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From Mark Thomas <>
Subject Re: IS that a good idea moving all the common libs?
Date Wed, 22 Dec 2010 11:49:39 GMT
On 21/12/2010 23:49, George Sexton wrote:
>> -----Original Message-----
>> From: Caldarale, Charles R []
>> Sent: Monday, December 20, 2010 1:08 PM
>> To: Tomcat Users List
>> Subject: Re: IS that a good idea moving all the common libs?
>> On Dec 20, 2010, at 12:16, "Luca Gervasi" <> wrote:
>>> I'm trying to lower the permgen needed by a large amount of webapps
>>> moving all the commonly used libs to the tomcat common libs.
>> That's a really, really bad idea. You would be intertwining all your
>> webapps, potentially introducing object leaks across webapps, and
>> creating insurmountable versioning issues. You'd also require a
>> complete Tomcat restart to update any single webapp.
> I'm really not seeing that. Anyhow, would the suggestion change if you were
> running 300+ web applications?

It is do-able if you are prepared to accept the associated constraints:
- all web apps must use the exact same version of every shared library
- updating the shared libraries requires a Tomcat restart
- all of the shared web apps must be "well behaved" in a multiple
class-loader environment
- one out of control web application can take down the whole instance

If the libraries are well behaved, it is possible to update/reload
individual web applications. As the new memory leak detection features
are showing, a fair number of libs and applications are not well behaved.

Personally, I think those constraints are too much for every web app so
I would probably use a mixture of multiple Tomcat instances,
self-container web apps and webapps that shared libraries but with a
preference for self-contained web apps over shared libraries.


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