Question

I am writing an app for iOS that uses data provided by a web service. I am using core data for local storage and persistence of the data, so that some core set of the data is available to the user if the web is not reachable.

In building this app, I’ve been reading lots of posts about core data. While there seems to be lots out there on the mechanics of doing this, I’ve seen less on the general principles/patterns for this.

I am wondering if there are some good references out there for a recommended interaction model.

For example, the user will be able to create new objects on the app. Lets say the user creates a new employee object, the user will typically create it, update it and then save it. I’ve seen recommendations that updates each of these steps to the server –> when the user creates it, when the user makes changes to the fields. And if the user cancels at the end, a delete is sent to the server. Another different recommendation for the same operation is to keep everything locally, and only send the complete update to the server when the user saves.

This example aside, I am curious if there are some general recommendations/patterns on how to handle CRUD operations and ensure they are sync’d between the webserver and coredata.

Thanks much.

Answer 1

You might want to read about ‘transactions’ – which is basically the grouping of multiple actions/changes as a single atomic action/change. This helps avoid partial saves that might result in inconsistent data on server.

Ultimately, this is a very big topic – especially if server data is shared across multiple clients. At the simplest, you would want to decide on basic policies. Does last save win? Is there some notion of remotely held locks on objects in server data store? How is conflict resolved, when two clients are, say, editing the same property of the same object?

With respect to how things are done on the iPhone, I would agree with occulus that “Done” provides a natural point for persisting changes to server (in a separate thread).