I am using iCloud with Core Data, based on the SQLite "Library-style" application design as specified by Apple. While the basic functionality works very well, I am concerned about the transaction logs and how they are managed.
While the database for my app is not large, it is very active and the core data stack is saved many times while the app is in use. I have noticed that a new transaction log is created for every core data save. The end result is that I have a TON of transaction logs and they take up much more space than the actual database.
1) Do the transaction logs ever get automatically pruned / coalesced, or will they continue to grow indefinitely, eventually numbering in the thousands and taking up many megabytes? It seems that the only way to manually purge the transaction logs and recreate a .baseline archive would be to disable and then re-enable iCloud (removing the ubiquity container and starting fresh). But this is obviously not a good solution.
2) My current architecture saves the core data stack often, even for minor changes. In general, this makes sense as my database is small and saving often ensures that the database file is always up-to-date. However, given the above issues with transaction logs, I am thinking that I should perhaps minimize saves to the database. Maybe doing so on a timed basis and/or on app transition states.
3) Even if I minimize the number of transaction logs by reducing how often I save the database, there seems to be an issue here, as the logs will continue to grow in number over time. Eventually the benefit of the "transaction log" design will become a burden in terms of the amount of iCloud storage used and the initial iCloud sync as a new device is added.
As Apple has provided very sparse information on iCloud and almost nothing in the form of "best practices", I would appreciate any insight from the community.
I filed a radar on this issue and received the following reply. They noted that it should be working correctly in iOS 5.1, though I have not yet verified this myself.
A clarification for any who might misunderstand the following. The transaction logs will be cleaned up by the core data internals. This is NOT something that should be performed by the application itself.
Engineering has provided the following feedback regarding this issue:
The transaction logs are intended to be deleted after all the active peers have had a chance to read them, and they exceed a threshold of space consumed. There was a previous issue that prevented the devices from correctly doing so.
来源：https://stackoverflow.com/questions/8704662/managing-core-data-icloud-transaction-logs