[Dovecot] quick question

[David Jonas]

On 01/22/2010 10:15 AM, Brandon Davidson wrote:
> We've thought about enabling IP-based session affinity on the load
> balancer, but this would concentrate the load of our webmail clients, as
> well as not really solving the problem for users that leave clients open
> on multiple systems. 

Webmail and IMAP servers are on the same network for us so we don't have
to go through the BigIP for this, we just use local round-robin DNS to
avoid any sort of clumping. Imapproxy or dovecot proxy local to the
webmail server would get around that too.

> I've done a small bit of looking at nginx's imap
> proxy support, but it's not really set up to do what we want, and would
> require moving the IMAP virtual server off our load balancers and on to
> something significantly less supportable. Having the dovecot processes
> 'talk amongst themselves' to synchronize things, or go into proxy mode
> automatically, would be fantastic.

Though we aren't using NFS we do have a BigIP directing IMAP and POP3
traffic to multiple dovecot stores. We use mysql authentication and the
"proxy_maybe" option to keep users on the correct box. My tests using an
external proxy box didn't significantly reduce the load on the stores
compared to proxy_maybe. And you don't have to manage another
box/config. Since you only need to keep users on the _same_ box and not
the _correct_ box, if you're using mysql authentication you could hash
the username or domain to a particular IP address:

SELECT CONCAT('192.168.1.', ORD(UPPER(SUBSTRING('%d', 1, 1))) AS host,
'Y' AS proxy_maybe, ...

Just assign IP addresses 192.168.1.48-90 to your dovecot servers. Shift
the range by adding or subtracting to the ORD. A mysql function would
likely work just as well. If a server goes down, move it's IP. You could
probably make pairs with heartbeat or some monitoring software to do it
automatically.

-David