NAME

Event::Lib - Perl extentions for event-based programming

SYNOPSIS

    use Event::Lib;
    use POSIX qw/SIGINT/;
    
    my $seconds;
    sub timer {
        my $event = shift;
        print "\r", ++$seconds;
        $event->add(1);
    }
    
    sub reader {
        my $event = shift;
        my $fh = $event->fh;
        print <$fh>;
        $event->add;
    }

    sub signal {
        my $event = shift;
        print "Caught SIGINT\n";
    }

    my $timer  = timer_new(\&timer);
    my $reader = event_new(\*STDIN, EV_READ, \&reader);
    my $signal = signal_new(SIGINT, \&signal);
        
    $timer->add(1);     # triggered every second
    $reader->add;
    $signal->add;
    
    event_mainloop;

DESCRIPTION

This module is a Perl wrapper around libevent(3) as available from <http://www.monkey.org/~provos/libevent/>. It allows to execute a function whenever a given event on a filehandle happens, a timeout occurs or a signal is received.

Under the hood, one of the available mechanisms for asynchronously dealing with events is used. This could be "select", "poll", "epoll", "devpoll" or "kqueue". The idea is that you don't have to worry about those details and the various interfaces they offer. Event::Lib offers a unified interface to all of them (but see "CONFIGURATION" further below).

Once you've skimmed through the next two sections (or maybe even now), you should have a look at "EXAMPLE: A SIMPLE TCP SERVER" to get a feeling about how it all fits together.

There's also a section briefly mentioning other event modules on the CPAN and how they differ from Event::Lib further below ("OTHER EVENT MODULES").

INITIALIZATION

This happens via loading the module via use() or require():

    use Event::Lib;

No further work is ever required.

Additionally, you may use the following two functions to retrieve some information regarding the underlying libevent. These functions are neither exported nor exportable so you have to call them fully package-qualified:

* Event::Lib::get_version()

This returns the version of libevent this module was compiled against.

* Event::Lib::get_method()

This returns the kernel notification method used by libevent. This will be one of "select", "poll", "epoll", "devpoll" and "kqueue".

EVENTS

The standard procedure is to create a few events and afterwards enter the loop (using event_mainloop()) to wait for and handle the pending events. This loop is truely global and shared even between forked processes. The same is true for events that you register. They will all be processed by the same loop, no matter where or how you create them.

Each event has a Perl function associated with itself that gets triggered when the event is due. Further event handling is delayed until the currently executing event handler is done. If you want an event to be handled as soon as it becomes imminent, it has to run in its own process so that it cannot be disturbed by other event handlers. This is particularly important for timer-based events when you expect those events to fire steadily every few seconds.

There's one more little thing to be aware of: Sometimes it may apear that your events aren't triggered because they produce no output in spite of your precious print() statements you put in. If you see no output, then you're a victim of buffering. The solution is to turn on autoflushing, so put

    $| = 1;

at the top of your program if no output appears on your screen or filehandles.

Event::Lib knows three different kind of events: a filehandle becomes readable/writeable, timeouts and signals.

Watching filehandles

Most often you will have a set of filehandles that you want to watch and handle simultaneously. Think of a webserver handling multiple client requests. Such an event is created with event_new():

* event_new($fh, $flags, $function, [@args])

$fh is the filehandle you want to watch. $flags may be the bit-wise ORing of "EV_READ", "EV_WRITE" and "EV_PERSIST". "EV_PERSIST" will make the event persistent, that is: Once the event is triggered, it is not removed from the event-loop. If you do not pass this flag, you have to re-schedule the event in the event-handler $function.

$function is the callback that is executed when the given event happened. This function is always called with at least two arguments, namely the event object itself which was created by the above event_new() and an integer being the event-type that occured (which could be "EV_WRITE", "EV_READ" or "EV_TIMEOUT"). @args is an optional list of additional arguments your callback will receive.

NOTE: $fh really ought to be a socket or a pipe. Regular files can't be handled by at least epoll(2). If, for some reason, you want to put an event on a regular file, you have to make sure that a kernel notification method is used that can deal with such file-handles. select(2) and poll(2) are good candidates as they don't have this limitation. So in order to prevent this limitation, you have to do:

    BEGIN {
        $ENV{ $_ } = 1 for qw/EVENT_NOEPOLL EVENT_NODEVPOLL EVENT_NOKQUEUE/;
    }
    use Event::Lib;

See "CONFIGURATION" further below for more details.

The function returns an event object (the very object that is later passed to the callback function).

