jean-frederic clere wrote:
Should general support for a query URL be provided in mod_proxy_balancer?  Or should this be left to mod_cluster?
Can you explain more? I don't get the question.
What I mean is
  1. Should mod_proxy_balancer be extended to provide a balancer algorithm in which one specifies a backend URL that will provide a single numeric health metric, throttle the number of such requests via a time-to-live associated with this information, and balance on this basis or
  2. Should mod_cluster handle this issue?
  3. Or both?
>From your response below, it sounds like you're saying it's #2, which is largely fine and good -- but this raises questions:
  1. How general is the health check metric in mod_cluster?
  2. Does this require special JBoss nuggets in Tomcat?
  3. When will mod_cluster support health metric based balancing of Tomcat?
  4. How "disruptive" to an existing configuration using mod_proxy_balancer/mod_proxy_ajp is mod_cluster?
  5. How portable is the mod_cluster code?
I say this is largely fine and good as I'd like to see just the health-metric based balancing algorithm in Apache 2.2.x itself.
Does mod_cluster provide yet another approach top to bottom (separate than mod_jk and mod_proxy/mod_proxy_ajp)?
Mod_cluster is just a balancer for mod_proxy but due to the dynamic creation of balancers and workers it can't get in the httpd-trunk code right now.
 It would seem nice to me if mod_jk and/or mod_proxy_balancer could do health checks, but you have to draw the line somewhere on growing any given module and if mod_jk and mod_proxy_balancer are not going in that direction at some point mod_cluster may be in my future. 
Jess Holle