Strategies on synching data and caching data between iphone and server

S开发者_StackOverfloway I have a TODO list iphone app, that can be edited/viewed from both a web application and the iphone application.

When on the iphone, when a user views all his todo lists, or sub-items, I would think that each time the user views a particular list it shouldn't be hitting the web applications API every-time, but rather cache locally the values and only hit the web when things change.

What strategies are there for this type of scenerio?

I agree with you in your dirty-otherwise-do-not-contact-the-server point. And I think this point is pretty straightforward and easy to implement.

However, be careful in this scenario: it gets dirty but at the same time, the device cannot reach the internet. In this scenario, I suggest you check the internet accessibility on a frequent basis (even when your app is in the background), and try to reach your server and update whenever possible.

This is a tricky problem. I'm currently working on an app that needs to perform a similar synchronization, and I haven't decided how I want to handle it yet.

You're right in that you don't want to be hitting the web repeatedly. It would slow the app down considerably. Keeping a local cache is the way to go.

One drawback is that the user could change/add an item on the web and you wouldn't see it on the phone. You'd need to have a refresh button (like in the Mail application, for example) to allow the user to get the changes.

Then you have an issue with conflict resolution. Say the same item is edited on both the phone and on the web. How does the user pick which one to keep, or do they get duplicated?

I think the best way to do this is to replicated your server's schema in CoreData. Then load a given element from the local DB, and in the background go out and check that element for updates if the device has an internet connection. You're hitting the db each time, but the user is not slowed down by the process.

You should not query the internet everytime you view the list.

But when you make updates to it, or edit it, you should update the server as well. That will make your life a whole lot simpler. That way when the user updates an item that he deleted in the web server, the server will just throw that request out...