Are there best practices/patterns for synchronizing local and remote data in SPA (html5 single page applications)?

I'm writing a simple "todo - helloworld" with jqueryMobile + knockout + breeze + WebAPI to learn about SPAs (Single Page Application) in a mobile context (unreliable internet connection)

To enable offline usage the WebApp will leverage

• Application Cache
• Localstorage

The App should use whenever possible remote database for loading and saving data, but should be able to seamlessly switch to localstorage when offline and synchronize local/remote changes when back online.

Now back to the question: the App will use Breeze's EntityManager for managing data (local cache and remote sync)

• "remoteDb"

To mitigate inconsistency/concurrency problems I would use 2 localstorage Keys:

• "localDb" for a local copy of the remote database (list of todos)
• "localPendingChanges" for changes the App wasn't able to submit to the remote database

so the flow would more or less be (pseudocode):

LoadData
  if(online)
    load remoteDb
    save localDb                   // save a local copy of the fresh loaded remotDb
    if(localPendingChanges)
      load localPendingChanges     // we are merging in the Breeze entityManager remote data with localPendingChanges
      Savedata                     // we are online and we have pending changes, so we should sync everything back to the remoteDb as soon as possible
  if(offline)
    load localDb
    if(localPendingChanges)
      load localPendingChanges     // we are merging in the Breeze entityManager local data with localPendingChanges

SaveData
  if(online)
    save remoteDb
    clear localPendingChanges      // until we are online there is no need to keep localPendingChanges
    LoadData                       // we are loading data from remoteDb to update our localDb to the latest version
  if(offline)
    save localPendingChanges       // we are saving only the changes to localstorage

What do you think about this approach? Is it a mess? Is it ok? What about concurrency problems in a multi user scenario?

This seems reasonable, not quite sure why you want two localStorage keys. The entityState (unmodified, modified, added, deleted) of every entity placed in localStorage is maintained so you can always 'extract' (see the EntityManager.getEntities method) just the pending changes from any local copy. So why not just save the entire entityManager state to localStorage before you shut down the app.

As for concurrency issues, you should definitely set a concurrency property on each entityType. As long as this exists, and if you are saving via the Entity Framework, breeze will detect any optimistic concurrency violations during a save and throw an exception. Obviously, you have determine the right behavior for your app after such an exception.

I'm sure you've seen it, but the QueryOptions.MergeStrategy (PreserveChanges, OverwriteChanges) can be very helpful in keeping or overwriting data on the local machine after any query.