PERL Beginners - Limiting a program to a single running instance

Author

Limiting a program to a single running instance

Mark Wagner

2007-10-03, 7:01 pm

I've got a Perl program that is run once an hour as a cron job.
Normally it finishes its task in about ten minutes, but occasionally
it takes more than an hour to complete. When this happens, a second
copy is started, the two copies step on each others' toes, and both
crash.

I'm looking for the second copy to know that another copy is running
and exit after logging a message to that effect. Any method needs to
work properly if the first copy crashes rather than exiting cleanly.

Thanks,
Mark Wagner

Chas. Owens

2007-10-03, 7:01 pm

On 10/3/07, Mark Wagner <carnildo@gmail.com> wrote:
> I've got a Perl program that is run once an hour as a cron job.
> Normally it finishes its task in about ten minutes, but occasionally
> it takes more than an hour to complete. When this happens, a second
> copy is started, the two copies step on each others' toes, and both
> crash.
>
> I'm looking for the second copy to know that another copy is running
> and exit after logging a message to that effect. Any method needs to
> work properly if the first copy crashes rather than exiting cleanly.
snip

Create a gatekeeper script* that runs the script in question. This
gatekeeper script should look at the currently running processes
(either using ps or some module suitable for your environment) and if
the script is currently running don't execute the script.

* or just add the functionality to your existing script, the benefit
of having a general gatekeeper script is that if you need to do this
again you can just pass different arguments in.

Elizabeth Cortell

2007-10-03, 7:01 pm

Checking for instances by grepping the output of ps tends to also catch instances of

vi myscript.pl

So that innocently editing the file on that box keeps it from running! Not what you have in mind, I think.

I suggest (untested, but we do something like this in the office):
Upon startup, check for the existence of a lockfile with agreed-upon name in some tmp directory. If it exists, exit immediately; otherwise create the file. Remove the file at end of execution or upon death of script (in the END block).

I haven't thought out all the ways that could go wrong. Comments welcome.

Cheers,

Liz

On Wednesday, October 03, 2007, at 01:42PM, "Chas. Owens" <chas.owens@gmail.com> wrote:
>On 10/3/07, Mark Wagner <carnildo@gmail.com> wrote:
>snip
>
>Create a gatekeeper script* that runs the script in question. This
>gatekeeper script should look at the currently running processes
>(either using ps or some module suitable for your environment) and if
>the script is currently running don't execute the script.
>
>* or just add the functionality to your existing script, the benefit
>of having a general gatekeeper script is that if you need to do this
>again you can just pass different arguments in.
>
>--
>To unsubscribe, e-mail: beginners-unsubscribe@perl.org
>For additional commands, e-mail: beginners-help@perl.org
>http://learn.perl.org/
>
>
>
>

Bob McConnell

2007-10-03, 7:01 pm

> -----Original Message-----
> From: Elizabeth Cortell [mailto:zrusilla@mac.com]=20
> Sent: Wednesday, October 03, 2007 4:48 PM
> To: beginners@perl.org
> Subject: Re: Limiting a program to a single running instance
>=20
> Checking for instances by grepping the output of ps tends to=20
> also catch instances of=20
>=20
> vi myscript.pl
>=20
> So that innocently editing the file on that box keeps it from=20
> running! Not what you have in mind, I think.
>=20
> I suggest (untested, but we do something like this in the office):
> Upon startup, check for the existence of a lockfile with=20
> agreed-upon name in some tmp directory. If it exists, exit=20
> immediately; otherwise create the file. Remove the file at=20
> end of execution or upon death of script (in the END block).
>=20
> I haven't thought out all the ways that could go wrong.=20
> Comments welcome.
>=20
>=20
> Cheers,
>=20
> Liz=20

If the process crashes, it won't always remove the file. Put the Process
ID in the file, then the next run can check to see if that process is
still alive and running the same application. If not, remove the file
and continue.

Bob McConnell

Chas. Owens

2007-10-03, 7:01 pm

On 10/3/07, Elizabeth Cortell <zrusilla@mac.com> wrote:
> Checking for instances by grepping the output of ps tends to also catch instances of
>
> vi myscript.pl
>
> So that innocently editing the file on that box keeps it from running! Not what you have in mind, I think.
snip

Which is why you don't grep for the script name. You look for
something like "^/usr/bin/perl myscript.pl$" in the ouput of your
system's equivalent to "ps -axo command".

Mark Wagner

2007-10-03, 7:01 pm

On 10/3/07, Elizabeth Cortell <zrusilla@mac.com> wrote:

> I suggest (untested, but we do something like this in the office):
> Upon startup, check for the existence of a lockfile with agreed-upon name in some tmp directory. If it exists, exit immediately; otherwise create the file. Remove the file at end of execution or upon death of script (in the END block).
>
> I haven't thought out all the ways that could go wrong. Comments welcome.

Will an END block get executed even if the script exits through an
error such as calling an undefined function?

--
Mark Wagner

Chas. Owens

2007-10-03, 7:01 pm

On 10/3/07, Mark Wagner <carnildo@gmail.com> wrote:
> On 10/3/07, Elizabeth Cortell <zrusilla@mac.com> wrote:
>
>
> Will an END block get executed even if the script exits through an
> error such as calling an undefined function?
snip

Yes, Perl always tries to run END blocks, but there are some errors
where END blocks cannot run (exhausted memory for instance). END
cannot run if the machine dies (for obvious reasons), so that is one
corner case you might need to be worried about. In general inspecting
the process list or using a file with the pid of the current process
in it (and if the file exists checking to see if that pid is the same
as the script in question) is the right solution.

Ken Foskey

2007-10-03, 7:01 pm

On Wed, 2007-10-03 at 13:34 -0700, Mark Wagner wrote:
> I've got a Perl program that is run once an hour as a cron job.
> Normally it finishes its task in about ten minutes, but occasionally
> it takes more than an hour to complete. When this happens, a second
> copy is started, the two copies step on each others' toes, and both
> crash.
>
> I'm looking for the second copy to know that another copy is running
> and exit after logging a message to that effect. Any method needs to
> work properly if the first copy crashes rather than exiting cleanly.

http://search.cpan.org/~elizabeth/S...Sys/RunAlone.pm

Another design for this sort of thing is a mutex. There are a few
modules for this. This means you can lock out smaller parts of code and
keep two copies running concurrently doing different work.

--
Ken Foskey
FOSS developer

Mark Wagner

2007-10-03, 7:01 pm

On 10/3/07, Bob McConnell <rvm@cbord.com> wrote:
>
> If the process crashes, it won't always remove the file. Put the Process
> ID in the file, then the next run can check to see if that process is
> still alive and running the same application. If not, remove the file
> and continue.

Thanks. I've set this up, and it seems to be working.

--
Mark Wagner

Jeff Pang

2007-10-04, 3:59 am

2007/10/4, Mark Wagner <carnildo@gmail.com>:

>
> Will an END block get executed even if the script exits through an
> error such as calling an undefined function?
>

If script die by itself,END is executed.
Some kill signals like SIGINT,SIGTERM can let script die and END be
executed,but some won't,like SIGKILL,when input `kill -9 pid` from
terminal,END won't be happened.
This is because kill signals like SIGINT,SIGTERM can be
intercepted,script has the chances to exit gracefully,so END can be
executed.But SIGKILL can't be intercepted,when sending kill
signal,script will be forced to die immediately.

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