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From "David Rees" <>
Subject mod_jk does not detect a hung Tomcat
Date Mon, 22 Sep 2003 17:09:22 GMT
I posted this to tomcat-users last week, but didn't get a reply...  I'm
hoping to get some feedback from the connectors developers on this issue I
occasionally run into...

I've got a setup where I've got two load balanced Tomcats running off of
Apache and mod_jk.

I've got a problem where one of the Tomcats will occasionally hang, but
not die.  This means that it will accept new connections, but will not
actually process anything.  This renders all clients using the hung Tomcat
completely stuck as they are not switched over to the other Tomcat.

mod_jk seems to assume that if it can connect to Tomcat, it must be ready
to respond to requests.

It seems that some sort of connection test (with a short socket timeout)
would be appropriate to validate that the connection is actually
responding.  While this would increase the latency of each request a bit,
it would improve the reliability.  Is there any provision in the AJP13
protocol to allow for testing of connections before sending a request over

It's easy to simulate this behavior by sending a STOP signal to Tomcat.

I've also attached a log from mod_jk showing the problem.  I marked the
point at which processing in mod_jk stopped until I sent a CONT signal to

Does mod_jk2 have this same problem?  Is there any interest in fixing
this? Does anyone have a workaround for this issue?


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