Here's an example how to create a listening socket that can accept connections from multiple clients:

    use IO::Socket::INET;
    use Event::Lib;
    
    sub accept_connection {
        my $event = shift;
        my $sock  = $event->fh;
        my $client = $sock->accept;
        ...
    }
        
    my $server = IO::Socket::INET->new(
        LocalAddr       => 'localhost',
        LocalPort       => 9000,
        Proto           => 'tcp',
        ReuseAddr       => SO_REUSEADDR,
        Listen          => 1,
        Blocking        => 0,
    ) or die $@;

    my $main = event_new($server, EV_READ|EV_PERSIST, \&accept_connection);

    # add the event to the event loop
    $main->add; 

    event_mainloop();

The above can be done without the "EV_PERSIST" flag as well:

    sub accept_connection {
        my $event = shift;
        my $sock = $event->fh;
        my $client = $sock->accept;
        ...
        # re-schedule event
        $event->add;
    }
    ...
    my $main = event_new($server, EV_READ, \&accept_connection);
    $main->add;
    event_mainloop();

* $event->add( [$timeout] )

Alias: event_add( $event, [$timeout] )

This adds the event previously created with event_new() to the event-loop. $timeout is an optional argument specifying a timeout given as floating-point number. It means that the event handler is triggered either when the event happens or when $timeout seconds have passed, whichever comes first.

Consider this snippet:

    use Event::Lib;
    
    sub handler {
        my ($ev, $type) = @_;
        if ($type == EV_READ) {
            ...
        }
        elsif ($type == EV_TIMEOUT) {
            ...
        }
    }
    
    # wait at most for 1.5 seconds
    event_new(\*STDIN, EV_READ, \&handler)->add(1.5);
    event_one_loop;

If "STDIN" becomes readable within 1.5 seconds, handler() will be called with $type set to "EV_READ". If nothing happens within these 1.5 seconds, it'll be called with $type set to "EV_TIMEOUT".

When $timeout is 0 it behaves as if no timeout has been given, that is: An infinite timeout is assumed. Any other timeout is taken literally, so 0.0 is not the same! In such a case, the event handler will be called immediately with the event type set to "EV_TIMEOUT".

It's a fatal error to add the same event multiple times:

    my $e = event_new(...);
    $e->add;
    $e->add;    # this line will die

When an event couldn't be added for some other reason, the event's exception handler is called. See "EXCEPTION HANDLING" further below on how exceptions raised by event_add() differ from other exceptions.

* $event->fh

Returns the filehandle this $event is supposed to watch. You will usually call this in the event-handler.

* $event->remove

This removes an event object from the event-loop. Note that the object itself is not destroyed and freed. It is merely disabled and you can later re-enable it by calling "$event->add".

Timer-based events

Sometimes you want events to happen periodically, regardless of any filehandles. Such events are created with timer_new():

* timer_new( $function, [@args] )

This is very much the same as event_new(), only that it lacks its first two parameters. $function is a reference to a Perl function that should be executed. As always, this function will receive the event object as returned by timer_new() as first argument, the type of event (always EV_TIMEOUT) plus the optional argumentlist @args.

* $event->add( [$timeout] )

Alias: event_add( $event, [$timeout] )

Adds $event to the event-loop. The event is scheduled to be triggered every $timeout seconds where $timeout can be any floating-point value. If $timeout is omitted, a value of one second is assumed.

It will throw an exception if adding the given event failed. If you still want your program to keep running, wrap this statement into an eval block:

    my $e = event_new(...);
    eval {
        $e->add;
    } or warn "Adding failed";

Note that timer-based events are not persistent so you have to call this method/function again in the event-handler in order to re-schedule it.

It's a fatal error to add the same event multiple times:

    my $e = timer_new(...);
    $e->add;
    $e->add;    # this line will die

* $event->remove

This removes the timer-event $event from the event-loop. Again, $event remains intact and may later be re-scheduled with event_add().

Signal-based events

Your program can also respond to signals sent to it by other applications. To handle signals, you create the corresponding event using signal_new().

Note that thusly created events take precedence over event-handlers defined in %SIG. That means the function you assigned to $SIG{ $SIGNAME } will never be executed if a "Event::Lib"-handler for $SIGNAME also exists.

* signal_new( $signal, $function, [@args] )

Sets up $function as a handler for $signal. $signal has to be an integer specifying which signal to intercept and handle. For example, 15 is "SIGTERM" (on most platforms, anyway). You are advised to use the symbolic names as exported by the POSIX module:

    use Event::Lib;
    use POSIX qw/SIGINT/;

    my $signal = signal_new(SIGINT, sub { print "Someone hit ctrl-c" });
    $signal->add;
    event_mainloop();

As always, $function receives the event object as first argument, the event-type (always EV_SIGNAL) as second. @args specifies an option list of values that is to be passed to the handler.

* $event->add( [$timeout] )

Alias: event_add( $event, [$timeout] )

Adds the signal-event previously created with signal_new() to the event-loop. $timeout is an optional argument specifying a timeout given as floating-point number. It means that the event handler is triggered either when the event happens or when $timeout seconds have passed, whichever comes first.

$timeout here has the exact same semantics as with filehandle-based events described further above.

