Re: Trigger File Behaviour

On 2011-02-17 17:43, Selva manickaraja wrote:
> Does this mean the fail-over will not be auto the moment the primary
> DB or Server goes down? Does it require for us to manually intervene
> to introduce the trigger_file?

Yes, and yes.

I depends upon the installation, as to how to detect that that the
primary has gone down (eg, lose network connection for how long?).
Therefore, it is left to the sysadmin (that's you and me) to write
external procedures to detect when it is time to fail-over, and write an
appropriate automated script to create the trigger file (or do it
manually). I do it manually, because I have multiple slaves and the
procedures are slightly more complex than for just one slave.

Setting up the configuration files is pretty trivial:

1. On each slave, create a recovery.conf that points to the primary.
2. Optionally, on the primary, create a recovery.done (note the
different extension) that points to whichever slave you plan on later
switching to a primary in case of a fail-over.

In the case of the failure of the primary:

1. Make sure the primary will not come back up (yet).
2. Create the trigger file (which triggers the switch) on whatever slave
you wish to become the new primary.
3. If you have only one slave, you are done. If you have more than one
slave, you will need to (on each other slave):
3.a. Stop the slave.
3.b. Edit the recovery.conf file to point to the new primary.
3.c. Restart the slave.
3.d. If the slave does not properly sync to the new primary, you may
have to (ugh) run the script below to resynchronize the slave to the new
primary.

When you are ready to have the old primary become a new slave, resync
the data on the old primary to the new primary (I use the following
script on the old primary):

service postgresql stop
{
cat <<-EOF
SELECT pg_start_backup( '$(date
--iso-8601)', true );
\! rsync -vaz --delete
new.primary.hostname:$PGDATA/ $PGDATA/
SELECT pg_stop_backup();
EOF
} | psql -e template1 postgres
mv $PGDATA/recovery.{done,conf}
service postgresql start

Presto! Your old primary is now back up as a slave.

>
> On Fri, Feb 18, 2011 at 1:17 AM, Dean Gibson (DB Administrator)
> <postgresql(at)ultimeth(dot)com <mailto:postgresql(at)ultimeth(dot)com>> wrote:
>
> On 2011-02-16 18:10, Selva manickaraja wrote:
>
> We tried setting the trigger file for fail-over purpose. But
> we just can't understand how it works. Each time the secondary
> is started the trigger file is removed. How can we introduce
> auto fail-over is this happens?
>
> Thank you. Regards, Selvam
>
>
> That's exactly what is supposed to happen. You will also find
> that the recovery.conf file gets renamed when the trigger file is
> created by you (and then is promptly deleted by PostgreSQL).
>
> Don't create the trigger file until you want the hot-standby
> server to switch roles and become the primary server (and thusly
> accept DB change requests).
>

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