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From Rainer Jung <>
Subject Re: Tomcat and Apache mod_jk For Failover
Date Wed, 06 Feb 2008 21:33:39 GMT
Hi James,

James Ellis schrieb:
> I am trying to set up a cluster of Tomcat servers where they
> replicate session btwn the tomcat servers. My questions are:
> 1) For failover, if I am using apache/mod_jk as a load balancer, will
> it automatically detect if one of the members in the cluster is down
> and not route requests to it?

Yes, if the problem is formally detectable, e.g. network problems, no 
responses etc. What problems get detected depends much on configuration. 
Have a look at

and for a complete list of attributes at

You should configure a status worker, such that you can inspect your 
load balancer status via a HTML GUI. See

> 2) Are there any issues assoicated with SSL and apache as the load
> balancer?

Not that I'm aware of, but maybe I don't get the point. Usually you 
terminate SSL on httpd and then simply use AJP13 as a protocol between 
mod_jk and Tomcat. More or less it's the only protocol mod_jk speaks.

The module then automatically tells Tomcat, if the original request was 
coming in via http or https.

> 3) Are there performance gains by using Apache/mod_jk as the load
> balancer, or should I just use another Tomcat instance and the
> "balancer" web app?

More often performance is not the key decision criterion. I assume the 
balancer webapp is not really production grade, but others might correct 
me here.

To choose the right load balancing solution, you have to decide which 
people should administer and troubleshoot it, and which technology best 
fits their skills (network appliances, Apache web server with mod_jk, ...).

Also: if you do clustering, you might be concerned about availability. 
The load balancer itself is a single point of failure, unless you 
implement some redunfdancy in that layer to.

> Thanks,Jim



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