[imp] High Availability / Load Balancing

[Eric Rostetter]

Quoting Dominic Ijichi <dom@ijichi.org>:

> correctamundo.  there's two ways you can think/go here:
> 
> 1) why create a pool of frontend servers that just handle http/php?  mysql at
> least has builtin replication that is good/solid/lightweight enough to
> justify policy of having sql server on each http server.

Anyone know if postgresql can also do this reliably?

> 2) why bother replicating the sql database when you've still got a single
> point of failure on the imap server?

Because you use Horde, not just IMP, and unless you are doing imap-based
authentication, then there is no reason the imap server should take down
nag, kronolith, gollem, etc.

Or, because you have a redundent imap server setup?

> mailstore is notoriously difficult to replicate
> in realtime good enough to have a hot failover (if anyone does this
> successfully, PLEASE speak up and share!!).

Shared scsi-devices on a cluster.  Only way to go.  Works great.  Been in
OpenVMS for years, in Tru64 unix, available but rather experimental for
linux.

Can also used shared scsi in a failover setup. That is rather well supported
in linux (compared to shared access).

> 3) why bother with replication redundancy?!  split your users -
> alphabetically/by domain for small installs, hash for larger installs - and
> have seperate servers for each chunk of users that have an entire
> mailstore/sql/ldap/php/http install and are self-sufficient.  then at least

Possible (but makes it harder to maintain users -- think renames for example)
and maybe harder to track down user problems, keep both machines uptodate,
etc.  But certainly doable.

> any service/server fails, the failure is isolated to a small chunk of users.

And how is it better if 1/nth of my users can't get access than if all of them
can't?  1/nth is still a large number of complaints.  The general idea is to
avoid any downtime, not to limit the number of users who see the downtime.

-- 
Eric Rostetter
The Department of Physics
The University of Texas at Austin

Why get even? Get odd!