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From Rainer Jung <rainer.j...@kippdata.de>
Subject Re: mod_jk not working as expected - is there a bug??
Date Wed, 25 Feb 2009 00:50:48 GMT
On 25.02.2009 00:00, Mohit Anchlia wrote:
> Reposting:
>
> Apache Server - 2.2
> Tomcat server 6
> Jboss - 4.2
>
> We have Web Servers talking to Jboss App Servers over mod_jk. When we
> do our patch or upgrade of software we do it in rolling fashion so
> that there is "0" customer impact. But it looks like mod_jk load
> balancer on Web server doesn't detect it as soon as Jboss App Server
> goes down. Our goal is to have 0 customer impact. So my question is
> what can we do to overcome this problem. Web Server sees Http Error
> Code 503.
>
> Information from log file:
>
> [Mon Feb 23 13:39:42.146 2009] [31682:4143745888] [error]
> ajp_connection_tcp_get_message::jk_ajp_common.c (966): (appfe4) can't
> receive the response message from tomcat, network problems or tomcat
> (10.10.81.89:8009) is down (errno=104)
> [Mon Feb 23 13:39:42.147 2009] [31682:4143745888] [error]
> ajp_service::jk_ajp_common.c (2097): (appfe4) Connecting to tomcat
> failed. Tomcat is probably not started or is listening on the wrong
> port

This means that mod_jk detected that your backend is down and thus puts 
it into an error state. All following requests will no longer be sent to 
this backend. Once a minute it will send a request there and try, but as 
long as it is down this test will not succeed and thus all requests will 
be sent to other nodes.

The first request that gets sent to the backend you stopped might get an 
error back. If you want to prevent that from happening, use Cping/Cpong:

http://tomcat.apache.org/connectors-doc/generic_howto/timeouts.html

so we will detect the broken node before actually sending a request 
there. More details are not possible to give without your JK 
configuration (Jk directive sin httpd configuration files, 
workers.properties and if used uriworkermap.properties).

The line number of the above message tells me you are using mod_jk 
1.2.25. Although there's nothing wrong in principal with 1.2.25, we 
always try to improve and you might consider switching to 1.2.27.

You should also increase your JkLogLevel to info. As long as only 
occasional info messages are in your log file everything is fine, but 
once error messages show up, the additional info messages contain useful 
formation.

Regards,

Rainer

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