Last time I was working on Wombat, I was trying to get its Core Data multi-threading use correct. Namely, I stopped sharing one context (NSManagedObjectContext) between all threads, and created a new context for each thread. This meant I didn’t have to lock the context each time it was touched, which resulted in better performance. I thought that it would be simple as that. Core Data would take care of the merging of the multiple contexts, and all would be good.
I was close, but it’s not quite that simple.
The first problem I ran into was the possibility of two different clients, each on its own on thread and thus context, posting the same message. Since contexts aren’t saved until a client disconnects, that would work for the first client who quit, but the second one would run into trouble. It would most likely succeed in its save, assuming it had no merge conflicts, but it would compromise the data integrity of the data store. That is, I shouldn’t have the same post in the database twice. That’s bad.
Now the NNTP protocol specifies that each post should have a unique ID, called a Message-ID. Its simply a blackbox string that is supposed to be globally unique. Typically it is a time stamp concatenated with the local host name, and enclosed in angle brackets (<>). Some NNTP clients attempt to generate the Message-ID themselves, but most are smart and allow the server to generate one on their behalf. That means that the odds of receiving duplicate messages from clients are pretty much nil, although the situation still has to be considered.
It’s far more likely that two peer servers connect to Wombat, and offer it the same message that was posted elsewhere on the network. This is still somewhat unlikely, because direct peers should be somewhat rare, and thus the likelihood of them connecting at the same time would be low.
I bring up the rarity of the these events for a reason. If these collisions were frequent it means I should probably go back and consider locking down and updating the contexts more often. That way I always know what messages I have and which ones I don’t. i.e. I should make sure I do preventative maintenance. The downside of preventative is obvious: its slower because I have to bottleneck all threads and update the data store. However, since the message collision events are (theoretically) rare, then I can just assume the thread’s current context is up-to-date. Then, when the thread goes to merge any changes to the context into the data store, it can handle any conflicts then.
Before this point, Wombat never made use of any of Core Data’s validation methods. That’s because everything was serialized through one context and I manually validated the incoming data before I inserted it into the context. For example, before I inserted a message I searched for any messages in the context with the same Message-ID. If I found one, I simply didn’t insert the new message, thus maintaining the integrity of the data store.
Now I needed to be able to catch any duplicates when I went to save the context to the data store. My first thought was: “wouldn’t it be great if Core Data modeling allowed me to specify an attribute as unique?” It would be, but alas, Core Data doesn’t allow it. If I could specify an attribute as being globally unique then I wouldn’t have to write any validation methods, but just let Core Data catch them for me. I also wonder if SQLite would be able to do anything with an attribute if it knew it was unique. For example, create an index on it so searching was quicker.
Anyway, dreams aside, I needed to write a validation method. The first thing I thought of was the validation method generated by Xcode for each attribute. The one of the form:
- (BOOL) validateForInsert:(NSError **)error;
It’s a pretty easy to use method, and it only gets called on an inserted object. I simply added code at this point to check for more than one message with the same Message-ID. If I found more than one, I returned NO and put an error in the out parameter. The error parameter is passed back to the call to [NSManagedObjectContext save:&error], so I can do useful things like stuff the offending object into the error and the person who called save will get it.
Now I’d like to take an intermission to rant a bit. The error mechanism here is ghetto at best. If you’ll notice the out error parameter only points to one error, as does the error parameter in save. So what happens if you have more than one error, because, say, you have six messages that are duplicates? You curl up in a corner and cry, that’s what. Then you have to use some convoluted logic to pretend the API was designed to support multiple errors.
First, you have to check to see if the error parameter is nil. If it is, you just jam a pointer to your error in it. If it’s not nil, then you have to check to see if it’s a special error that’s designated as “multiple errors.” If it’s the special “multiple errors” error, then you create a entirely new “multiple errors” error with all the old stuff in it, plus your error added to its array of multiple errors. If the current error is not the special “multiple errors” error, then you have to create one, and jam the old error and your new error in it. Fun stuff.
Hey Apple, you wanna know what else would have worked? An NSMutableArray of NSError’s. Crazy idea, I know.
Anyway, after I figured out how to get errors back to the caller of save, I needed to do something about them. Fortunately processing duplicates are easy. You just deleted them. I thought about doing this inside of validateForInsert, but some of the Apple documentation advises against mutating the context inside of validation routines. Instead, the caller of save just walks the list of errors (which has its own special logic to produce an array of errors out of a single error) and deletes the duplicates, then attempts the save again.
At this point, I thought I was home free. But save kept returning merge errors even after I had deleted the duplicates, and I didn’t know why. The answer turned out to be the merge policy on the context. By default, the merge policy is “don’t merge, and report anything that doesn’t merge.” I’m sure that’s a fine policy for applications who never use more than one context per data store, but it doesn’t work so well in Wombat.
There are actually several merge policies described in Apple’s documentation. NSErrorMergePolicy simply returns an error for each merge conflict, and is the default. NSMergeByPropertyStoreTrumpMergePolicy and NSMergeByPropertyObjectTrumpMergePolicy are similar to each other in that they merge on a property by property basis. They only differ when the property has been changed in both the store and context. In that case, NSMergeByPropertyStoreTrumpMergePolicy takes whatever was in the store. Conversely, NSMergeByPropertyObjectTrumpMergePolicy takes whatever was in the object context. NSOverwriteMergePolicy simply forces all the changes in the object context into the data store. Finally, NSRollbackMergePolicy discards any object context changes that conflict with what’s in the data store.
For Wombat, I chose NSMergeByPropertyObjectTrumpMergePolicy because it does finer grain merging, and because there’s a slim chance what’s in the object context is more up-to-date than what’s in the store.
All in all, merging multiple contexts was harder than I thought, and harder than I thought it needed to be. It would be nice if Core Data could do some more validation automatically (like unique attributes) and if the error handling was better. I also think picking a better default merge policy would help, because when getting the random merge errors, it wasn’t all that obvious to me that the merge policy was the problem.