[Mod-pubsub-developer] A note on making the Perl PubSub Client library long-lived.

[Adam R. <Ad...@Kn...>]

Offline from this list someone noted that the Perl PubSub Client
library regularly kills its calling process whenever errors occur,
using a call to the Perl "die" function.  (There are 57 calls to
"die" in the library, for those keeping track at home. :)

Sometimes we want an application using this library (such as a daemon)
to run for months on end.  I asked Kragen how to do this and below is
his response.  Kragen, is it ok if we put this on the Wiki?

   Adam



-----Original Message-----
From: Kragen Sitaker
Sent: Monday, January 12, 2004 5:14 PM
To: Adam Rifkin

The Perl "die" function is similar to the "throw" keyword in Java or C++
--- it raises an exception that propagates up the call stack until
somebody handles it explicitly.  If nobody handles it explicitly, Perl
prints an error message and ends the program.  [This person] is
complaining about this default handling, but just as in Java or C++,
it's possible to handle the exception differently.

The Perl construct that corresponds to Java or C++'s "catch" is called
"eval" in Perl; confusingly, it shares its name and provenance with a
similar, but more dangerous, construct that allows your program to =
compile
and run code at runtime, much like the ClassLoader functionality in =
Java.
"perldoc -f eval" can give you more details.

If you want your daemon to live for months, you can do that by wrapping
its main loop like this:
sub daemon_main_loop_routine {
  # ...
  eval {
    $pubsub_client_object->handle_events();
  };
  warn "Pubsub error: $@" if $@;
  # ...
}

Perl's usual perspicuous naming convention reigns here; eval saves the
exception for you in a variable helpfully named $@. =20

I usually observe an exception in the client library code when the event
router closes the connection, so it may be a good idea to close and =
reopen
the connection (recreating all subscriptions) when it raises an =
exception.

Hope this helps.  You may want to forward an edited version of this to
the original sender and/or the mod_pubsub mailing list.

- Kragen