PostgreSQL 11 Administration Cookbook

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A standby is an exact copy of the master. But how do you synchronize things so that the query results you get from a standby are guaranteed to be the same as those you'd get from the master? Well, that in itself is not possible. It's just the physics of an eventually consistent system. We need it to be eventually consistent because otherwise, the synchronization would become a performance bottleneck.

What we can do is synchronize two requests on different servers, for example, if we wish to issue a write on the master and then later issue a read from a standby. Such a case is automatically handled by synchronous replication. If we aren't using sync rep, then we can wait for the standby to catch up with an action on the master, ...