Implement REST async request-reply pattern in .Net 8

Decouple backend processing from a frontend host, where backend processing needs to be asynchronous, but the frontend still needs a clear response. More details are in the article.

ToDo API

I'm going to show implementation based on ToDo API. We have REST methods to get ToDo(s). The typical implementation - gets ToDo item by ID and gets all ToDo items

group.MapGet("/", (Store store) => store.Todos.Values.ToArray());
group.MapGet("/{id}", (string id, Store store) =>
  store.Todos.TryGetValue(id, out var todo)
    ? Results.Ok(todo)
    : Results.NotFound());

More interested POST method, because it is asynchronous:

• Accept a request and generate jobId
• Create a job and send it to the queue
• Return 202 accepted status code with response header Location URL where get the result and RetryAfter to delay next request.

group.MapPost("/", async (Todo model, 
   BackgroundJobQueue queue,
   Store store, HttpResponse response) =>
{
    model = model with { Id = Guid.NewGuid().ToString() };
    var job = new Job(
     Guid.NewGuid().ToString(),
     DateTime.UtcNow,
     JobStatus.InProgress,
     $"/api/todos/{model.Id}"
    );

    store.Jobs.Add(job.Id, job);

    Func<CancellationToken, ValueTask> workItem = async (token) =>
    {
      await Task.Delay(TimeSpan.FromSeconds(10), token);
      store.Todos.Add(model.Id, model);

    };
    await queue.QueueJobAsync(job.Id, workItem);

    response.Headers.RetryAfter = TimeSpan.FromSeconds(2).ToString();
    response.Headers.Location = $"/api/jobs/{job.Id}";
    response.StatusCode = StatusCodes.Status202Accepted;
});

Background service

In this example, I'm using ASP Net Core build-in BackgroundService as a job processor and Channel as an in-process queue. But it may be any job processor like hangfire, lambda, etc. Also, it may be any queue in-process or hosted. Just take into account all the pros and cons related to queues and job processors.

private async Task BackgroundProcessing(CancellationToken stoppingToken)
{
  while (!stoppingToken.IsCancellationRequested)
  {
    var (id, jobTask) = await queue.DequeueJobAsync(stoppingToken);
    if (!store.Jobs.TryGetValue(id, out var job))
      return;

    try
    {
      await jobTask(stoppingToken);
      job = job with { CompletedAt = DateTime.UtcNow, Status = JobStatus.Completed };
    }
    catch (Exception ex)
    {
      job = job with { CompletedAt = DateTime.UtcNow, Error = ex.Message, Status = JobStatus.Failed };
    }

    store.Jobs[id] = job;
  }
}

Job API

This is a job status API that redirects to Location where get the job completion results or returns the job with an error when it is failed. Also, return the job with RetryAfter header when it is still processing.

group.MapGet("/{id}", (string id, Store store, HttpResponse httpResponse) =>
{
  if (!store.Jobs.TryGetValue(id, out var job) || job is null)
    return Results.NotFound();

  var okResult = () =>
  {
    httpResponse.Headers.RetryAfter = TimeSpan.FromSeconds(5).ToString();
    return Results.Ok(job);
  };

  return job.Status switch
  {
    JobStatus.Completed => Results.Redirect(job.Location),
    JobStatus.InProgress => okResult(),
    JobStatus.Failed => Results.BadRequest(job),
    _ => throw new NotImplementedException(),
  };
});

return group;

Test using the .http file

.http files are a simple way to quickly invoke your API endpoints

1. Get all ToDo items
2. Create a new ToDo
3. Using Location to get job status
4. Get ToDo item by id

GET {{base_url}}/api/todos

###
# @name createTodo
POST {{base_url}}/api/todos
Content-Type: application/json

{
    "dueBy": "2024-02-14",
    "title": "Write an article"
}

###
@jobLocation = {{createTodo.response.headers.Location}}

# @name job
GET {{base_url}}{{jobLocation}}
Accept: application/json

###
@todoId = {{job.response.body.$.id}}

GET {{base_url}}/api/todos/{{todoId}}

Conclusion

I provided an example, of how to implement the async request-reply pattern. Important things such as queues and job processors should be chosen related to the requirements. Also, do not forget to expire job items in the DB. Btw API clients should use status codes and headers 😎. And finally, the source code in the GitHub repository

ohalay / async-request-reply

Asynchronous Request-Reply pattern implementation

Decouple backend processing from a frontend host, where backend processing needs to be asynchronous, but the frontend still needs a clear response

Requirements

• .NET8

View on GitHub

Help links

• Async Request-Reply pattern
• Queued background job
• Rest client .http file