Why don't some apps have the "Allow app to run in the background" option?

Question

When trying to reproduce the issue in this question, I noticed that some apps lack the "Allow app to run in the background" and the "... even when Battery Saver is on" options in Battery Saver. At first I thought those apps weren't even meant to run in the background, but then I noticed one of the apps lacking those options is Facebook Messenger, which clearly does run in the background (since I get notifications about new messages).

So does that mean I'm unable to prevent Facebook Messenger from running in the background? Or does it somehow piggyback on the main Facebook app (which does have the option and which I've allowed to run in the background)? What exactly determines whether or not the option(s) in question show up?

Answer 1

The option you're seeing in Battery Saver only appears when the selected app has a Background Agent.

To be clear, no app can run in the background on Windows Phone. This is for battery saving and performance reasons. But apps can have a background agent which is a separate piece of functionality that runs separately from the main app itself.

Background agents are commonly used to update the live tile of an app, update the lockscreen, etc but there are many other uses (update contact list, etc).

Due to the restrictions on how background tasks are run, especially the frequency (eg. when the phone is not being used), they are not suitable for messaging applications like Facebook Messenger or Whatsapp.

Instead, these types of messaging apps use push notifications to alert users of new messages and other events.

Voice-over-IP apps like Skype have a special kind Background Agent. It allows the app to wake up and handle incoming Skype calls without the main app needing to run all the time.