Multi-master replicates same database to all nodes in cluster and allows writes to each node. Transaction isolation is enforced cluster-wide, so in case of concurrent updates on different nodes database will use the same conflict resolution rules (mvcc with repeatable read isolation level) as single node uses for concurrent backends and always stays in consistent state. Any writing transaction will write to all nodes, hence increasing commit latency for amount of time proportional to roundtrip between nodes nedded for synchronization. Read only transactions and queries executed locally without measurable overhead. Replication mechanism itself based on logical decoding and earlier version of pglogical extension provided for community by 2ndQuadrant team.
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