CODEWITHSUNDEEP
iphoneobjective-cmultithreadingcore-data
Douglas Mayle
Douglas Mayle

Reputation: 21755

Multithreaded use of Core Data (NSOperationQueue and NSManagedObjectContext)

In Apple's Core Data documentation for Concurrency with Core Data, they list the preferred method for thread safety as using a seperate NSManagedObjectContext per thread, with a shared NSPersistentStoreCoordinator.

If I have a number of NSOperations running one after the other on an NSOperationQueue, will there be a large overhead creating the context with each task?

With the NSOperationQueue having a max concurrent operation count of 1, many of my operations will be using the same thread. Can I use the thread dictionary to create one NSManagedObjectContext per thread? If I do so, will I have problems cleaning up my contexts later?

What’s the correct way to use Core Data in this instance?

Upvotes: 14

Views: 9491

Answers (3)

RonLugge
RonLugge

Reputation: 5184

While this question is old, it's actually at the top of Google's search results on 'NSMangedObjectContext threading', so, I'll just drop in a new answer.

The new 'preferred' method is to use the initWithConcurrencyType: and tell the MOC whether it's a main thread MOC or a secondary thread moc. You can then use the new performBlock: and performBlockAndWait: methods on it and the MOC will take care of serializing operations on it's 'native' thread.

The issue then becomes how do you intelligently handle merging the data between the various MOCs your application may spawn, along with a thousand other details that make life 'fun' as a programmer.

Upvotes: 3

Barry Wark
Barry Wark

Reputation: 107774

The correct way to use Core Data in this case is to create a separate NSManagedObjectContext per operation or to have a single context which you lock (via -[NSManagedObjectContext lock] before use and -[NSManagedObjectContext unlock] after use). The locked approach might make sense if the operations are serial and there are no other threads using the context.

Which approach to use is an empirical question that can't be asnwered without data. There are too many variables to have a general rule. Hard numbers from performance testing are the only way to make an informed decision.

Upvotes: 19

Philippe Leybaert
Philippe Leybaert

Reputation: 171914

Operations started using NSOperationQueue using a maximum concurrent operation count of 1 will not run all operations on the same thread. The operations will be executed one after the other, but a new thread will be created every time.

So creating objects in the thread dictionary will be of little use.

Upvotes: 3

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