[imp] load balancing / temporary files..

[Andrew Morgan]

On Wed, 18 Dec 2002, Liam Hoekenga wrote:

> >From what I've read, it sounds like using a SQL based session manager, and
> using VFS to store uploaded files are two useful things in trying to
> spread the load across a pool of servers.  I'm curious, though, what about
> the temp files that aspell (and the other helper apps) create?  I'm most
> concerned about aspell... but what if a user gets bounced to a new server
> in the middle of spell checking a message?
>
> Liam

For all the effort you would put into making all the little details work,
you'd be better off spending a little money to get a hardware load
balancing appliance.  We're using a Foundry Networks ServerIronXL here.
It has "sticky" sessions, so once it directs the client to one of the
servers, they will keep going to that server (after some period of
inactivity it will timeout the sticky).

Then the only time you run into trouble is when you take down one of the
servers unexpectedly, which would leave their attachments, etc on the
downed server.  If you have a planned outage, you can tell the
ServerIronXL to disable that server ahead of time, and it will stop
sending new sessions to that server but continue for active sessions.

We do MySQL based sessions here, with MySQL running on a single server
separate from our webmail servers.  Someone else mentioned placing the
MySQL server on the IMAP server so you only have a single point of failure
instead of two single points of failure.  I like this idea in general.  In
our case though, we host webmail for several different IMAP servers so it
isn't as much of an advantage.

	Andy