The recommended front-end module is mod_cluster but mod_jk or mod_proxy could be used as in Tomcat or other JBoss AS version.

To use AJP define a ajp listener in the undertow subsystem like:

               <http-listener name="default" socket-binding="http"/>
               <ajp-listener name="ajp" socket-binding="ajp"/>

To the ajp in the in the socket-binding-group like:

<socket-binding-group name="standard-sockets" default-interface="public">
         ....
           <socket-binding name="http" port="8080"/>
           <socket-binding name="ajp" port="8009"/>
           <socket-binding name="https" port="8443"/>