eXtremeDB HA synchronization procedure

Here is the description of synchronization procedure:

1. Masterand replica databases have a ha_sequencer – 64bit integer which is initializedby zero at startup.

2. Masterincrements the sequencer at each commit of a WRITE transaction and sends thesequencer value along with transaction data to the replica(s)

3. Ifthe replica successfully applies the received transaction, it sets itssequencer to the received value. So after the transaction was committed, thevalues of the ha_sequencer are the same on the master and replica.

4. Whenthe replica connects to the master, it sends the value of ha_sequencer. Mastercompare its own value with received one. If values are equal, this means thatthe master and replica have the same database version and no synchronization isrequired – replica reports MCO_REPL_NOTIFY_DB_EQUAL notification code.

5. Otherwise(if master’s and replica’s sequencer values are different), the masterinitiates the synchronization procedure depending on the mode_flags – static,hot-sync or stateful initialization.

So, the DB_EQUAL code means that the values ofha_sequencers on the master and replica are equal. If the replica’s database isin-memory and newly created, the replica’s sequencer is 0. This means that themaster’s sequencer is also 0. But this is possible only if the master didn’tperform write transactions since creation of the database.

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