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e65760ca85
This commit adds basic syncing capabilities with peers if they are both online. It updates the work done on MVDS, but I decided to create the code in status-go instead, since it's very tight to the application (similarly the code that was the inspiration for mvds, bramble, is all tight together at the database level). I reused parts of the protobufs. The flow is: 1) An OFFER message is sent periodically with a bunch of message-ids and group-ids. 2) Anyone can REQUEST some of those messages if not present in their database. 3) The peer will then send over those messages. It's disabled by default, but I am planning to add a way to set up the flags.
How to write migrations?
We only write up migrations, down migrations are not always possible in sqlite or
too complex/too expensive. For example to remove a column you would have to duplicate
the table, copy over the data, delete and recreated.
This can be very expensive for some tables (user_messages for example), so should not be attempted.
Notes
One issue we faced multiple times is that updates to user_messages can be very
expensive, leading to slow upgrade times and interrupted migrations. So avoid
writes if not necessary.