SLONIK FAILOVER

Description

The FAILOVER command causes the backup node to take over all sets that currently originate on the failed node. slonik will contact all other direct subscribers of the failed node to determine which node has the highest sync status for each set. If another node has a higher sync status than the backup node, the replication will first be redirected so that the backup node replicates against that other node, before assuming the origin role and allowing update activity.

After successful failover, all former direct subscribers of the failed node become direct subscribers of the backup node. The failed node is abandoned, and can and should be removed from the configuration with SLONIK DROP NODE.

If multiple set origin nodes have failed, then you should tell FAILOVER about all of them in one request. This is done by passing a list like NODE=(ID=val,BACKUP NODE=val), NODE=(ID=val2, BACKUP NODE=val2) to FAILOVER.

Nodes that are forwarding providers can also be passed to the failover command as a failed node. The failover process will redirect the subscriptions from these nodes to the backup node.

This uses schemadocfailednode(p_failed_nodes integer, p_backup_node integer, p_failed_node integer[]).

Example

FAILOVER (
   ID = 1,
   BACKUP NODE = 2
);

#example of multiple nodes   
FAILOVER(  
   NODE=(ID=1, BACKUP NODE=2),
   NODE=(ID=3, BACKUP NODE=4)
);

	 

Locking Behaviour

Exclusive locks on each replicated table will be taken out on both the new origin node as replication triggers are changed. If the new origin was not completely up to date, and replication data must be drawn from some other node that is more up to date, the new origin will not become usable until those updates are complete.

Dangerous/Unintuitive Behaviour

This command will abandon the status of the failed node. There is no possibility to let the failed node join the cluster again without rebuilding it from scratch as a slave. If at all possible, you would likely prefer to use SLONIK MOVE SET instead, as that does not abandon the failed node.

If a second failure occours in the middle of a FAILOVER operation then recovery might be complicated.

Slonik Event Confirmation Behaviour

Slonik will submit the FAILOVER_EVENT without waiting but wait until the most ahead node has received confirmations of the FAILOVER_EVENT from all nodes before completing.

Version Information

This command was introduced in Slony-I 1.0

In version 2.0, the default BACKUP NODE value of 1 was removed, so it is mandatory to provide a value for this parameter

In version 2.2 support was added for passing multiple nodes to a single failover command