Wednesday, May 21, 2014

Problems With Core Data Migration Manager and Journal_mode WAL

Pablo Bendersky:

When you use a Migration Manager, Core Data will create a new database for you, and start copying the entities one by one from the old DB to the new one.

As we are using journal_mode = WAL, there’s an additional file besides DB.sqlite called DB.sqlite-wal.

From what I can tell, the problem seems to be that Core Data creates a temporary DB, inserts everything there, and when it renames it to the original name, the -wal file is kept as a leftover from the old version. The problem is that you end up with an inconsistent DB.

A different part of Core Data is aware of the multiple files, though:

To safely back up and restore a Core Data SQLite store, you can do the following:

  • Use the following method of NSPersistentStoreCoordinator class, rather than file system APIs, to back up and restore the Core Data store:

    - (NSPersistentStore *)migratePersistentStore:(NSPersistentStore *)store toURL:(NSURL *)URL options:(NSDictionary *)options withType:(NSString *)storeType error:(NSError **)error

    Note that this is the option we recommend.

  • Change to rollback journaling mode when adding the store to a persistent store coordinator if you have to copy the store file.

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