[Mediawiki-l] Sending DB reads to master if slave replication fails?

[Jani Patokallio]

Thanks for the fast and authoritative reply!

Brion Vibber <brion at pobox.com> wrote:
> It sounds like you mainly want to use replication to maintain a hot standby
> database, not for load balancing. It may actually be best here to not tell
> MediaWiki about the slave at all: use the master only, and consider using
> some other HA proxy or whatever to hot-swap the old slave in for the master
> if the master stops responding.

So the slave becomes the master and starts accepting writes?  Doesn't
this imply that the direction of replication on the MySQL level has to
be reversed after the former master comes back online, and now has to
become the slave to get any changes that were made in the meantime?
This sounds fairly painful, especially if the failure is intermittent.
 Alternatively, if you're saying that the slave should be read-only
even when it takes over, then this isn't really much of an improvement
on the current state of affairs.

> Very early on we did try to fall back to master if no non-lagged slaves were
> available, however it can be highly problematic if you're using replication
> for the purpose of load balancing -- which is what MediaWiki's explicit
> support for multiple database servers is designed for.

Problem is, while our system does occasionally get spikes of load
usually involving heavy reads of swathes of the database (which is why
we've got the master/slave split), the replication typically fails
randomly for some reason other than load: network glitches, out of
disk space, etc.  So it's not that lag is high, it's that replication
has failed entirely.

Anyway, I gather that the best thing to do is still set max lag, since
at least this way the non-replicating slave switches MediaWiki into
read-only mode and the user gets a clear failure message instead of
just wondering why their edits seem to disappear into the ether.

Cheers,
-jani