Can GRDB safely be used from multiple (extension) targets in a shared app group?

[hannesoid]

Let's say we have

• an iOS app with a main app target and a widget extension (or more extensions).
• the app and the extension both access the same SQLite file in a shared app group folder.

Usually we need NSFileCoordinator to coordinate access, but I read somewhere that SQLite has some multi-process protections when reading or using transactions.

As I couldn't find specific info in the documentation - does SQLite itself or GRDB take care of coordinating access in such a scenario, or do we still need to use NFileCoordinator?

[groue]

Hello @hannesoid, please check the Sharing a Database guide, full of NSFileCoordinator and other tips and techniques.

Don't miss the initial advice of the guide:

Preventing errors that may happen due to database sharing is difficult. It is extremely difficult on iOS. And it is almost impossible to test.
Always consider sharing plain files, or any other inter-process communication technique, before sharing an SQLite database.

[groue]

I guess I need to add a link to the main README ;-)

The link was already there, in the main README:

Sharing a Database: Recommendations for App Group Containers and sandboxed macOS apps.

Maybe you could not find it because it does not use the keywords would were expecting - any suggestion that would improve the copy is welcomed!

[hannesoid]

Right, I'm not sure why I didn't see it, It's hard to reconstruct, I suppose I was searching for keywords like

• app extensions
• extensions
• multiple
• multiple targets
• NSFileCoordinator
• file coordination
• coordinated
• file access

Perhaps extending that sentence to cover some of that would have made it a direct hit for me, something like:

Sharing a Database: Recommendations for sandboxed macOS apps and App Group Containers, using a single database from multiple app extensions, multiple targets or apps, file coordination / NSFileCoordinator.

[groue]

Thank you for your generous and helpful retrospection 👍

NSFileCoordinator is a little bit low-level and overly specific, but I like "file coordination" which has an excellent information scent!

Sharing a Database: How to share an SQLite database between multiple processes - recommendations for App Group containers, App Extensions, App Sandbox, and file coordination.

Yeah, name-dropping of Apple technologies is exactly what we need here. "Does GRDB support App Extensions? Can I store my database in an app group container?"

I'll use the same exact sentence in the article header.

[hannesoid]

You're very welcome!
Sounds exactly right and nicely findable, thanks for taking so much care to improve the documentation.
(the documentation is overall outstandingly good and complete by the way, I love it, great work)

[groue]

You're welcome and thank you very much!