Re: [ADMIN] Replication mode Master-Slave - maintenance question.

You may need to study up then PostgreSQL replication. Synchronous attaches
the slave to the master through a replication slot, if you're using 9.4 or
newer, you can check that on the master by querying pg_stat_replication,
but the slave's recovery.conf file has a setting for which slot to use.
Prior to 9.4 the named slot was not present but it still used a replication
channel, but that would appear in the same table on the master. The other
way to do replication is simply with WAL file shipping, but you can and
should use WAL file shipping even if a replication channel or slot is in
use. Think of it as insurance. With a WAL replica, all the slave does is
read and process WAL files as they arrive. The master has no concept that a
slave is attached. Thus it is possible over time for a master to expire WAL
segments that the slave never received (if it was unavailable). The move to
replication slots in 9.4 eliminated this possibility as master knows of the
slave(s) and will not expire WAL segments until all of them have received
the files. That's the quick and dirty explanation, but like I began, you
maybe ought to study how to do replication the way that suits your firm
best.

On Fri, Mar 24, 2017 at 6:05 AM, czezz <czezz(at)o2(dot)pl> wrote:

> Hi John,
> thanks for this reply.
>
> Honestly I do not know is it synchronous or asynchronous. I didnt set any
> parameter like this.
> Is there a quick way to verify that?
>
> Test case scenario I tested so far was: stop SLAVE for some time (2-3
> hours) and do a lot of INSERTS on MASTER.
> Afterwards, when SLAVE is up again it synced delta from MASTER's WAL
> files.
> Do it worked as I expected.
>
> Why are you saying that "extended period of time" will be an issue? Is
> there any time limit for SLAVE to be down or you are just saying about
> worse case scenario?
> I assume that I will never lose any WAL files on MASTER.
>
> Best regards,
> czezz
>
> Dnia 23 marca 2017 19:06 John Scalia <jayknowsunix(at)gmail(dot)com> napisał(a):
>
> You did not indicate whether you're Master-Slave setup is synchronous or
> asynchronous, but yes, what you described should work. The slave should
> have a recovery.conf file and the presence of that will cause that slave to
> wind through all WAL files present before it resumes further processing. At
> least this is what happens in synchronous streaming mode. Your problem
> might be if the slave is off for an extended period of time or if one of
> the WAL files disappears. At that point the slave would not have all the
> transactions for consistency.
> Again,to your question near the end. It depends on how you're set up.
> Generally though slaves do make the request to their master.
>
> On Thu, Mar 23, 2017 at 1:47 PM, czezz < <czezz(at)o2(dot)pl>czezz(at)o2(dot)pl> wrote:
>
> Hi,
> I have set my Postgres into replication mode Master-Slave and it works
> good.
> Though, I have some question about maintaining it.
>
> Let me start from the beginning. How I set it up (briefly):
> 1. Stand Alone postgres (future MASTER) has been set to archive data to:
> /datastore/WAL (archive_command = 'cp %p /datastore/WAL/%f && gzip
> /datastore/WAL/%f ')
> 2. I have shared /datastore/WAL over NFS
> 3. I prepared SLAVE machine and created initial database out of MASTER's
> WAL-archival (over NFS) by preparing following file:
>
> $ vi /var/lib/pgsql/9.2/data/recovery.conf
> restore_command = 'gunzip -c /datastore/WAL/%f > %p'
> standby_mode = 'on'
> primary_conninfo = 'host=MASTER_IP_ port=5432 user=rep
> password=password'
> trigger_file = '/var/lib/pgsql/MASTER.trigger'
>
> 4. One last thing - I keep archive_mode = off on the SLAVE.
>
> The above works perfect.
>
> Now, here I need your help to understand what is happening in case
> following scenario.
>
> SCENARIO:
> I turn off SLAVE for some time (and MASTER receives a lot of new data) and
> later I turn SLAVE on.
> Is it true/correct:
> - SLAVE restores missing data/delta over NFS (from MASTER's archival).
> This is due to existing /var/lib/pgsql/9.2/data/recovery.conf and entry:
> restore_command = 'gunzip -c /datastore/WAL/%f > %p'
> - Once restore is done, SLAVE will keep replicate new data from MASTER
> (btw. it is SLAVE who request replication, correct?)
>
> Can anyone please confirm that my above assumption is correct and if not
> correct it?
>
> Best regards,
> czezz
>
>
>