Minimal android foreground service killed on high-end phone

A service started by startForeground belongs to the second most important group visible process:

2. A visible process is doing work that the user is currently aware of, so killing it would have a noticeable negative impact on the user experience. A process is considered visible in the following conditions:

3. It is running an Activity that is visible to the user on-screen but not in the foreground (its onPause() method has been called). This may occur, for example, if the foreground Activity is displayed as a dialog that allows the previous Activity to be seen behind it.

4. It has a Service that is running as a foreground service, through Service.startForeground() (which is asking the system to treat the service as something the user is aware of, or essentially visible to them).

5. It is hosting a service that the system is using for a particular feature that the user is aware, such as a live wallpaper, input method service, etc.

The number of these processes running in the system is less bounded than foreground processes, but still relatively controlled. These processes are considered extremely important and will not be killed unless doing so is required to keep all foreground processes running.

That being said, you can never be sure that your service is not killed at any time. E.g. memory pressure, low battery etc. See who-lives-and-who-dies.

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For how to handle it, basically you answered the question yourself. The way to go is START_STICKY:

For started services, there are two additional major modes of operation they can decide to run in, depending on the value they return from onStartCommand(): START_STICKY is used for services that are explicitly started and stopped as needed, while START_NOT_STICKY or START_REDELIVER_INTENT are used for services that should only remain running while processing any commands sent to them. See the linked documentation for more detail on the semantics.

As a general guideline you should do as little as possible in the background (ore foreground) service, i.e. only do the location tracking and keep everything else in your foreground activity. Only the tracking should require very little configuration an can be loaded quickly. Also the smaller your service is the less likely it is to be killed. Your activity will be restored by the system in the state that is was before it went into background, as long as it is not killed as well. A "cold-start" of the foreground activity on the other hand should not be a problem.
I don't consider that as ugly, because this guarantees that the phone always provides the best experience to the user. This is the most important thing it has to do. That some devices close services after 30 minutes (possibly without user interaction) is unfortunate.

So, as you stated, you have to

Persist everything in the service in e.g. a Room database. Every variable, every custom class, every time any of them changes and then start the service with START_STICKY.

See creating a never ending service

Implicit question:

Depending on how long it takes for Android to re-create the service after killing it, a large portion of locations may be lost.

This usually takes only a really short time. Especially because you can use the Fused Location Provider Api for the location updates, which is an independent system service and very unlikely to be killed. So it mainly depends on the time you need to recreate the service in onStartCommand.

Also take note that from Android 8.0 onwards you need to use a forground service because of the background location limits.

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Edit: As recently covered in the news: Some manufacturers may give you a hard time to keep your service running. The site https://dontkillmyapp.com/ keeps track of the manufacturers and possible mitigations for your device. Oneplus is currently (29.01.19) one of the worst offenders.

When releasing their 1+5 and 1+6 phones, OnePlus introduced one of the most severe background limits on the market to date, dwarfing even those performed by Xiaomi or Huawei. Not only did users need to enable extra settings to make their apps work properly, but those settings even get reset with firmware update so that apps break again and users are required to re-enable those settings on a regular basis.

Solution for users

Turn off System Settings > Apps > Gear Icon > Special Access > Battery Optimization.

sadly there is

No known solution on the developer end

I know it's late but this may help some one. I too faced the same problem of keeping the foreground service alive without being killed by OS from different manufacturers. Most of the Chinese manufacturer's OS kills the foreground service even if it is added to the exception lists (Battery, Cleaner, etc) and allowed to auto start.

I found this link which solved me this long time problem of keeping the service alive.

All you have to do is run your foreground service in a seperate process. That's it.

You can do that by adding android:process to your service in your AndroidManifest.xml.

For Example:

<service android:name=".YourService"
        android:process=":yourProcessName" />

You can refer the docs to know more about android:process

EDIT: SharedPreferences won't work across multiple processes. In this case, you must go for IPC (Inter Process Communication) methods or you can use ContentProviders for storing and accessing your data to be used across processes. Referred from docs.