OpenAPI Specifications

Arc provides built-in support for generating OpenAPI 3.0 specification documents from your registered HTTP endpoints. This allows you to document your API and integrate with tools like Swagger UI, Postman, and other OpenAPI-compatible clients.

Overview

The OpenAPI support in Arc automatically generates a specification document based on:

Registered routes (GET, POST, PUT, DELETE, PATCH)
Endpoint metadata (operation IDs, summaries, tags)
Authentication requirements
Response codes

Getting Started

To add OpenAPI support to your Arc application, you need to:

Add a reference to the Cratis.Arc.OpenApi package
Call the MapOpenApi() extension method on your ArcApplication

Installation

Add the package reference to your project:

dotnet add package Cratis.Arc.OpenApi

Basic Configuration

using Cratis.Arc;
using Cratis.Arc.OpenApi;

var builder = ArcApplication.CreateBuilder(args);
var app = builder.Build();

// Map the OpenAPI endpoint
app.MapOpenApi();

await app.RunAsync();

This will expose the OpenAPI specification document at /openapi.json by default.

Configuration Options

The MapOpenApi() method accepts several optional parameters to customize the generated document:

Custom Endpoint Path

Change where the OpenAPI document is served:

app.MapOpenApi(pattern: "/api/swagger.json");

API Title and Version

Customize the API information in the document:

app.MapOpenApi(
    title: "My API",
    version: "2.0.0");

Complete Example

app.MapOpenApi(
    pattern: "/api/openapi.json",
    title: "Customer Management API",
    version: "1.0.0");

Generated Document Structure

The OpenAPI document includes:

API Information

Title: The name of your API
Version: The API version

Servers

Default server URL (/)

Paths

All registered routes with their:

HTTP methods (get, post, put, delete, patch)
Operation IDs (from endpoint metadata)
Summaries (from endpoint metadata)
Tags (from endpoint metadata)
Response codes:
- 200 - Success
- 401 - Unauthorized (for authenticated endpoints)
- 500 - Internal Server Error

Security

If any endpoints require authentication (have AllowAnonymous = false), the document includes:

Bearer token security scheme
JWT format specification
Security requirements per operation

Endpoint Metadata

To provide rich OpenAPI documentation, use endpoint metadata when registering routes:

app.MapGet("/api/customers", async context =>
{
    // Handler implementation
},
new EndpointMetadata(
    Name: "GetCustomers",
    Summary: "Retrieves all customers",
    Tags: ["Customers"],
    AllowAnonymous: false));

Metadata Properties

Name: Becomes the operationId in the OpenAPI document
Summary: Becomes the summary in the OpenAPI document
Tags: Used for grouping operations in API documentation tools
AllowAnonymous: When false, adds security requirements to the operation

Integration with API Clients

Swagger UI

You can use the generated OpenAPI document with Swagger UI:

<!DOCTYPE html>
<html>
<head>
    <title>API Documentation</title>
    <link rel="stylesheet" href="https://unpkg.com/swagger-ui-dist@5/swagger-ui.css">
</head>
<body>
    <div id="swagger-ui"></div>
    <script src="https://unpkg.com/swagger-ui-dist@5/swagger-ui-bundle.js"></script>
    <script>
        SwaggerUIBundle({
            url: '/openapi.json',
            dom_id: '#swagger-ui',
        });
    </script>
</body>
</html>

Postman

Import the OpenAPI document into Postman:

Open Postman
Click Import
Choose Link and enter your OpenAPI URL (e.g., http://localhost:5000/openapi.json)
Click Continue to import the collection

Code Generation

Use OpenAPI generators to create client libraries:

# Using openapi-generator-cli
openapi-generator-cli generate \
    -i http://localhost:5000/openapi.json \
    -g csharp \
    -o ./generated-client

Example Application

Here's a complete example of an Arc application with OpenAPI support:

using Cratis.Arc;
using Cratis.Arc.Http;
using Cratis.Arc.OpenApi;

var builder = ArcApplication.CreateBuilder(args);
var app = builder.Build();

// Configure OpenAPI endpoint
app.MapOpenApi(
    pattern: "/openapi.json",
    title: "Product Catalog API",
    version: "1.0.0");

// Register API endpoints
app.MapGet("/api/products", async context =>
{
    var products = new[]
    {
        new { Id = 1, Name = "Product 1", Price = 29.99 },
        new { Id = 2, Name = "Product 2", Price = 39.99 }
    };
    
    await context.WriteResponseAsJsonAsync(products, products.GetType());
},
new EndpointMetadata(
    Name: "ListProducts",
    Summary: "Get all products in the catalog",
    Tags: ["Products"],
    AllowAnonymous: true));

app.MapPost("/api/products", async context =>
{
    var product = await context.ReadBodyAsJsonAsync(typeof(object));
    context.SetStatusCode(201);
    await context.WriteResponseAsJsonAsync(product, product?.GetType() ?? typeof(object));
},
new EndpointMetadata(
    Name: "CreateProduct",
    Summary: "Create a new product",
    Tags: ["Products"],
    AllowAnonymous: false));

await app.RunAsync();

Best Practices

Use Descriptive Names

Provide clear operation IDs and summaries:

new EndpointMetadata(
    Name: "GetCustomerById",
    Summary: "Retrieves a customer by their unique identifier",
    Tags: ["Customers"])

Organize with Tags

Group related endpoints together:

// All customer-related endpoints use the "Customers" tag
Tags: ["Customers"]

// Order-related endpoints use the "Orders" tag
Tags: ["Orders"]

Document Security Requirements

Be explicit about authentication:

// Public endpoint
new EndpointMetadata(
    Name: "GetPublicData",
    AllowAnonymous: true)

// Protected endpoint
new EndpointMetadata(
    Name: "GetUserData",
    AllowAnonymous: false)

Keep Versions Updated

Update the version when making breaking changes:

app.MapOpenApi(
    title: "My API",
    version: "2.0.0");  // Incremented for breaking changes

Limitations

The current OpenAPI implementation:

Generates basic request/response schemas (no detailed type information)
Does not include request body schemas
Does not include response body schemas
Supports standard HTTP methods (GET, POST, PUT, DELETE, PATCH)

For more advanced OpenAPI features with full schema generation, consider using Cratis.Arc.Swagger with ASP.NET Core.

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