Using H2 safely on Android

[Robert Lebel]

I need to use JDBC and H2 (instead of sqlite) for an Android app. 

The app will be composed of multiple Activities and will target modern android tablets (Android 3+). 

The activities lifecycle on Android is tricky, and what i want to avoid is H2 being killed by the OS and risking a corrupt database. 

What is the best approach to be safe? Should I always open the database in the onResume() callback of an Activy and close it in the onPause() callback? It would have to be done asynchronously from the main thread, and syncrhonized so another activity from the same app doesn't try to open H2 while it's being closed. I think it would be safe, but i am afraid of performance issues when navigating from one activity to another (opening h2 can take 500ms to 1s on my tegra 2 tablet)

Or should I manage H2 from an android Service that all my activities would bind to? The service would be stopped by android when no activities are bound to it.

I am new to android and maybe i am overcomplicating things... I would appreciate advices from other developpers who have used h2 on android.

[Thomas Mueller]

Hi,

The activities lifecycle on Android is tricky, and what i want to avoid is H2 being killed by the OS and risking a corrupt database. 

The database shouldn't get corrupt if the application is killed. It's still better if the application isn't killed however, because this will speed up database recovery. But I don't know too much about Android to help you I'm afraid.

Regards,

Thomas

[Bastian Schöpp]

I think this is a pain point. If you open the database and the OS kills your app, you won't get a corrupted database but the next time you open it, it will take much longer (I assume h2 needs to do some cleanup on the unclosed database). Does anyone know a way to avoid this behavior?

[Michael Rogers]

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You can minimise the chances of your process being killed by running a
service that calls startForeground(). Android will only kill a process
containing a foreground service as a last resort.

https://developer.android.com/guide/components/processes-and-threads.html#Lifecycle

https://developer.android.com/reference/android/app/Service.html#ProcessLifecycle

The service's onLowMemory() method will be called when the system is
low on memory, before killing any processes containing foreground
services, so that might be a good place to close the database cleanly.

https://developer.android.com/reference/android/app/Service.html#onLowMemory%28%29

However, running a service in the foreground isn't very polite to
other apps, which will be killed to make room for your app if
necessary. A more polite alternative would be to run a service that
doesn't call startForeground(), and to close the database cleanly if
either onLowMemory() or onDestroy() is called.

I don't recommend closing the database from the activity's onPause()
or onDestroy() methods, since those will be called whenever the user
switches to another activity or rotates the screen.

Cheers,
Michael

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[Noel Grandin]

You could try running a
"CHECKPOINT SYNC"
command from onPause().

At the very least, that would dramatically reduce the amount of work
that H2 has to do during recovery.

[Michael Rogers]

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To improve durability I've been experimenting with WRITE_DELAY and
CHECKPOINT SYNC, as recommended here:
http://h2database.com/html/advanced.html#durability_problems

Can you tell me whether it's useful to call CHECKPOINT SYNC after each
commit if WRITE_DELAY is set to 0? Or conversely, is it useful to set
WRITE_DELAY to 0 if I'm going to call CHECKPOINT SYNC after each commit?

Thanks,
Michael

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[Robert Lebel]

I opted to manage H2 from a service. All my activities bind themselves to this service in activity.onResume(), unbind themselves in onPause(). When no activity have been bound to the service for 5 seconds, the service stop itself, stop h2 and do some other cleanup.

So when my app goes in the background without beeing destroyed, h2 is stopped after 5 seconds. If the user restore my app in the foreground, h2 is resumed (i restart the service if needed).

My app has been published, and It seems to be working fine. But I still think i may have missed a simpler solution. It would be nice if h2 had an official recipe for android. 

[Sunny Leaves]

Hi Robert

I know half a year went by since your post but as we're roughly going through a similar though-process regarding our app, I was wondering whether you'd be able/wish to update your latest feedback on your enacted strategy.

Your insights would be most valuable to us.

Many thanks

Sunny.