following on from the previous thread "re: optimal persistent IMAP proxy
load-balanced architecture".
A distillation of the problem discussed in the above thread is that
load-balancers are good for resilience but cause a problem when forwarding
non-persisted sessions.
(user) ----- [webmail] -- -- -- <lb> -- -- -- [IMAP Proxy] -------- [IMAP
server]
\-- -- -- [IMAP Proxy] -------- [IMAP
server]
\-- -- -- [IMAP Proxy] -------- [IMAP
server
That is - load-balancers will spray start-do-stop sessions generated by each
IMAP webmail page click. This means a single end user will have sessions across
many backend IMAP servers unecessarily. The solution to this problem is to only
push persisted long-term IMAP sessions over a load-balancer.
(user) ----- [webmail] -- -- -- [IMAP Proxy] -------- <lb> ------ [IMAP
server]
\- no resilience -/
The problem then becomes one of directing short HTTP related start-do-stop IMAP
from the webmail servers to the same set of IMAP proxies for further
persisting. Of course if the primary choice fails, then failover choices can be
taken. This is a form of "stickiness" in directing traffic.
(user A) ----- [webmail] -- -- [director] -- -- [IMAP Proxy 1] (user A)
(user B) \-- -- [IMAP Proxy 2] (user B)
Does anyone know of a hardware/software tool that can do this?
Tariq Rashid
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