On 2/23/2011 11:19 AM, Tech Madhu wrote:
> hello all,
>
> Iam somewhat new to Slony. Have a question on controlled switchover.
>
> I have 1 master/ 1slave setup . When i want to do a switchover, using lock
> set, move set, how do i know the 'slave' has caught up with the master? Say
> the slave is behind the master by N transactions, and i issue lock set, move
> set, will slony automatically ensure the slave gets in sync before doing the
> switchover? or is it the 'applications' responsibility.. If its the latter,
> how can i tell when the slave has caught up? I looked at:
> http://www.slony.info/documentation/monitoring.html
> I can't easily tell looking at contents of the sl_status, sl_confirm,
> sl_event if the slave has caught up.. The sl_setsync table shows 0 rows,
> (sl_setsync : Contains information about the state of synchronization of
> each replication set, including transaction snapshot data. ). if its 0, does
> it mean everything in sync?

The MOVE_SET event, generated on the old origin, will cause it to become 
a subscriber. After that, the event is propagated to the new origin, 
which on processing it generates the ACCEPT_SET event, which in turn 
travels back. So if you do

LOCK SET ...
MOVE SET ...
WAIT FOR EVENT (ORIGIN = <old_origin>, CONFIRMED = <new_origin>);
SYNC (ID = <new_origin>);
WAIT FOR EVENT (ORIGIN = <new_origin>, CONFIRMED = <old_origin>);

you are guaranteed that the switchover is complete and everything is 
caught up.


Jan

-- 
Anyone who trades liberty for security deserves neither
liberty nor security. -- Benjamin Franklin