Apache Load Balancer Persistence

Wednesday, July 08 2009 @ 11:14 AM CDT

Contributed by: nat

Apache 2.2's load balancer is pretty neat. However, to get persistence to work properly, you have to be careful. Here we are setting the balancer manager to watch a client cookie called BALANCEID, and each member has a particular route string tied to it that is set in the cookie. It's important to note that the cookie format must be: something.routestring, ie, nat.server1. If it is just server1, it will not work.


  ProxyPass / balancer://mycluster/ stickysession=BALANCEID
  ProxyPassReverse / http://localhost:71/
  ProxyPassReverse / http://localhost:72/
  <Proxy balancer://mycluster>
        BalancerMember http://localhost:71 route=server1
        BalancerMember http://localhost:72 route=server2
  </Proxy>

Now there's the issue of the client cookie being set. What if you are load balancing a third party app or webserver and can't easily get the cookie set on the client? No problem! While it didn't work in 2.2.3 (in particular, the BALANCER_ROUTE_CHANGED var being set), when I tried in 2.2.11, I was able to set the cookie myself based on which balancer member was selected:


  # Set session cookie if BALANCER_ROUTE_CHANGED, containing BALANCER_WORKER_ROUTE env variable, which is set to the route above
  # Note that cookie value should be a session id, followed by a period, followed by the route.
  # Since session id cookie usually not advised to be mutable, best create own cookie with anything you want
  # for the session part, just make sure to have a period and route part last

  ### Used for setting cookie
  LoadModule headers_module modules/mod_headers.so
  Header add Set-Cookie "BALANCEID=balancer.%e; path=/;" env=BALANCER_ROUTE_CHANGED

  # Just give some debug info in the header, don't use once you have it working
  Header add X-Var "BALANCER_ROUTE_CHANGED=%e" env=BALANCER_ROUTE_CHANGED
  Header add X-Var "BALANCER_WORKER_ROUTE=%e" env=BALANCER_WORKER_ROUTE
  Header add X-Var "BALANCER_SESSION_ROUTE=%e" env=BALANCER_SESSION_ROUTE

Presto! Session persistence, all handled at the reverse proxy load balancer level.

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