syntax, the specific DataLoader object associated with the unique identifier "bookLoader" can be obtained and assigned to the bookLoader variable.

Example: Utilizing Multiple DataLoaders in a GraphQL Service
import ballerina/graphql;
import ballerina/graphql.dataloader;
import ballerina/http;

@graphql:ServiceConfig {
    contextInit: isolated function (http:RequestContext requestContext, http:Request request) returns graphql:Context {
        graphql:Context context = new;
        context.registerDataLoader("postsLoader", new dataloader:DefaultDataLoader(postsLoaderFunction));
        context.registerDataLoader("rePostsLoader", new dataloader:DefaultDataLoader(rePostsLoaderFunction));
        context.registerDataLoader("followersLoader", new dataloader:DefaultDataLoader(followersLoaderFunction));
        return context;
    }
}
service on new graphql:Listener(9090) {
    resource function get users() returns User[] {
        return getAllUsers();
    }
}

isolated distinct service class User {
    private final int userId;

    isolated function init(int userId) {
        self.userId = userId;
    }

    isolated resource function get prePosts(graphql:Context ctx) {
        dataloader:DataLoader postsLoader = ctx.getDataLoader("postsLoader");
        postsLoader.add(self.userId);

        dataloader:DataLoader rePostsLoader = ctx.getDataLoader("rePostsLoader");
        rePostsLoader.add(self.userId);
    }

    isolated resource function get posts(graphql:Context ctx) returns Post[]|error {
        dataloader:DataLoader postsLoader = ctx.getDataLoader("postsLoader");
        Post[] posts = check postsLoader.get(self.userId);

        dataloader:DataLoader rePostsLoader = ctx.getDataLoader("rePostsLoader");
        Post[] rePosts = check rePostsLoader.get(self.userId);

        return [...posts, ...rePosts];
    }

    isolated resource function get preFollowers(graphql:Context ctx) {
        dataloader:DataLoader followersLoader = ctx.getDataLoader("followersLoader");
        followersLoader.add(self.userId);
    }

    isolated resource function get followers(graphql:Context ctx) returns Follower[]|error {
        dataloader:DataLoader followersLoader = ctx.getDataLoader("followersLoader");
        return check followersLoader.get(self.userId);
    }
}

isolated function postsLoaderFunction(readonly & anydata[] ids) returns Post[][]|error {
    final readonly & int[] keys = <readonly & int[]>ids;
    // Logic to retrieve posts from the data source for the given user ids
    // Post[][] posts = ...
    return posts;
};

isolated function rePostsLoaderFunction(readonly & anydata[] ids) returns Post[][]|error {
    final readonly & int[] keys = <readonly & int[]>ids;
    // Logic to retrieve re posted items from the data source for the given user ids
    // Post[][] rePosts = ...
    return rePosts;
};

isolated function followersLoaderFunction(readonly & anydata[] ids) returns Follower[][]|error {
    final readonly & int[] keys = <readonly & int[]>ids;
    // Logic to retrieve followers from the data source for the given user ids
    // Follower[][] followers = ...
    return followers;
};

The above example utilizes three DataLoader instances: postsLoader, rePostsLoader, and followersLoader. These DataLoaders are associated with the batch load functions postsLoaderFunction, rePostsLoaderFunction, and followersLoaderFunction. The 'post' field in the example utilizes the postsLoader and rePostsLoader DataLoaders, while the 'followers' field utilizes the followersLoader DataLoader. This demonstrates how different fields can utilize specific DataLoaders to efficiently load and retrieve related data in GraphQL resolvers.