Automatic failover with PostgreSQL 9.1

Question

PostgreSql 9.1 has master-slave synchronous replication. Suppose the master is machine A and the slave is machine B.

If the master fails, how does PostgreSQL know when to make the slave the master? What if the slave incorrectly thought the master was down because of a temporary network glitch on the master where the client program could still contact the master though.

And moreover, how would my client program know the slave in the new master and more importantly is ready to accept writes. Does the slave send a message to the client?

Answer 1

检查repmgr ，它的工作之一就是解决这个问题。

Answer 2

Typically you want to use a promotion-management system like repmgr or patroni. Then you want to use some sort of a high availability proxy (could be pgbouncer or haproxy) to handle the actual abstraction so your applications do not need to know what system is master.

In answer to your question, most of these systems use a heartbeat to determine if there is a problem. Patroni goes out over the etcd heartbeat. Repmgr has its own heartbeat check. With Repmgr you need to write hook scripts to take care of stonith, and so forth.