Ballerina OpenAPI tool makes it easy for you to start the development of a service documented in an OpenAPI contract in Ballerina by generating a Ballerina service and client skeletons. It enables you to take the code-first API design approach by generating an OpenAPI contract for the given service implementation.

The Ballerina OpenAPI tool support provides the following capabilities.

Prerequisite: Install the latest Ballerina Swan Lake distribution.

1. Generate Ballerina service/client stubs from a given OpenAPI contract file using the CLI tool.
2. Export the OpenAPI definition from a given Ballerina service implementation using the CLI tool.
3. Validate the service implementation compliance with a provided OpenAPI contract using the OpenAPI annotation.

• The OpenAPI compiler plugin allows you to validate a service implementation against an OpenAPI contract during the compile time. This plugin ensures that the implementation of a service does not deviate from its OpenAPI contract.

Generate Ballerina services from OpenAPI Contracts

If you are an API developer who prefers the design-first approach, you can use an existing or your OpenAPI definition to generate Ballerina services using the OpenAPI CLI command as follows.

$ bal openapi -i <openapi-contract> --mode service

The generated service can be used as a code template to start the service implementation.

For example,

$ bal openapi -i hello.yaml --mode service

This generates a Ballerina service in a file named hello_service.bal and relevant schemas in a file named types.bal for the hello.yaml OpenAPI contract as depicted below. The above command can be run from anywhere on the execution path. It is not mandatory to run it from within a Ballerina package.

The service generation process is complete. The following files were created.
-- hello_service.bal
-- types.bal

Generate from tags

To generate the Ballerina service stub with a subset of tags defined in an OpenAPI contract, use the --tags option and specify the tags you need as specified in the OpenAPI definition.

$ bal openapi -i <openapi-contract> [--tags <"tag1","tag2">]

For example,

$ bal openapi -i hello.yaml --tags "pets", "list"

Once you execute the command, only the operations related to the given tags get included in the generated service file.

Info: For more information on the command options, see OpenAPI to Ballerina.

Export OpenAPI contracts from Ballerina services

If you prefer to follow the code-first approach, you can convert your Ballerina service APIs into human-readable or machine-readable documents such as OpenAPI documents by using the Ballerina to OpenAPI CLI Tool as follows.

Export the Ballerina service to an OpenAPI Specification 3.0.0 definition. For the export to work properly, the input Ballerina service should be defined using the basic service and resource-level HTTP annotations.

$ bal openapi [-i | --input] <ballerina-service-file-path> [(-o | --output) <output-location>]

The ballerina-service-file-path parameter specifies the path of the ballerina service file (e.g., my_api.bal) and is mandatory.

If your Ballerina file includes multiple services, this command generates the OpenAPI contract for each service in the Ballerina file.

Export in JSON format

Use the --json flag If you need the Ballerina service to OpenAPI output in JSON. The default is YAML.

$ bal openapi -i <ballerina-resource-file> [--json]

Export for a specific service

If you need to document an OpenAPI contract for only one given service (when there are multiple), use the following command, specifying the service name as the absolute-resource-path.

$ bal openapi -i <ballerina-resource-file> [-s|--service] <service-name>

For example,

$ bal openapi -i helloService.bal

This generates the OpenAPI contracts for the Ballerina services in the hello_service.bal Ballerina file.

$ bal openapi -i helloService.bal -s "/hello"

This generates the OpenAPI contracts for the Ballerina service in the hello_service.bal Ballerina file of which the absolute-resource-path is /hello.

Export with a given title and version

You can use the @openapi:ServiceInfo annotation for specifying the title and version information of the OpenAPI contract as follows.

@openapi:ServiceInfo {
    contract: "/path/to/openapi.json|yaml",
    title: "Store Management",
    'version: "0.1.0"
}

Info: These contract, title, and 'version are all optional attributes and can be used as described below.

For example,

Ballerina service file with the OpenAPI annotation

@openapi:ServiceInfo {
    title: "Store Management APIs",
    'version: "1.1.0"
}
service /greet on new http:Listener(9090) {
...
}

Generated OpenAPI contract with the given details

openapi: 3.0.1
info:
  title: Store Management APIs
  version: 1.1.0
...

OpenAPI validator compiler plugin

The OpenAPI Validator Compiler plugin validates a service against a given OpenAPI contract. The Compiler Plugin gets activated if a service has the @openapi:ServiceInfo annotation. This plugin compares the service and the OpenAPI contract and validates both against a pre-defined set of validation rules. If any of the rules fail, the plugin provides compilation errors.

