Re: Streaming replication status

Stefan Kaltenbrunner wrote:
> Greg Smith wrote:
>>
>> The other popular request that keeps popping up here is providing an
>> easy way to see how backlogged the archive_command is, to make it
>> easier to monitor for out of disk errors that might prove
>> catastrophic to replication.
>
> I tend to disagree - in any reasonable production setup basic stulff
> like disk space usage is monitored by non-application specific matters.
> While monitoring backlog might be interesting for other reasons,
> citing disk space usage/exhaustions seems just wrong.

I was just mentioning that one use of the data, but there are others.
Let's say that your archive_command works by copying things over to a
NFS mount, and the mount goes down. It could be a long time before you
noticed this via disk space monitoring. But if you were monitoring "how
long has it been since the last time pg_last_archived_xlogfile()
changed?", this would jump right out at you.

Another popular question is "how far behind real-time is the archiver
process?" You can do this right now by duplicating the same xlog file
name scanning and sorting that the archiver does in your own code,
looking for .ready files. It would be simpler if you could call
pg_last_archived_xlogfile() and then just grab that file's timestamp.

I think it's also important to consider the fact that diagnostic
internals exposed via the database are far more useful to some people
than things you have to setup outside of it. You talk about reasonable
configurations above, but some production setups are not so reasonable.
In many of the more secure environments I've worked in (finance,
defense), there is *no* access to the database server beyond what comes
out of port 5432 without getting a whole separate team of people
involved. If the DBA can write a simple monitoring program themselves
that presents data via the one port that is exposed, that makes life
easier for them. This same issue pops up sometimes when we consider the
shared hosting case too, where the user may not have the option of
running a full-fledged monitoring script.

--
Greg Smith 2ndQuadrant Baltimore, MD
PostgreSQL Training, Services and Support
greg(at)2ndQuadrant(dot)com www.2ndQuadrant.com