In synchronous multi-master replication, each server can accept write requests, and modified data is transmitted from the original server to every other server before each transaction commits. Heavy write activity can cause excessive locking, leading to poor performance. In fact, write performance is often worse than that of a single server. Read requests can be sent to any server. Some implementations use shared disk to reduce the communication overhead. Synchronous multi-master replication is best for mostly read workloads, though its big advantage is that any server can accept write requests – there is no need to partition workloads between master and slave servers, and because the data changes are sent from one server to another, there is no problem with non-deterministic functions like random().

Postgres Plus Advanced Server does not offer this type of replication, though Postgres Plus Advanced Server two-phase commit (PREPARE TRANSACTION and COMMIT PREPARED) can be used to implement this in application code or middleware.