plumber – file system for interprocess messaging
plumber [ –p plumbing ]
The plumber is a user–level file server that receives, examines,
rewrites, and dispatches plumb(6) messages between programs. Its
behavior is programmed by a plumbing file (default /usr/$user/lib/plumbing)
in the format of plumb(6).
Its services are mounted on the directory /mnt/plumb (/mnt/term/mnt/plumb on the CPU server) and consist of two pre–defined files, send and rules, and a set of output ports for dispatching messages to applications. The service is also published as a srv(4) file, named in $plumbsrv, for mounting elsewhere.
Programs use write (see read(2)) to deliver messages to the send file, and read(2) to receive them from the corresponding port. For example, sam(1)'s plumb menu item or the B command cause a message to be sent to /mnt/plumb/send; sam in turn reads from, by convention, /mnt/plumb/edit to receive messages about files to open.
A copy of each message is sent to each client that has the corresponding port open. If none has it open, and the rule has a plumb client or plumb start rule, that rule is applied. A plumb client rule causes the specified command to be run and the message to be held for delivery when the port is opened. A plumb start rule runs the command but discards the message. If neither start or client is specified and the port is not open, the message is discarded and a write error is returned to the sender.
The set of output ports is determined dynamically by the specification in the plumbing rules file: a port is created for each unique destination of a plumb to rule.
The set of rules currently active may be examined by reading the
file /mnt/plumb/rules; appending to this file adds new rules to
the set, while creating it (opening it with OTRUNC) clears the
rule set. Thus the rule set may be edited dynamically with a traditional
text editor. However, ports are never deleted
dynamically; if a new set of rules does not include a port that
was defined in earlier rules, that port will still exist (although
no new messages will be delivered there).
/usr/$user/lib/plumbing default rules file
/sys/lib/plumb directory to search for files in include statements
/mnt/plumb mount point for plumber(4).
plumb(1), plumb(2), plumb(6)
Plumber's file name space is fixed, so it is difficult to plumb
messages that involve files in newly mounted services.