First off, when all instances are up and running, mod_jk/mod_proxy_ajp load balancing is straightforward. Each servlet engine appends its jvmRoute suffix to its sessions. When mod_jk/mod_proxy_ajp balancer is in sticky session mode it sends requests with jvmRoute suffixes to the worker of the same name (i.e. jvmRoute=worker name).
All that said I am honestly not 100% certain what happens when a given worker is down and mod_jk/mod_proxy_ajp receives a request that is supposed to be handled by it.
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