Bad Gateway 502 error with Apache mod_proxy and Tomcat

Question

We\'re running a web app on Tomcat 6 and Apache mod_proxy 2.2.3. Seeing a lot of 502 errors like this:

Bad Gateway! The proxy server received an in

Answer 1

If you want to handle your webapp's timeout with an apache load balancer, you first have to understand the different meaning of timeout. I try to condense the discussion I found here: http://apache-http-server.18135.x6.nabble.com/mod-proxy-When-does-a-backend-be-considered-as-failed-td5031316.html :

It appears that mod_proxy considers a backend as failed only when the transport layer connection to that backend fails. Unless failonstatus/failontimeout is used. ...

So, setting failontimeout is necessary for apache to consider a timeout of the webapp (e.g. served by tomcat) as a fail (and consecutively switch to the hot spare server). For the proper configuration, note the following misconfiguration:

ProxyPass / balancer://localbalance/ failontimeout=on timeout=10 failonstatus=50

This is a misconfiguration because:

You are defining a balancer here, so the timeout parameter relates to the balancer (like the two others). However for a balancer, the timeout parameter is not a connection timeout (like the one used with BalancerMember), but the maximum time to wait for a free worker/member (e.g. when all the workers are busy or in error state, the default being to not wait).

So, a proper configuration is done like this

1. set timeout at the BalanceMember level:

 
     BalancerMember http://member1:8080/svc timeout=6 
 ... more BalanceMembers here

2. set the failontimeout on the balancer

ProxyPass /svc balancer://mycluster failontimeout=on

Restart apache.