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From Jess Holle <>
Subject Re: mod_proxy / mod_proxy_balancer
Date Wed, 06 May 2009 11:57:06 GMT
jean-frederic clere wrote:
> Jess Holle wrote:
>> An ability to balance based on new sessions with an idle time out on 
>> such sessions would be close enough to reality in cases where 
>> sessions expire rather than being explicitly invalidated (e.g. by a 
>> logout).
> Storing the sessionid to share the load depending on the number of 
> active sessions, brings a problem of security, no?
To the degree that you consider Apache vulnerable to attack to retrieve 
these, yes.

I prefer the health check request approach below for this and other 
reasons (amount of required bookkeeping, etc).
>> Of course that redoes what a servlet engine would be doing and does 
>> so with lower fidelity.  An ability to ask a backend for its current 
>> session count and load balance new requests on that basis would be 
>> really helpful.  Whether this ability is buried into AJP, for 
>> instance, or is simply a separate request to a designated URL is 
>> another question, but the latter approach seems fairly general and 
>> the number of such requests could be throttled by a time-to-live 
>> setting on the last such count obtained.
>> Actually this could and should be generalized beyond active sessions 
>> to a back-end health metric.  Each backend could compute and respond 
>> with a relative measure of busyness/health and respond and the load 
>> balancer could then balance new (session-less) requests to the least 
>> busy / most healthy backend.  This would seem to be *huge* step 
>> forward in load balancing capability/fidelity.
>> It's my understanding that mod_cluster is pursuing just this sort of 
>> thing to some degree -- but currently only works for JBoss backends.
> This wrong it works with Tomcat too.
mod_cluster works with Tomcat, but according to the docs I've seen the 
dynamic (health/session metric based rather than static) load balancing 
only worked with JBoss backends.

Or has this changed?

Jess Holle

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