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Hello Matt,
Wednesday, May 30, 2001, 11:31:03 PM, you wrote:
> Another thought on this: What happens when you are using multiple
> SQL servers (for HA) and the primary is down? With having a small
> program that gets called by tcpserver, it's going to try to connect
> to the first SQL server and will have to timeout before it tries
> the second. That's painful, and every POP and SMTP session will
> then get a "timeout" length pause before their connection succeeds.
> When tcpserver opens the connection, if the persistent connection
> to the primary server drops, it'll try to reopen it. If it failes
> to reopen, I could add some logic to get it to connect to a
> secondary server. Then only the connections that arrive during the
> time tcpserver's connection is retrying have a pause, once it's
> failed over to the backup MySQL server(s) all connection resume as
> normal.
This is the most important point about MySQL and HA, IMHO. I've been
wondering
about this topic since I first set up replication among our DB
machines.
IMHO, it pretty much flawed to try to hack each and every MySQL
client so
it can do fail over to a backup machine, much more sense would be a
patch
to libmysql that automagically tries to connect to a backup server if
the primary is down. Thoughts?
The biggest head-ache about using the backup machine is that it can
end up
in a data integrity disaster as it is perfectly ok to run selects
on the backup (they don't change any data) but no updates at all
should
be done to the backup machine(s) since those won't get propagated
to the master. I think the whole replication model should include an
algorithm that elects a new master after the failure of the standard
one is detected, the standard one then can come back up and stay as
slave til the admin decides what to do about the situation.
(Fortunately, MySQL is in our case so the replication is only done
for
data safety reasons, not HA so far...)
The point about patching tcpserver is of course true but then you
should start thinking about integrating vpopmail completely into
tcpserver from
begin as executing vchkpw everytime also causes additional load.
Best regards,
Gabriel
(·(·Ô
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