multiple instances of the same script?

[Scott James Remnant]

On Sat, 2007-03-10 at 14:13 +0100, paul wrote:

> Can multiple instance of the same script run at the same  time?
> 
Yes.

As you've noted, this isn't the default behaviour because the "one copy
of one job running at one time" behaviour turns out to be more useful
for most cases; especially since it provides free locking for cron
replacement later on, etc.

Since 0.3.5, Upstart has also supported having jobs that may have
multiple copies running at any one time; I would recommend 0.3.7 though,
since that massively improves the initctl tool's handling of them.

Such jobs (where multiple instances may be running at any one time) can
be declared by using the "instance" stanza in the job definition.

Example:

	start on block-device-added

	instance

	script
	    ...
	end script

Now every time the start event happens, or a manual start is attempted,
a new instance will be created and that will run.  Such jobs are
expected to handle interlocking between each other themselves.

Example:

	# status --show-ids foo
	foo [#100] (instance)

	# start --show-ids foo
	foo [#123] (start) waiting
	foo [#123] (start) starting
	  :

	# start --show-ids foo
	foo [#124] (start) waiting
	foo [#124] (start) starting

	# status --show-ids foo
	foo [#100] (instance)
	    [#123] (start) running, process 54321
	    [#124] (start) running, process 56789

	# status --show-ids --by-id 123
	foo [#123] (start) running, process 54321

Notice how the new instances have a different id to the primary one, but
the same name.  The primary job can never be anything other than
stop/waiting since attempting to start it always spawns a new instance.

Running "stop foo" would stop all instances of it; you can stop
individual instances by their id.


There's future plans for a possible "watershed" variety on this.
Watershed jobs wouldn't spawn new instances whenever the event occurred,
instead they would simply recognise that they need to be respawned after
they have stopped.

This is useful for where the task being run is expensive, and it's only
necessary to run it if there was any change since the last time it was
started -- and not for each event.

Scott
-- 
Have you ever, ever felt like this?
Had strange things happen?  Are you going round the twist?
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