How to do C# Ajax Comet via WebAPI2?

Question

I have a C# ASP.Net (non MVC) project that uses Ajax Comet long-polling. A web page makes a HTTP call to an endpoint handled by a class implementing IHttpAsyncHandler.

If there is nothing to report (within n seconds) to the web page, an empty HTTP response is sent, and the webpage re-calls. If there is something to send, and update is sent, and the webpage processes and re-calls. This is pretty standard push technology, and works very well.

Now I am trying to add API endpoints, using WebAPI2, non-MVC. I have synchronous controllers working, based on the ApiController class.

I would like to set up push technology for the API calls, so that API users do not have to poll for updates.

Similar to the method above, the API endpoint call is received, and the context is stored. If a timeout expires, the call is returned empty, and the caller is expected to call again. If data updates within the timeout, the data is returned to the caller, and the caller is then expected to call again and wait for more updates.

The problem is that there seems to be no asynchronous version of ApiController. The aim is to free the thread that is handling the API call, to return it to the pool, and then when there is data available, or the timeout expires, a worker thread is used to return the response.

How can I set up an ApiController so that the thread processing the call is freed, the call context is stored, and I can send a response to the call at a later point in time?

Long polling works and is very efficient, specially for poor quality connections. It is working well. I just need to figure out how to hang up a WebAPI2 connection as if it were a standard HTTP request. — Steve Hibbert, Dec 07 '16 at 12:02

score 2 · Accepted Answer · edited May 23 '17 at 12:25

2

You can use async/await to achieve what you want i.e.:

[HttpPost]
public async Task<HttpResponseMessage> LongRunningOperation([FromBody]Input obj)
{
    // Do what ever you need with input data        

    await WaitForEvent();        

    // Do what ever you need to return a response

    return someResponse;
}

In this example the Web API method is declared as async and in its body await operator was used to return a thread to a pool.

I assumed that in order to implement Comet you uses some kind of events. As far as I remember many years ago I used ManualResetEvent to do so. However, it can be anything else.

What is important is that WaitForEvent method should return something awaitable. In other words ManualResetEvent or other wait handle should be wrapped in a task. You can do that with AsyncFactory.FromWaitHandle method.

It is also worth reading this discussion about asyn/await in the context of Web API.

edited May 23 '17 at 12:25

Community

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answered Dec 13 '16 at 22:21

Michał Komorowski

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I use events elsewhere, for normal thread wrangling, but had not thought to use them here. I will try this out, thanks for the post. – Steve Hibbert Dec 14 '16 at 10:27
For website comet, currently I use async Http handlers. It creates a derived AsyncResult object that wraps the http context. The AsyncResult objects are stored in a collection until either a timeout elapses, or data arrives. When either happens, the context request is completed. The client experiences either an empty response at the timeout, or a response with data at some time before the timeout (ie classic comet), and either way, client loops and re-requests. The client is almost always connected, passively waiting for data, without tying up a thread. – Steve Hibbert Dec 14 '16 at 10:27

How to do C# Ajax Comet via WebAPI2?

1 Answers1