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From Michael Echerer <m...@tngtech.com>
Subject Re: Multiple instances of Tomcat on same machine
Date Mon, 01 May 2006 08:04:47 GMT
keith wrote:
> Hi all,
> 
> I'm wondering if anyone knows what are the advantages of running the
> same web application in multiple instances of Tomcat on the same machine
> (with a dual duo-core Intel processor).
> 
> Do having multiple instances affect performance positively or negatively
> taking in to account that the machine has multiple processors?
I doubt there's a performance gain as you could also increase the
default settings of just one Tomcat. Of course, each JVM adds memory
overhead. Depending on how many webapps you deploy this might not be the
issue compared to your webappss' memory consumption however.

Pro:

1) However in case you have more memory in your machine than a JVM can
possible use with Windows/Linux you might want to start multiple
Tomcats/JVMs and distribute webapps or even the same webapp to
distribute the memory consumption of one app's sessions and loadbalance.

2) You might want two Tomcat instances with loadbalancing in front in
order to do maintainance work. Of course, you could just have one
hot-standby instead of using both. So probably only a good idea if
combined with #2.

3) If you don't trust your VM you might hope if one crashes you still
got one ;-) However OOMs might be difficult to detect properly by a
loadbalancer and that Tomcat might still hang around, but not stable.

4) you prefer to distribute webapps, need a complex vhost setup, <add
what you like>, but dislike configuring just one Tomcat. Instead you
want one Tomcat per "special" setup.

Cons:
1) keeping config in sync
2) webapps installed multiple times (or symlinked /webapp with other
effects...). Tomcat updates shouldn't be more effort as long as you
stick just to patch versions as you'll still only have on binary
installation CATALINA_HOME vs. multiple CATALINA_BASE configs.
3) more complex, might want session clustering to get session replications


Cheers,
Michael
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