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From "Caldarale, Charles R" <Chuck.Caldar...@unisys.com>
Subject RE: Tomcat threads
Date Tue, 02 Mar 2010 23:57:34 GMT
> From: Bharath Vasudevan [mailto:bharath.cng@gmail.com]
> Subject: Tomcat threads
> 
> Are they user level threads or kernel threads?

Depends on the particular JVM being used, not on Tomcat.  Pretty much all modern JVMs use
a kernel thread for each java.lang.Thread instance on which start() has been called.

> I see that the requests are kicked off through the threads from a pool
> (which has a configured size), how can this be scalable?

Because you can make the limit as large as you want.  Making it too large leaves you open
to DOS attacks and running out of resources on the server.

> Also, from my understanding in servlets, the response for a 
> request in has to be done then and there (as in I cant respond
> for a request later in future from some random thread) .

That's the nature of HTTP, not servlets in particular.  A properly designed application facing
a long-duration situation will usually send an interim status back to the client, and kick
off an auxiliary thread to do whatever is going to run for a while.  The auxiliary thread
places the final response in some location associated with the client when it's ready.  The
client polls for status and the final response using JavaScript or some other mechanism.

> If there is contention for a resource which might take time
> (causing the response to take time), wont tomcat run out of
> threads?

Yes; Tomcat won't care, but your users might.  Don't use poorly designed applications.

 - Chuck


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