The @openapi:ServiceInfo annotation is used to bind the service with an OpenAPI contract. You need to add this annotation to the service file with the required values for enabling the validations.

Note: Providing a contract path attribute is mandatory for the OpenAPI validator. The other attributes are optional.

The following is an example of the annotation usage in the Ballerina file.

import ballerina/openapi;

@openapi:ServiceInfo {
    contract: "/path/to/openapi.json|yaml",
    tags: ["store"],
    operations: ["op1", "op2"],
    failOnErrors: true // (default value => true),
    excludeTags: ["pets", "user"],
    excludeOperations: ["op1", "op2"]
}
service /greet on new http:Listener(9090) {
  ...
}

Info: For annotation attributes details, see Annotation reference.

Generate Ballerina clients from OpenAPI definitions

The client generated from an OpenAPI definition can be used in your applications to call the service defined in the OpenAPI file. If you want to generate only the Ballerina client, you can set the mode as the client when running the OpenAPI tool.

Note: Before generating your client using the command-line tool, please check if a pre-generated client for your API already exists in the Ballerina Central. (If so, you can refer to the client's API documentation for more information on how to use the pre-generated client in your code.)

$ bal openapi -i <openapi-contract> --mode client

For example,

$ bal openapi -i hello.yaml --mode client

This generates a Ballerina client stub (client.bal), a util file (utils.bal) for the relevant utils methods related to the client stub, and a schemas file (types.bal) for the hello.yaml OpenAPI contract. The above command can be run from anywhere on the execution path. It is not mandatory to run it from within a Ballerina package.

Client generated successfully. The following files were created. 
-- client.bal
-- types.bal
-- utils.bal

Generate with boiler-plate tests

Use the --with-tests flag in the client mode to generate a Ballerina client with boilerplate test cases for all the remote methods available in it.

$ bal openapi -i <openapi-contract> [--mode client] [--with-tests]

For example,

$ bal openapi -i hello.yaml --mode client --with-tests

In addition to the above-mentioned generated file, this generates a test.bal file in the default client generation.

Generate with nillable types

Info: This is an optional flag in the OpenAPI to Ballerina command.

If your OpenAPI specification includes JSON schema properties that are not marked as nullable:true, they may be returned as null in some responses which results in a JSON schema to Ballerina record data binding error. If you suspect this can happen for any property, it is safe to generate all data types in the generated record with Ballerina nil support by turning this flag on.

$ bal openapi -i <openapi-contract> [-n |--nullable]

Generate with a given method type

Use the --client-methods <resource|remote> option to select the client method type, which can be resource or remote. (The default option is remote).

$ bal openapi -i <openapi-contract> --mode client --client-methods <resource|remote>

Info: For more command options, see OpenAPI to Ballerina.

Publish your client

To see your new client in Ballerina central in the future, follow the steps below to send a GitHub Pull Request to the WSO2 openapi-connectors repository to publish it.

1. Fork and clone the openapi-connectors repository. Connectors generated from the Ballerina OpenAPI tool are managed in this repository.

2. Open the cloned repository and navigate to the /openapi directory.

3. Run the bal new <connector_name> -t lib command to create a new Ballerina package.

4. Copy the generated files (i.e., client.bal, types.bal, and utils.bal) into the <connector_name> directory.

5. Run the bal build command to check whether the newly-generated connector is compiling successfully.

6. Run the necessary unit tests to verify the functionality of the generated connector.

7. Add the license header at the top of each .bal file.

8. Add the Package.md and Module.md files.

9. Add a connector icon to the root of the connector. The icon needs to be a .png of 200x200 px size named as icon.png.

10. Update the Ballerina.toml file.

11. Run the ./gradlew build command.

12. Push the changes and create a Pull Request to the master branch of the openapi-connectors repository.

Annotation reference

The @openapi:ServiceInfo annotation

The @openapi:ServiceInfo annotation supports several usages in the Ballerina OpenAPI tool.

@openapi:ServiceInfo {
    contract: "/path/to/openapi.json|yaml",
    tags: ["store"],
    operations: ["op1", "op2"],
    failOnErrors: true // (default value => true),
    excludeTags: ["pets", "user"],
    excludeOperations: ["op1", "op2"],
    title: "store",
    'version: "0.1.0",
    embed: true // (default value => true)
}
service /greet on new http:Listener(9090) {
   ...
}

The attributes of the annotation are optional and can be used for each particular purpose as described below.