Core Data cloud sync - need help with logic

Question

I'm in the middle of brainstorming a cloud sync solution for a Core Data app that I am currently developing. I'm planning to open source the code for this once its done, for anyone to use with their Core Data apps, so input from the community on how this system should work is much appreciated :-) Here's what I'm thinking:

Server Side

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Storage Provider

As with all cloud sync systems, storage is a major piece of the puzzle. There are many ways to handle this. I could set up my own server for storage, or use a service like Amazon S3, but because I'm starting out with $0 capital, at this moment, a paid storage solution isn't a viable option. After some thought, I decided to settle with Dropbox (an already well established cloud sync application and storage provider). The pros of using Dropbox are:

• It's free (for a limited amount of space)
• In addition to being a storage service, it also handles cloud sync
• They recently released an Objective-C SDK which makes it much easier to interface with it in Mac and iPhone apps

In case I decide to switch to a different storage provider in the future, I intend to add "services" to this cloud sync framework, basically allowing anyone to create a service class to interface with their choice of storage provider, which can then simply be plugged into the framework.

Storage Structure

This is a really difficult part to figure out, so I need as much input as I can here. I've been thinking about a structure like this:

CloudSyncFramework
======> [app name]
==========> devices
=============> (device id)
================> deviceinfo
================> changeset
==========> entities
=============> (entity name)
================> (object id)

A quick explanation of this structure:

• The master "CloudSyncFramework" (name undecided) folder will contain separate folders for each app that uses the framework
• Each app folder contains a devices folder and an entities folder
• The devices folder will contain a folder for each device that is registered with the account. The device folder will be named according to the device ID, obtained using something like [[UIDevice currentDevice] uniqueIdentifier] (on iOS) or a serial number (on Mac OS).
• Each device folder contains two files: deviceinfo and changeset. deviceinfo contains information about the device (e.g. OS version, last sync date, model, etc.) and the changeset file contains information about objects that have changed since the device last synchronized. Both files will just be simple NSDictionaries archived into files using NSKeyedArchiver.
• Each Core Data entity has a subfolder under the entities folder
• Under each entity folder, every object that belongs to that entity will have a separate file. This file will contain a JSON dictionary with the key-value pairs.

Simultaneous Sync

This is one of the areas where I am almost completely clueless. How would I handle 2 devices connecting and syncing with the cloud at the same time? There seems to be a high risk of things getting out of sync here, or even data corruption.

Handling migrations

Once again, another clueless area here. How would I handle migrations of the Core Data managed object model? The easiest thing to do here seems to be just to wipe the cloud data store clean and upload a new copy of the data from a device which has undergone the migration process, but this seems somewhat risky, and there may be a better way.

Client Side

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Converting NSManagedObjects into JSON

Converting attributes into JSON isn't a very hard task (theres lots of code for it floating around the web). Relationships are the key problem here. In this stackoverflow post, Marcus Zarra posts code in which the relationship objects themselves are added to the JSON dictionary. However, he mentions that this can cause an infinite loop depending on the structure of the model, and I'm not sure if this would work with my method, because I store each object as an individual file.

I've been trying to find a way to get an ID as a string for an NSManagedObject. Then I could save relationships in JSON as an array of IDs. The closest thing I found was [[managedObject objectID] URIRepresentation], but this isn't really an ID for an object, its more of a location for the object in the persistent store, and I don't know if its concrete enough to use as a reference for an object.

I suppose I could generate a UUID string for each object and save it as an attribute, but I'm open for suggestions.

Syncing changes to the cloud

The first (and still best) solution that popped into my head for this was to listen for the NSManagedObjectContextObjectsDidChangeNotification to get a list of changed objects, then update/delete/insert those objects in the cloud data store. After the changes have been saved, I would need to update the changeset file for every other registered device to reflect the newly changed objects.

One problem that comes up here is, how would I handle a failed or interrupted sync?. One idea I have is to first push changes to a temporary directory on the cloud, then once that has been confirmed as successful, to merge it with the master data on the cloud so that an interruption in the middle of the sync won't corrupt data. Then I would save records of the objects that need to be updated in the cloud into a plist file or something, to be pushed during the next time the app is connected to the internet.

Retrieving changed objects

This is fairly simple, the device downloads its changeset file, figures out which objects need to be updated/inserted/deleted, then acts accordingly.

