(ignore issues of support for particular versions of Apache)
google pretends to know a lot about this general question, but it seems to be in discussions between general users with the same curiosity and there is little or no developer input. OTOH, there is a great indication that Tomcat connector FAQ and httpd documentation could help a lot of people by presenting the same conclusion, whatever that happens to be ;) Are there key design issues which will make one better than the other assuming each has enough developer attention to fix code bugs? Are there still fundamental pieces missing from mod_proxy_ajp + mod_proxy_balancer which have to be resolved before mod_proxy_ajp is the natural solution for anybody on Apache >= 2.2? When should somebody use mod_proxy_http [+ mod_proxy_balancer] instead of one of these solutions? The Tomcat connectors FAQ says this: ---snip---- # mod_jk is great and should be used for production. It is getting fixes as needed (which is now rare). # mod_proxy. A cheap way to proxy without the hassles of configuring JK. This solution lacks sticky session load balancing. If you don't need some of the features of jk, jk2 - this is a very simple alternative. # mod_proxy_ajp. With apache 2.2, mod_proxy was rewritten to support load balancing as well as a new transport called mod_proxy_ajp. This module is distributed with the Apache http server, not the Tomcat server. ---snip--- Isn't pass-through of client SSL connection information another problem with mod_proxy? (servlets can't access cipher or client certificate)