5.9. Failover and Failback

Note: The "master node" referes to a node which has the "youngest (or the smallest) node id" among live the database nodes. In streaming replication mode, this may be different from primary node. In Table 5-6, %m is the new master node chosen by Pgpool-II. It is the node being assigned the youngest (smallest) node id which is alive. For example if you have 3 nodes, namely node 0, 1, 2. Suppose node 1 the primary and all of them are healthy (no down node). If node 1 fails, failover_command is called with %m = 0.

Note: When a failover is performed, basically Pgpool-II kills all its child processes, which will in turn terminate all the active sessions to Pgpool-II. After that Pgpool-II invokes the failover_command and after the command completion Pgpool-II starts new child processes which makes it ready again to accept client connections.

However from Pgpool-II 3.6, in the steaming replication mode, client sessions will not be disconnected when a fail-over occurs any more if the session does not use the failed standby server. If the primary server goes down, still all sessions will be disconnected. Health check timeout case will also cause the full session disconnection. Other health check error, including retry over case does not trigger full session disconnection.

Note: You can run psql (or whatever command) against backend to retrieve some information in the script, but you cannot run psql against Pgpool-II itself, since the script is called from Pgpool-II and it needs to run while Pgpool-II is working on failover.

Note: You can run psql (or whatever command) against backend to retrieve some information in the script, but you cannot run psql against Pgpool-II itself, since the script is called from Pgpool-II and it needs to run while Pgpool-II is working on failover.

Note: If follow_master_command is not empty, then after failover on the primary node gets completed in Master Slave mode with streaming replication, Pgpool-II degenerates all nodes excepted the new primary and starts new child processes to be ready again to accept connections from the clients. After this, Pgpool-II executes the command configured in the follow_master_command for each degenerated backend nodes.

Note: It is recommended to turn on the backend health checking (see Section 5.8) when fail_over_on_backend_error is set to off. Note, however, that Pgpool-II still triggers the failover when it detects the administrative shutdown of PostgreSQL backend server. If you want to avoid a fail over even in this case, you need to specify DISALLOW_TO_FAILOVER on backend_flag.