There are three rules: connect, also-connect and connect-exclusive.
(connect "SuperCollider:out_(.*)" "alsa_pcm:playback_\1")
ensures that whenever scsynth(1) is running any output ports it registers are connected to appropriate ALSA playback ports. The connect rule only makes a connection if none already exist.
(also-connect "alsa_pcm:playback_1" "jack.udp-[0-9]*:in_1")
ensures that when jack.udp(1) is running in send mode all signals that are ordinarily sent to the local audio interface will also be sent to the network destination. The also-connect aliasing applies to both the left and right hand side of connect rules.
(connect-exclusive "SuperCollider:out_(.*)" "ardour:sc3_in_\1/in 1")
ensures that if SuperCollider and an ardour(1) session with appropriate tracks are both running that SuperCollider gets connected to ardour and disconnected from everything else.
This type of connection daemon has the following advantages over both environment variables and command line arguments: 1. it does not require any client connection support or logic at all, 2. it provides a uniform user interface to all clients, and 3. it has a somewhat declarative style.
This type of daemon is very lightweight and non-intrusive: it requires no arguments, it runs from an unprivileged thread, if the rule set is empty it is a no-op, it does not write any files, it does not require any modification to clients or to how clients are started.
When a set of port registrations is made it checks the modification time of the rule set files, /etc/jack.plumbing and ~/.jack.plumbing, re-reads the rule set if it has been updated, then makes the JACK graph obey the rules.
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