I have a standard, non-async action like:
[HttpPost]
public JsonResult StartGeneratePdf(int id)
{
PdfGenerator.Current.GenerateAsync(id);
return Json(null);
}
The idea being that I know this PDF generation could take a long time, so I just start the task and return, not caring about the result of the async operation.
In a default ASP.Net MVC 4 app this gives me this nice exception:
System.InvalidOperationException: An asynchronous operation cannot be started at this time. Asynchronous operations may only be started within an asynchronous handler or module or during certain events in the Page lifecycle. If this exception occurred while executing a Page, ensure that the Page is marked <%@ Page Async="true" %>.
Which is all kinds of irrelevant to my scenario. Looking into it I can set a flag to false to prevent this Exception:
<appSettings>
<!-- Allows throwaway async operations from MVC Controller actions -->
<add key="aspnet:AllowAsyncDuringSyncStages" value="true" />
</appSettings>
https://stackoverflow.com/a/15230973/176877
http://msdn.microsoft.com/en-us/library/hh975440.aspx
But the question is, is there any harm by kicking off this Async operation and forgetting about it from a synchronous MVC Controller Action? Everything I can find recommends making the Controller Async, but that isn't what I'm looking for - there would be no point since it should always return immediately.
The
InvalidOperationException
is not a warning.AllowAsyncDuringSyncStages
is a dangerous setting and one that I would personally never use.The correct solution is to store the request to a persistent queue (e.g., an Azure queue) and have a separate application (e.g., an Azure worker role) processing that queue. This is much more work, but it is the correct way to do it. I mean "correct" in the sense that IIS/ASP.NET recycling your application won't mess up your processing.
If you absolutely want to keep your processing in-memory (and, as a corollary, you're OK with occasionally "losing" reqeusts), then at least register the work with ASP.NET. I have source code on my blog that you can drop in your solution to do this. But please don't just grab the code; please read the entire post so it's clear why this is still not the best solution. :)