plumber – file system for interprocess messaging|
The plumber is a user-level file server that receives, examines,
rewrites, and dispatches plumb(7) messages between programs. Its
behavior is programmed by a plumbing file (default $HOME/lib/plumbing)
in the format of plumb(7).
Its services are posted via 9pserve(4) as plumb. and consist of
two pre-defined files, plumb/send and plumb/rules, and a set of
output ports for dispatching messages to applications.
Programs use fswrite (see 9pclient(3)) to deliver messages to
the send file, and fsread 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 plumb/send; sam in turn reads from, by
convention, 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
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 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).|