I'm making an iOS App, well, two actually. App A will be able to write TO-DOs to a remote MySQL database, and App B will be able to read what App A has written/modified. App B needs to be always up-to-date and there can be more than one instance of App B running on different iDevices. App B stored the almost-exact copy of what App A has written on the remote MySQL DB, in a local SQLite DB. MySQL operations are handled by a simple PHP script. The problem is how to update App B' local DB when something changes on the remote MySQL database (that is, when App A writes something new or modifies something existing). I came up with two possible solutions: The first is to tell the PHP the last_row_id (the id of the last row that App B has got from the server) so that the PHP can ask MySQL to get all the rows with a greater ID (ID is an auto increment value on MySQL table).
But... This won't help to discover the changed items.
The second solution I thought was to put a "last_updated" column on the MySQL with a timestamp and update it every time the row changes. App B should store the timestamp of the last sync, and report back this value to PHP so that the it can request the MySQL all the rows that have a timestamp greater than the one it receives from App B.
But... Both solutions fail when the MySQL is sharded and clustered (and this is what I'm interested in the most). The auto-increment id approach fails because it will no longer be progressive (see auto_increment_offset and auto_increment_increment), and the timestamp approach would be unreliable because the clock can be out of sync on any MySQL server.
How would you solve this problem in a reliable way?