Core Data and JSON question

Question

I know this question has been possed before, but the explanation was a little unclear to me, my question is a little more general. I'm trying to conceptualize how one would periodically update data in an iPhone app, using a remote web service. In theory a portion of the data on the phone would be synced periodically (only when updated). While other data would require the user be online, and be requested on the fly.

Conceptually, this seems possible using XML-RPC or JSON and Core data. I wonder if anyone has an opinion on the best way to implement this, I am a novice iPhone developer, but I understand much of the process conceptually.

Thanks

Answer 1

To synchronize a set of entities when you don't have control over the server, here is one approach:

1. Add a touched BOOL attribute to your entity description.

2. In a sync attempt, mark all entity instances as untouched (touched = [NSNumber numberWithBool:NO]).

3. Loop through your server-side (JSON) instances and add or update entities from your Core Data store to your server-side store, or vice versa. The direction of updating will depend on your synchronization policy, and what data is "fresher" on either side. Either way, mark added, updated or sync'ed Core Data entities as touched (touched = [NSNumber numberWithBool:YES])

4. Depending on your sync policy, delete all entity instances from your Core Data store which are still untouched. Untouched entities were presumably deleted from your server-side store, because no addition, update or sync event took place between the Core Data store and the server for those objects.

Synchronization is a fair amount of work to implement and will depend on what degree of synchronization you need to support. If you're just pulling data, step 3 is considerably simpler because you won't need to push object updates to the server.

Answer 2

Syncing is hard, very hard. Ideally you would want to receive deltas of the changes from the server and then using a unique id for each record in Core Data, update only those records that are new or changed.

Assuming you can do that, then the code is pretty straight forward. If you are syncing in both directions then things get more complicated because you need to track deltas on both sides and handle collisions.

Can you clarify what type of syncing you are wanting to accomplish? Is it bi-directional or pull only?

Answer 3

I have an answer, but it's sucky. I'm currently looking for a more acceptable/reliable solution (i.e. anything Marcus Zarra cooks up).

What I've done needs some work ... seriously, because it doesn't work all the time...

The mobile device has a json catalog of entities, their versions, and a url pointing to a json file with the entity contents.

The server has the same setup, the catalog listing the entities, etc.

Whenever the mobile device starts, it compares the entity versions of it's local catalog with the catalog on the server. If any of those versions on the server are newer, it offers the user an opportunity to download the entity updates.

When the user elects to update, the mobile device now has the url for each of the new/changed entities and downloads it. Once downloaded, the app will blow away all objects for each of the changed entities, and then insert the new objects from JSON. In the event of an error, the deletions/insertions are rolled back to pre-update status.

This works, sort of. I can't catch it in a debug session when it goes awry, so I'm not sure what might cause corruption or inconsistency in the process.