Using the OpenAPI Tools

OpenAPI Specification is a specification that creates a RESTFUL contract for APIs, detailing all of its resources and operations in a human and machine-readable format for easy development, discovery, and integration. Ballerina OpenAPI tooling will make it easy for users to start the development of a service documented in an OpenAPI contract in Ballerina by generating Ballerina service and client skeletons.

Using the Capabilities of the OpenAPI Tools

The OpenAPI tools provide the following capabilities.

  1. Generate the Ballerina Service or Client code for a given OpenAPI definition.
  2. Generate the client stub for an existing Ballerina service at build time.
  3. Export the OpenAPI definition of a Ballerina service.

The openapi command in Ballerina is used for OpenAPI to Ballerina and Ballerina to OpenAPI code generation. Code generation from OpenAPI to Ballerina can produce ballerina mock services and ballerina client stubs.

For build time client stub generation, annotation support is provided.

Mock Service from OpenAPI

ballerina openapi gen-service <moduleName>:<serviceName> 
                               [-c: copy-contract] 
                               [-o: outputFile]

Generates a Ballerina service for the OpenAPI file.

This generated service is a mock version of the actual Ballerina service. Generated sources contain the service definition in src/<module-name>/ and the OpenAPI contract that used to generate will be copied to src/<module-name>/resources.

Client Stub from OpenAPI

ballerina openapi gen-client [<moduleName>]:<clientName> 
                   <openapi-contract> [-o <dir-path> | --output <dir-path>]

Generates a Ballerina client stub for the service defined in a OpenAPI file.

This client can be used in client applications to call the service defined in the OpenAPI file.

Service to OpenAPI Export

ballerina openapi gen-contract [<moduleName>:]<serviceName> 
                                [-i: <ballerinaFile> | --ballerina-file <ballerina-file>] 
                                [-o: <openapi-contract> | --output <openapi-contract>] 
                                [-s | --skip-bind]

Export the Ballerina service to a definition of OpenApi Specification 3.0. For the export to work properly, the input Ballerina service should be defined using basic service and resource level HTTP annotations.

Client Stub for Service

Generates a Ballerina client stub to communicate with a Ballerina service.

All endpoint(s) that are used for client stub generation should be marked with the @openapi:ClientEndpoint annotation. If not, there might be errors during client stub generation. Endpoints that are not marked with this annotation are not picked for client stub generation. The @openapi:ClientConfig { generate: true } annotation is used to enable or disable client stub generation per service.


Mock Service from OpenAPI Sample

ballerina openapi gen-service helloworld:helloService hello_service.yaml

This will generate a Ballerina service, for hello_service.yaml OpenAPI contract, named helloService in the module named helloworld. This command should be executed inside a Ballerina project.

Client Stub from OpenAPI Sample

ballerina openapi gen-client hello_client hello_service.yaml

This will generate a Client named hello_client in a module named client for the service documented in hello_service.yaml. This command should be executed inside a Ballerina project.

OpenAPI from Service Sample

ballerina openapi gen-contract helloworld:helloService -i src/helloworld/helloService.bal

This will generate the OpenAPI contract for the Ballerina service hello which is in hello.bal Ballerina file.

Client Stub from Service Sample

Apply annotation to say that client generation is enabled by adding @openapi:ClientConfig { generate: true } and point the client endpoint to be applied on generation by adding @openapi:ClientEndpoint annotation to the client endpoint.

import ballerina/http;
import ballerina/log;
import ballerina/openapi;

// Define this endpoint as a selected endpoint for client generation.
listener http:Listener helloEp = new(9090);

// Enable client code generation for this service.
@openapi:ClientConfig {
    generate: true
@http:ServiceConfig {
    basePath: "/sample"
service Hello on helloEp {    
    @http:ResourceConfig {
        methods: ["GET"],
        path: "/hello"
    resource function hello(http:Caller caller, http:Request req) {
        http:Response res = new;
        var result = caller->respond(res);
        if (result is error) {
            log:printError("Error when responding", err = result);