Introducing Azure Functions OpenAPI Extension

In this post, let's have a look at a nice feature that's now available with Azure Functions. That is being able to expose HTTP trigger functions via an Open API specification endpoint.

Let's go with an example. I already have an In-Process .NET Azure Function created with a single HTTP Trigger function. 

The first step to enable Open API specification is to install the following NuGet package.

dotnet add package Microsoft.Azure.WebJobs.Extensions.OpenApi -v 1.0.0

Once that is done, if we run the Function App locally, we should be seeing something interesting.

Additional Endpoints

Suddenly we have 4 new endpoints. If I navigated to the SwaggerUI endpoint, I can see this.

OpenAPI Document

That looks good. Now we just need to decorate the function to enrich the OpenAPI specification.

using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Azure.WebJobs;
using Microsoft.Azure.WebJobs.Extensions.Http;
using Microsoft.Azure.WebJobs.Extensions.OpenApi.Core.Attributes;
using Microsoft.Azure.WebJobs.Extensions.OpenApi.Core.Enums;
using Microsoft.Extensions.Logging;
using Microsoft.OpenApi.Models;
using System.Net;
 
namespace FunctionApp1;
 
public static class Function1
{
    [FunctionName("Function1")]
    [OpenApiOperation(operationId: "Run", tags: new[] { "GETTERS" })]
    [OpenApiSecurity("function_key", SecuritySchemeType.ApiKey, Name = "code", In = OpenApiSecurityLocationType.Query)]
    [OpenApiParameter(name: "name", 
        In = ParameterLocation.Query, 
        Required = true, 
        Type = typeof(string), 
        Description = "The **Name** parameter")]
    [OpenApiResponseWithBody(statusCode: HttpStatusCode.OK, 
        contentType: "text/plain", 
        bodyType: typeof(string), 
        Description = "The OK response")]
    public static IActionResult Run(
        [HttpTrigger(AuthorizationLevel.Function, "get")] HttpRequest req,
        ILogger log)
    {
        string name = req.Query["name"];
 
        string responseMessage = $"Hello, {name}. This HTTP triggered function executed successfully.";
 
        return new OkObjectResult(responseMessage);
    }
}

And now if I run the function app, I should be seeing my HTTP Trigger function.

OpenAPI Document

That looks nice. I can test my function from the Swagger UI itself.

OpenAPI Document

That's quite nice, isn't it? With Visual Studio 2022, you can get yourself a head start by using this template.

.NET 6 (In-Process) - Http trigget with OpenAPI

That was enabling OpenAPI specification for In-Process .NET Functions. 

If you want to enable Open API specification for Isolated (Out-of-process) .NET Azure Functions, you need to install the following NuGet package.

dotnet add package Microsoft.Azure.Functions.Worker.Extensions.OpenApi -v 1.0.0

Once that's done, you need to update the Program.cs as follows.

Program.cs

using Microsoft.Azure.Functions.Worker.Extensions.OpenApi.Extensions;
using Microsoft.Extensions.Hosting;
 
namespace FunctionApp3;
 
public class Program
{
    public static void Main()
    {
        IHost host = new HostBuilder()
            .ConfigureFunctionsWorkerDefaults(worker => worker.UseNewtonsoftJson())
            .ConfigureOpenApi()
            .Build();
 
        host.Run();
    }
}

And then decorate the Functions the same way as in In-Process model.

using Microsoft.Azure.Functions.Worker;
using Microsoft.Azure.Functions.Worker.Http;
using Microsoft.Azure.WebJobs.Extensions.OpenApi.Core.Attributes;
using Microsoft.Azure.WebJobs.Extensions.OpenApi.Core.Enums;
using Microsoft.Extensions.Logging;
using Microsoft.OpenApi.Models;
using System.Collections.Specialized;
using System.Net;
using System.Web;
 
namespace FunctionApp3;
 
public class Function1
{
    private readonly ILogger _logger;
 
    public Function1(ILoggerFactory loggerFactory)
    {
        _logger = loggerFactory.CreateLogger<Function1>();
    }
 
    [Function("Function1-Isolated")]
    [OpenApiOperation(operationId: "Run", tags: new[] { "GETTERS" })]
    [OpenApiSecurity("function_key", SecuritySchemeType.ApiKey, Name = "code", In = OpenApiSecurityLocationType.Query)]
    [OpenApiParameter(name: "name",
        In = ParameterLocation.Query,
        Required = true,
        Type = typeof(string),
        Description = "The **Name** parameter")]
    [OpenApiResponseWithBody(statusCode: HttpStatusCode.OK,
        contentType: "text/plain",
        bodyType: typeof(string),
        Description = "The OK response")]
    public HttpResponseData Run([HttpTrigger(AuthorizationLevel.Function, "get")] HttpRequestData req)
    {
        NameValueCollection query = HttpUtility.ParseQueryString(req.Url.Query);
        string name = query["name"];
 
        HttpResponseData response = req.CreateResponse(HttpStatusCode.OK);
        response.Headers.Add("Content-Type", "text/plain; charset=utf-8");
 
        string responseMessage = $"Hello, {name}. This HTTP triggered function executed successfully.";
 
        response.WriteString(responseMessage);
 
        return response;
    }
}

That's about it. Now if we run this, it should be working the same way as in In-Process mode.

OpenAPI Document

Hope this helps.

Happy Coding.

Regards.

Jaliya