Keeping Core Data Objects in multiple stores

Question

I'm developing an iOS application using Core Data. I want to have the persistent store located in a shared location, such as a network drive, so that multiple users can work on the data (at different times i.e. concurrency is not part of the question).

But I also want to offer the ability to work on the data "offline", i.e. by keeping a local persistent store on the iPad. So far, I read that I could do this to some degree by using the persistent store coordinator's migration function, but this seems to imply the old store is then invalidated. Furthermore, I don't necessarily want to move the complete store "offline", but just a part of it: going with the simple "company department" example that Apple offers, I want users to be able to check out one department, along with all the employees associated with that department (and all the attributes associated with each employee). Then, the users can work on the department data locally on their iPad and, some time later, synchronize those changes back to the server's persistent store.

So, what I need is to copy a core data object from one store to another, along with all objects referenced through relationships. And this copy process needs to also ensure that if an object already exists in the target persistent store, that it's overwritten rather than a new object added to the store (I am already giving each object a UID for another reason, so I might be able to re-use the UID).

From all I've seen so far, it looks like there is no simple way to synchronize or copy Core Data persistent stores, is that a fair assessment?

So would I really need to write a piece of code that does the following:

• retrieve object "A" through a MOC
• retrieve all objects, across all entities, that have a relationship to object "A"
• instantiate a new MOC for the target persistent store
• for each object retrieved, check the target store if the object exists
• if the object exists, overwrite it with the attributes from the object retrieved in steps 1 & 2
• if the object doesn't exist, create it and set all attributes as per object retrieved in steps 1 & 2

While it's not the most complicated thing in the world to do, I would've still thought that this requirement for "online / offline editing" is common enough for some standard functionality be available for synchronizing parts of persistent stores?

Your point of views greatly appreciated, thanks, da_h-man

Answer 1

I was just half-kidding with the comment above. You really are describing a pretty hard problem - it's very difficult to nail this sort of synchronization, and there's seldom, in any development environment, going to be a turn-key solution that will "just work". I think your pseudo-code description above is a pretty accurate description of what you'll need to do. Although some of the work of traversing the relationships and checking for existing objects can be generalized, you're talking about some potentially complicated exception handling situations - for example, if updating an object, and only 1 out 5 related objects is somehow out of date, do you throw away the update or apply part of it? You say "concurrency" is not a part of the question, but if multiple users can "check out" objects at the same time, unless you plan to have a locking mechanism on those, you would start having conflicts when trying to make updates.

Something to check into are the new features in Core Data for leveraging iCloud - I doubt that's going to help with your problem, but it's generally related.

Since you want to be out on the network with your data, another thing to consider is whether Core Data is the right fit to your problem in general. Since Core Data is very much a technology designed to support the UI and MVC pattern in general, if your data needs are not especially bound to the UI, you might consider another type of DB solution.

If you are in fact leveraging Core Data in significant ways beyond just modeling, in terms of driving your UI, and you want to stick with it, I think you are correct in your analysis: you're going to have to roll your own solution. I think it will be a non-trivial thing to build and test.

An option to consider is CouchDB and an iOS implementation called TouchDB. It would mean adopting more of a document-oriented (JSON) approach to your problem, which may in fact be suitable, based on what you've described.

Answer 2

From what I've seen so far, I reckon the best approach is RestKit. It offers a Core Data wrapper that uses JSON to move data between remote and local stores. I haven't fully tried it yet, but from what the documentation reads, it sounds quite powerful and ideally suited for my needs.

Answer 3

You definetly should check these things:

Parse.com - cloud based data store

PFIncrementalStore https://github.com/sbonami/PFIncrementalStore - subclass of NSIncrementalStore which allows your Persistent Store Coordinator to store data both locally and remotely (on Parse Cloud) at the same time

All this stuff are well-documented. Also Parse.com is going to release iOS local datastore SDK http://blog.parse.com/2014/04/30/take-your-app-offline-with-parse-local-datastore/ wich is going to help keep your data synced.