This query is about the optimal arrangement of IMAP proxies between IMAP
webmail clients and "real IMAP servers" which use a shared NAS/SAN for storage
of mail.
The Problem
~~~~~~~~~
The problem is caused by the fact that IMAP webmail clients are heavy on
backend IMAP servers. IMAP webmail clients, unlike some native clients, do not
persist IMAP sessions, and so the result is that every user "click" leads to a
whole open-do-close IMAP session. Ideally, a persistent IMAP session would be
opened when a user logs in, and is re-used during the webmail users activity,
and is closed when the user logs out.
(user) ----- [webmail] -- -- -- [IMAP server]
Solution
~~~~~~
The solution is often seen to lie in IMAP proxies which maintain a session to
the real IMAP servers for a given timeout period.
(user) ----- [webmail] -- -- -- [IMAP Proxy] -------- [IMAP server]
Problem with Solution
~~~~~~~~~~~~~~~~
The problems arise when IMAP proxies are used in architectures which use load
balancers and load-balanced clusters.
Imagine a group of real IMAP servers, which use a SAN shared storage for the
mail data, headed by a load balancer which distributes IMAP connections across
these IMAP servers. Note that additional load-balancer mechanisms such as
"stickiness" are not required as IMAP connections are stateful unlike common
HTTP requests.
Now if a set of IMAP proxies is placed behind the load-balancers then it is
possible that many IMAP sessions generated by a users webmail session can end
up load balanced to many IMAP proxies. This is not good. It means that *one*
users's webmail session has been sprayed across *multiple* IMAP proxies and
each of these will persist a session with the real IMAP servers (for at least
the timeout period).
(user) ----- [webmail] -- -- -- <lb> -- -- -- [IMAP Proxy] -------- [IMAP
server]
\-- -- -- [IMAP Proxy] -------- [IMAP
server]
\-- -- -- [IMAP Proxy] -------- [IMAP
server]
So we could place the IMAP proxies closer to the webmail servers - before the
load balancers. This means that each webmail server must be preconfigured to
talk to a single IMAP proxy. This way the persisted sessions survive through
the load-balancers and only one session is maintained to the backend real IMAP
servers. However, the problem with this is that there is reduced resilience -
if the proxy fails then no users can use that particular webmail server.
(user) ----- [webmail] -- -- -- [IMAP Proxy] -------- <lb> ------ [IMAP
server]
Constraint
~~~~~~~~
An answer is to place the IMAP proxy co-resident with the webmail application,
effectively giving the webmail application a persisting capability. However
this option is not available in this instance as the webmail servers cannot be
modified.
What are people's opinions here? Are there any magic load-balancing mechanisms
that can be used to good effect here? The load-balancers are Foundry ServerIron
machines.
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