Sync Android contacts with server - syncing algorithm

I want to sync my android device's contacts with a server (1-way syncing: update server according to the device's contacts DB).

I've seen the SampleSyncAdapter example: http://developer.android.com/resources/samples/S开发者_运维百科ampleSyncAdapter/index.html

and a very good blog for understanding the pieces in the puzzle: http://ericmiles.wordpress.com/2010/09/22/connecting-the-dots-with-android-syncadapter/

I understand that the sync logic itself is supposed to be in overriding onPerformSync() in a class extending AbstractThreadedSyncAdapter.

I was not, however, able to find an example for the syncing algorithm. For instance, how do I sync just the diffs from the previous sync? am I supposed to keep a cache for the latest synced contacts and diff it with the current contacts DB?

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Ideally, you offload much of that work to the server.

If you're talking about a REST api type of server, you have two options.

1. First, go fetch an index of the rows in your dataset, and pass a parameter as part of the request to filter out rows changed since a date, and then fetch those specific rows only.

If that isn't supported by their REST API,

1. Fetch each row at its own URL, and use HTML header fields to identify only what's changed. See "If-Modified-Since" field for example. This will tell the server that if the page you fetch is unchanged, not to bother returning it, but rather to send a 304 (not modified) response. If it's 304, you know you don't need to change your DB.