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Subject DO NOT REPLY [Bug 47011] New: mod_proxy/mod_proxy_balancer hot-standby BalancerMembers not taking over immediately
Date Thu, 09 Apr 2009 20:32:34 GMT

           Summary: mod_proxy/mod_proxy_balancer hot-standby
                    BalancerMembers not taking over immediately
           Product: Apache httpd-2
           Version: 2.2.8
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: regression
          Priority: P2
         Component: mod_proxy_balancer

After upgrading from Apache 2.2.6 to Apache 2.2.8 or greater, when gracefully
shutting down one of our Embedded Tomcats on our application server, we notice
a temporary outage (503) from our proxy balancer before the hot-standby
(status=+H) takes over.

- Application proxy server (Apache 2.2.8)
   - Proxies requests via mod_proxy/mod_proxy_balancer/mod_proxy_ajp to
application server
- Application server (Java)
   - Runs master Java application server with Embedded Tomcat (Tomcat/5.5.17)
on port 8009
   - Runs slave Java application server with Embedded Tomcat (Tomcat/5.5.17) on
port 8008

When the application proxy server was Apache 2.2.6, we were able to gracefully
shutdown the master Tomcat server (calling embedded.stop()), and the Hot
Standby (status=+H) BalancerMember would immediately start serving requests. 
After the upgrade to 2.2.8, we see "HTTP/1.1 503 This application is not
currently available" while the active BalancerMember is shutting down before
the hot-standby takes over (~1 second). These errors appear in both the Tomcat
access log and the Apache access log.

I have recompiled and tested Apache versions 2.2.6, 2.2.8, 2.2.9 and 2.2.11 to
verify that this problem exists after version 2.2.6.  I still had the problem
when switching the BalancerMember protocol from ajp:// to http://, so I think
this rules out AJP-specific issues.  I was unable to reproduce the problem
using Apache servers as the BalancerMembers, so I'm speculating this has
something to do with the interaction with Tomcat.

Proxy conf:
ProxyPassMatch /(.+/)?application.server$    balancer://production_server

<Proxy balancer://production_server/>
    BalancerMember ajp://server.domain.tld:8009/    lbset=1    retry=10
    BalancerMember ajp://server.domain.tld:8008/    lbset=2    retry=10

    ProxySet lbmethod=bytraffic

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