<Listener> element (and the more common variants described below) configures the "remoting" software that connects the in-process and out-of-process portions of the SP. For most purposes, you can ignore it as an internal detail that doesn't require your intervention.
This remoting layer is an inter-process communication mechanism, but is not optimized to be used between machines. It is accessed frequently, typically at least once per web request. The best analogy would be the AJP protocol connector used between Apache and Tomcat, but is generally even less efficient. Used over a domain socket or loopback connection, the speed is not a factor, but across a network, performance will suffer.
Can be omitted on Version 2.4 and above, in which case TCP (on Windows) or Unix (everywhere else) plugins will be used, with the default settings described on their respective topic pages.
The SP includes a pair of socket-based implementations, one recommended for Unix/Linux use and the other for Windows.
All existing SP variants require a maximum one (and only one) of the following elements be specified:
- Listener implementation that relies on a Unix domain socket.
- Listener implementation that relies on a TCP socket. Can be used across a network, but this is not recommended, as noted above.
- Pluggable extension point for new alternatives, requires a
type attribute specifying the plugin's unique type. Will contain other content specific to the plugin.