Long running task in WebAPI

Question

Here's my problem: I need to call multiple 3rd party methods inside an ApiController. The signature for those methods is Task DoSomethingAsync(SomeClass someData, SomeOtherClass moreData). I want those calls to continue running in the background, after the ApiController has sent the data back to the client. When DoSomethingAsync completes I want to do some logging and maybe save some data to the file system. How can I do that? I'd prefer to use the asyny/await syntax.

Answer 1

Great news, there is a new solution in .NET 4.5.2 called the QueueBackgroundWorkItem API. It's really simple to use:

HostingEnvironment.QueueBackgroundWorkItem(ct => DoSomething(a, b, c)); 

Here's an article that describes it in detail.

https://blogs.msdn.microsoft.com/webdev/2014/06/04/queuebackgroundworkitem-to-reliably-schedule-and-run-background-processes-in-asp-net/

And here's anohter article that mentions a few other approaches not mentioned in this thread. http://www.hanselman.com/blog/HowToRunBackgroundTasksInASPNET.aspx

Answer 2

You almost never want to do this. It is almost always a big mistake.

ASP.NET (and most other servers) work on the assumption that it's safe to tear down your service once all requests have completed. So you have no guarantee that your logging will be done, or that your data will be written to disk. Particularly with the disk writes, it's entirely possible that your writes will be corrupted.

That said, if you are absolutely sure that you want to implement this extremely dangerous design, you can use the BackgroundTaskManager from my blog.

Update: I've written a blog series that goes into detail on a proper solution for request-extrinsic code. In summary, what you really want to do is move the request-extrinsic code out of ASP.NET. Introduce a durable queue and an independent processor; the ASP.NET controller action will place a request onto the queue, and the independent processor will read requests and execute them. This "processor" can be an Azure Function/WebJob, Win32 Service, etc.