This module provides comprehensive annotations and utilities to enhance the seamless interoperability between Ballerina and OpenAPI specifications.

The OpenAPI module delivers the following key capabilities:

1. Contract Enhancement: Annotations to enrich and customize OpenAPI contracts generated from Ballerina services
2. Contract Validation: Annotations to facilitate compile-time validation of Ballerina services against existing OpenAPI contracts
3. Runtime Introspection: Built-in support for exposing OpenAPI documentation as introspection resources

The @openapi:ServiceInfo annotation configures service-level metadata for OpenAPI contract generation, including title, description, contact information, and version details.

@openapi:ServiceInfo {
    contract: "/path/to/openapi.json|yaml",
    title: "Store Management API",
    version: "1.0.0",
    description: "Comprehensive API for managing retail store operations"
}
service /store on new http:Listener(8080) {
    // Service implementation
}

Note: All fields in the @openapi:ServiceInfo annotation are optional.

The @openapi:ResourceInfo annotation enriches individual resource functions with operation-specific metadata such as operation IDs, summaries, tags, and examples.

@openapi:ResourceInfo {
    operationId: "createStoreInventory",
    summary: "Add new inventory items to store",
    tags: ["inventory", "retail", "management"],
    examples: {
        requestBody: {
            "application/json": {
                value: {
                    itemId: "ITEM001",
                    quantity: 50,
                    price: 29.99
                }
            }
        }
    }
}
resource function post inventory(Inventory payload) returns InventoryResponse|error {
    // Resource implementation
}

Note: All fields in the @openapi:ResourceInfo annotation are optional.

The @openapi:Example annotation provides concrete examples for types, record fields, and parameters to improve API documentation and developer experience.

// Type-level example
@openapi:Example {
    value: {
        id: 12345,
        name: "Jessica Smith",
        email: "jessica.smith@example.com",
        role: "admin"
    }
}
type User record {
    int id;
    string name;
    string email;
    string role;
};

// Field-level examples
type Product record {
    @openapi:Example { value: "PROD-001" }
    string productId;
    
    @openapi:Example { value: "Premium Wireless Headphones" }
    string name;
    
    @openapi:Example { value: 199.99 }
    decimal price;
};

// Parameter example
resource function get orders(
    @openapi:Example { value: "approved" } 
    "approved"|"pending"|"cancelled"|"shipped" status,
    
    @openapi:Example { value: 10 }
    int? 'limit = 20
) returns Order[]|error {
    // Implementation
}

Enable runtime exposure of OpenAPI contracts by setting the embed field to true in the @openapi:ServiceInfo annotation. This creates introspection endpoints that serve the OpenAPI specification and interactive documentation.

@openapi:ServiceInfo {
    embed: true,
    title: "Hello World API",
    description: "A simple greeting service with embedded OpenAPI documentation"
}
service /hello on new http:Listener(9090) {
    resource function get greeting(string? name = "World") returns string {
        return string `Hello, ${name}!`;
    }
}

An OPTIONS request to the service reveals available introspection endpoints:

curl -v localhost:9090/hello -X OPTIONS

Response:

HTTP/1.1 204 No Content
allow: GET, OPTIONS
link: </hello/openapi-doc-dygixywsw>;rel="service-desc", 
      </hello/swagger-ui-dygixywsw>;rel="swagger-ui"
server: ballerina
date: Thu, 13 Jun 2024 20:04:11 +0530

The response includes links to:

• OpenAPI Specification (rel="service-desc"): Raw OpenAPI JSON/YAML document
• Swagger UI (rel="swagger-ui"): Interactive API documentation interface

Use the @openapi:ServiceInfo annotation to validate Ballerina services against existing OpenAPI contracts at compile time, ensuring API compliance and consistency.

@openapi:ServiceInfo {
    contract: "petstore-api.yaml",
    failOnErrors: true  // Strict validation (default)
}
service /petstore on new http:Listener(8080) {
    resource function get pets() returns Pet[]|error {
        // Implementation must match contract specification
    }
    
    resource function post pets(NewPet payload) returns Pet|error {
        // Implementation validated against contract
    }
}

• Strict Mode (failOnErrors: true): Contract violations generate compilation errors, preventing deployment of non-compliant services
• Warning Mode (failOnErrors: false): Contract violations are reported as warnings, allowing deployment with notifications

Best Practice: For contract-first development, consider generating service contract types from the OpenAPI specification and implementing services against these types. This approach provides stronger type safety and compile-time guarantees. See Generate service contract from OpenAPI for detailed guidance.