Asynchronous callback from web api controller

Question

I'm very new to Web API and I have an unusual pattern that I need to implement. In the Post method of my controller, it is to take an object which includes a CallbackURL. It will then immediately return an HTTP response to the caller. Afterwards, it will use a 3rd party, off-site API to perform some work with the object. Once that work is done, the controller is to post the results of that work to the CallbackURL.

However, I do not know how to implement this in Web API. Once I return the HTTP response, the controller's lifecycle is over, correct? If so, how do I perform the work I need to do after I return the response?

Jason P · Accepted Answer · 2013-12-16 22:41:34Z

4

If you only need to post results to a url and not to the client that initiated the call, you could possibly do something as easy as this:

public string MyAPIMethod(object input)
{
    Task.Factory.StartNew(() =>
    {        
        //call third-party service and post result to callback url here.        

    });

    return "Success!";
}

The api call will return right away, and the Task you created will continue the processing in a different thread.

answered Dec 16, 2013 at 22:41

Jason P

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2 Comments

Joshua Dixon Over a year ago

Thank you very much. I believe this is what I needed. However, after implementing it, I'm noticing crashes whenever multiple requests are made simultaneously. The issue with the crashes seems almost random: various objects that should never be null being null, and it's never the same objects. Do you happen to know why starting a task like this would cause that? Alternately, let me ask if this method is safe to use on an API that could be hit with a very large number of simultaneous requests?

Jason P Over a year ago

It depends on whether the code in the task is thread-safe. All the multi-threading pitfalls will need to be avoided. One thing to consider in particular is that you won't have access to the HttpContext inside the task thread.

Ruben Ramirez Padron · Accepted Answer · 2015-02-18 21:25:02Z

Creating a task to finish up the request (as suggested by Jason P above) will most likely solve the problem, thread-safety provided. However that approach might hurt the performance of your Web service if calls to the 3rd party API take a significant amount of time to complete and/or you are expecting many concurrent clients. If that was the case, your problem seems to be the perfect candidate for a service pattern called "Request/Acknowledge/Callback" (also "Request/Acknowledge/Relay"). Using that pattern, your Web API method will just store each request (including the callback URL) into a queue/database and return quickly. A separate module (possibly running on more than one machine, depending on the number and complexity of the tasks) will take care of completing the tasks, and subsequently notifying completion through the callback URL (please see http://servicedesignpatterns.com/ClientServiceInteractions/RequestAcknowledge).

damienc88 · Accepted Answer · 2013-12-16 23:04:34Z

This is presuming you want to return the results of your 3rd-party query to the caller.

You're correct, this is outside of what's possible with WebAPI. Once you return the HTTP Response, the client also has no connection to your server.

You should look into Asp.Net SignalR, which allows a persistent connection between the client and server, working in modern browsers, and even back to IE7 (though officially unsupported), as well as supporting non-browser clients.

You can then do a couple of things, all of which require the client to connect to SignalR first.

Option 1: You can call your WebApi controller, which can return, but not before launching a task. This task can query the 3rd party api, then invoke a function on the caller via SignalR with the results that you want to provide.

Option 2: You can call a SignalR Hub action, which can talk back to your client. You can tell your client the immediate response, query the 3rd-party api, then return the results you want to provide.

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