One process acts as a 'timebase master', and others may choose to sync to this masters' time. Aside from current frame numbers, a timebase master may also emit more abstract information such as the current location in 'Beat/Bar/Tick' format and the current BPM.
While this system isn't perfect (it requires all clients' time signature maps to be synced up), it does open up some interesting possibilities: in theory one application (the timebase master) could govern tempo changes, and other applications could be written in such a way to follow tempo changes automatically.
Right now most timebase clients seem to assume a constant BPM (disallowing tempo changes entirely), but as a 'hello world' I wrote a JACK MIDI metronome that syncs to a BBT-emitting timebase master. Note that it only emits note-on events, so be sure to choose a non-sustained instrument
http://arnout.engelen.eu/files/dev/linu ... metronome/
