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From jean-frederic clere <>
Subject Re: mod_proxy / mod_proxy_balancer
Date Wed, 06 May 2009 09:42:25 GMT
Jess Holle wrote:
> Rainer Jung wrote:
>> In most situations aplications need stickyness. So balancing will not
>> happen in an ideal situation, instead it tries to keep load equal
>> although most requests are sticky.
>> Because of the influence of sticky requests it can happen that
>> accumulated load distributes very uneven between the nodes. Should the
>> balancer try to correct such accumulated differences?
>>   Other applications are memory bound. Memory is needed by request
>> handling but also by session handling. Data accumulation is mor
>> eimportant here, because of the sessions. Again, we can not be perfect,
>> because we don't get a notification, when a session expires or a user
>> logs out. So we can only count the "new" sessions. This counter in my
>> opinion also needs some aging, so that we won't compensate historic
>> inequality without bounds. I must confess, that I don't have an example
>> here, how this inequality can happen for sessions when balancing new
>> session requests (stickyness doesn't influence this), but I think
>> balancing based on old data is the wrong model here too.
> 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?

> 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.



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