Are the benefits to using background workers in ASP.NET if there isn't app recycling?

Question

Background: I have a simple ASP.NET Core 3.1 site. Very rarely (three or four times per week), a user might fill out a form that triggers an email to be sent.

I don't want to delay the page response while running the 'send email' operation (even though it only takes a second or two), so from everything I've read, it seems like the code that should handle the email should be a background worker/hosted service, and the Razor pages code should place the data object to be sent in a collection that gets monitored by the background service.

What I'm not fully understanding is why this is necessary in modern ASP.NET Core.

If I was doing this in a normal C# application (not ASP), I'd simply make the 'send email' method async (it's using MailKit, which has async methods), and call the async method without awaiting, allowing the the work be done on the threadpool while allowing the response thread to continue.

But existing answers and blog posts say that calling an async method without an await in ASP is dangerous, due to the fact that IIS can restart ASP processes (application pool recycling).

Yet, most things I've read say Application Recycling is an artifact of old ASP when memory leaks were common, and it's not really a thing on .Net Core. Additionally, many ASP applications aren't even hosted in IIS anymore.

Further, as far as I can tell, IHostedService/Background Worker objects aren't doing anything special - they don't seem to add any additional threading; they just look like singletons that have additional notification for environment startup and shutdown.

So:

Is calling a fire-and-forget async method in ASP.NET Core still considered poor practice, especially if the fire and forget task is short-lived? If so, why? [see edit below for clarification]
Other than notifications for shutdown, is there any reason why a background service is considered better than borrowing a managed threadpool thread (via Task.Run or QueueBackgroundWorkItem)? Wouldn't waking a background service (if it was awaiting on object to be placed in a collection) consume a pool thread in the same way?

Edit: I acknowledge that starting a task, and reporting success to the user, when there's a chance that operation could be terminated, is poor form. There's benefit to being notified of a shutdown and being able to finalize tasks.

Perhaps a better question is, does the old behavior of cycling still exist in modern ASP (on IIS or Kestrel)? Are there other reasons an orderly shutdown might be triggered (other than server shutdown/manual stop)?

user_10620317 · Answer

I would still call it a poor practice.

The main concern here as well as in the referenced post is mainly about the promise of task completion.

Without being aware of the ghost background tasks, the runtime will not be able to notify the tasks to gracefully stop. This may or may not cause serious issues depending on the status of the tasks at the point the termination occurs.

Using fire forget task often means, your task is at the risk of having to start all over again when the process restarts. And sometimes this is not possible due to loss of context. Imagine your fire-forget task is calling another web API with parameters provided by a web request. The parameters are likely to get wiped out from memory if the process restarts.

And remember, the recycling is not always triggered by IIS / server. It could also be triggered by people. Say when your application runs into a memory leak issue, and you may want to recycle the app process every 1 hour as a temporary relief. Then you need to make sure you don't break your background tasks.

In terms of hosting - it is still possible to host ASP.Net Core applications in-process, in which the app pool gets recycled by IIS after a configured time period, or by default 29 hours.

In terms of lifetime - hosted services are types you register to DI, so DI features could be used, for example, this built-in hosted service implements IDisposable, which means proper clean up could be done upon shutting down.

Frankly, background tasks and hosted services both allow you to do fire and forget. But when you need reliability and resilience, hosted services win.

user_316108 · Answer

To answer the second half of your question, the app will wait for all hosted services' StopAsync methods to finish before shutting down. As long as you await your Tasks in the hosted service, this effectively means you can assume your Tasks will be allowed to finish running before the app shuts down. The app could still be force-shutdown, which in that case, nothing is guaranteed anymore.

If you need more guarantees about your background tasks, you should move them to run in a separate process. You could use something like Runly to make it easier to break out functionality into background jobs. It also makes it easy to provide real-time feedback to the user so that you are not lying to the user when you say "everything is done" while something is still running in the background.

Full disclosure: I cofounded Runly.

Are the benefits to using background workers in ASP.NET if there isn't app recycling?

2 Answers