And that sums up my thoughts for the logic that this system will use :-) Any insight, suggestions, answers to problems, etc. is greatly appreciated.

UPDATE

After lots of thinking, and reading TechZens suggestions, I have come up with some modifications to my concept.

The largest change I've thought up is to make each device have a separate data store in the cloud. Basically, every time the managed object context saves (thanks TechZen), it will upload the changes to that device's data store. After those changes are updated, it will create a "changeset" file with change details, and save it into the changeset folders of the OTHER devices that are using the application. When the other devices connect to sync, they will go through the changeset folder and apply each changeset to the local data store, then update their respective data stores in the cloud as well.

Now, if a new device is registered with the account, it will find the newest copy of the data out of all the devices and download that for use as its local storage. This solves the problem of simultaneous sync and reduces the chances for data corruption because there is no "central" data store, each devices touches only its data and just updates changes rather than every device accessing and modifying the same data at the same time.

There's some obvious conflict situations to deal with, mainly in relation to deleting objects. If a changeset is downloading instructing the app to delete an object that is currently being edited, etc. there needs to be ways to deal with this.

Answer 1

You want to look at this pessimistic take on cloud sync: Why Cloud Sync Will Never Work. It covers a lot of the issues that you are wrestling with. Many of them are largely intractable.

It is very, very, very difficult to synchronize information period. Adding in different devices, different operating systems, different data structures, etc snowballs the complexity often fatally. People have been working on variants of this problem since the 70s and things really haven't improve much.

The fundamental problem is that if you leave the system flexible and customizable, then the complexity of synchronizing all the variations explodes exponentially as a function of the number of customization. If you make it rigid, you can sync but you are limited in what you can sync.

How would I handle 2 devices connecting and syncing with the cloud at the same time?

If you figure that out, you will be rich. It's a big issue for current cloud sync providers. They real problem here is that your not "syncing" your merging. Software sucks at merging because its very hard to establish a predefined rule set to describe all the possible merges.

The simplest system is to establish either a canonical device or a device hierarchy such that the system always knows which input to choose. This however, destroys flexibility.

How would I handle migrations of the Core Data managed object model?

The migration of the Core Data model is largely irrelevant to the server. That's something that Core Data manages internally to itself. Model migration updates the model i.e. the entity graph, not the actual data.

Converting NSManagedObjects into JSON

Modeling relationships is hard especially with tools that don't support it as easily as Core Data does. However, the URI of a permanent managed object ID is supposed to serve as a UUID that nails the object down to a specific location in a specific store on a specific device. It's not technically guaranteed to be universally unique but its close enough for all practical purposes.

Syncing changes to the cloud

I think you're confusing implementation details of Core Data with the cloud itself. If you use NSManagedObjectContextObjectsDidChangeNotification you will evoke network traffic every time the observed context changes regardless of whether those changes are persisted or not. Depending on the app, this could drive connections thousands of times in a few minutes. Instead, you only want to sync when context is saved at the most.

One problem that comes up here is, how would I handle a failed or interrupted sync?

You don't commit changes until the sync completes. This is a big problem and leads to corrupt data. Again, you can have flexibility, complexity and fragility or inflexibility, simplicity and robustness.

Retrieving changed objects: This is fairly simple, the device downloads its changeset file, figures out which objects need to be updated/inserted/deleted, then acts accordingly

It's only simple if you have an inflexible data structure. Describing changes to a flexible data structure is a nightmare.

Not sure if I have helped any. None of the problems have elegant solutions. Most designer end up with rigidity and/or slow, brute force iterative merging.

Answer 2

Take a serious look at RestKit.

It is an open source project that aims to help with integrating iOS apps with cloud data, including but not limited to the scenario where there is a core-data model for that data on the client.

I have recently started to use it in one of my projects, and found it to be quite useful. In the core-data scenario, you implement declarative mappings between your data model and the content you GET from and POST to the server, and it takes care of things like injecting objects from the cloud into your client model, posting new objects to the server and incorporating server-generated objects IDs into your client-side model, doing all of this in a background thread and taking care of all the core-data context threading issues and so on.

RestKit by no means is a mature product, but is has a fairly good foundation and quite a few things that can use help from other contributors. Especially, if your goal is to create an open source solution, it would be great to contribute and improve something like this rather than re-invent a new solution. Unless of course, your see serious differences between what you have in mind and other existing solutions :-)