The OpenXPC™ project is an independent and source-compatible reimplementation of Apple's XPC, an IPC mechanismm. It is hosted on the D-Bus Message Oriented Middleware. OpenXPC is intended principally for Airyx OS, but is also compatible with other free-software BSDs and GNU+Linux. It is not yet complete.
Architecture is subject to change to improve compatibility. The described architecture isn't fully implemented yet, but describes the basic plan of action.
XPC services run in one of three kinds of domain: the singleton system domain for services available to all apps and libraries; the per-user session domain, and the per-app app domain. The first two are directly mapped to the D-Bus system and session domains. The third, being app-specific, is implemented by the setting of an environment variable by LaunchServices during the launch of an app, that variable naming a private folder for that app. Unix domain sockets are created in that folder bearing the service name of each XPC service, and Libdbus is used to directly speak the D-Bus protocol over these, without involving D-Bus Daemon.
xpcd/xpcctl are provisional daemons which provide a subset of the launchd interfaces. Whether they will be necessary is yet to be determiend.
OpenXPC is free and open source software.