Note that signal-events are always persistent unless $timeout was given. That means that you have to delete the event manually if you want it to happen only once:

    sub sigint {
        my $event = shift;
        print "Someone hit ctrl-c";
        $event->remove;
    }
    
    my $signal = signal_new(SIGINT, \&sigint);
    $signal->add;
    event_mainloop();

Subsequently, a persistent and timeouted signal-handler would read thusly:

    sub sigint {
        my $event = shift;
        print "Someone hit ctrl-c";
        $event->add(2.5);
    }

    my $signal = signal_new(SIGINT, \&sigint);
    $signal->add(2.5);
    event_mainloop();

It's a fatal error to add the same event multiple times:

    my $e = signal_new(...);
    $e->add;
    $e->add;    # this line will die

* $event->remove

The same as their counterparts for filehandle-events, so please see above.

COMMON METHODS

* $event->pending

This will tell you whether $event is still in the event-queue waiting to be processed. More specifically, it returns a false value if $event was already handled (and was not either persistent or re-scheduled). In case $event is still in the queue it returns the amount of seconds as a floating-point number until it is triggered again. If $event has no attached timeout, it returns "0 but true".

* $event->args( [@args] )

When called with no arguments, it will in scalar context return the number of additional arguments associated with $event. In list context, it returns those arguments as one list.

When @args is given, the current list of arguments for $event is replaced with @args and nothing is returned.

* $event->args_del

This will remove all additional arguments from $event so the next time the event handler is called, the list of additional arguments passed to it will be empty.

* $event->callback

Returns the callback associated with this event as code-reference so that you can call it manually in case you think you need that:

    $event->callback->($event, $event->fh, @args);

* $event->except_handler( $function )

You can associate an exception handler with each event which gets called in case the callback for that event dies. $function is a Perl code-reference which will - when called - receive the event as first argument, the error message with which the event handler died, the type of event and any additional arguments associated with that event. That way you can inspect the circumstances and provide your own error-handling.

Please see "EXCEPTION HANDLING" for some background and more details.

ENTERING THE EVENT-LOOP

Event::Lib offers three functions to process pending events.

* event_mainloop ()

This function will start the event-loop and never return, generally. More precisely, it will return if either the program ran out of events in which case event_mainloop() returns a true value. In case of an error during event-processing, it will return a false value in which case you should check $!.

IMPORTANT: When any of your events register new events they will be added to the global queue of events and be handled in the same loop. You are therefore not allowed to call event_mainloop() more than once in your program. Attempting to do so will yield a warning and the operation is silently turned into a no-op.

* event_one_loop( [$timeout] )

This function will do exactly one loop which means the next pending event is handled. In case no event is currently ready for processing, it will block and wait until one becomes processible.

If $timeout is specified, it will wait at most $timeout seconds and then return.

* event_one_nbloop ()

This is the non-blocking counterpart to event_one_loop(): It returns immediately when no event is ready to be processed. Otherwise the next imminent event is handled.

You want to use either event_one_loop() or event_one_nbloop() instead of event_mainloop() if you want to write your own event-loop. The core of such a program could look like this:

    event_new(...)->add;
    event_new(...)->add;
    timer_new(...)->add;
    ...
    
    while () {
        event_one_nbloop();
        ...
        select undef, undef, undef, 0.05;   # sleep for 0.05 seconds
    }

EVENT LIFECYCLE

It is important to understand the lifetime of events because concepts such as scope and visibility have little meaning with respect to events.

When you add an event to the queue using event_add(), this event will remain there until it is triggered, no matter what you do with the object returned by event_new(), timer_new() and signal_new() respectively. Consider this code:

    use Event::Lib;
    $| = 1;
    
    my $event = timer_new(sub { print "timer called\n" });
    
    # schedule the timer to go off in ten seconds
    $event->add(10);
    
    undef $event;
    event_mainloop;

This program will, regardless of the "undef $event", print "timer called". As a consequence, there is only one true and correct way to cancel an event, namely by calling remove() on it. Likewise:

    use warnings;
    use Event::Lib;
    $| = 1;
    
    my $event = timer_new(sub { print "Called after two seconds\n" });
    $event->add(2);
    $event = timer_new(sub { print "Called after three seconds\n" });
    $event->add(3);

    event_mainloop;
    
    __END__
    Explicit undef() of or reassignment to pending event at - line 8.
    Called after two seconds
    Called after three seconds

So even though you have only one Perl object container $event, you have two events!

As this can become hard to maintain in complex programs, Event::Lib will emmit a warning if any of the above cases is detected and if you have warnings enabled. If you don't want this warning turn it off temporarily. The above program then becomes:

    use warnings;
    use Event::Lib;
    $| = 1;
    
    my $event = timer_new(sub { print "Called after two seconds\n" });
    $event->add(2);
    
    {
        no warnings 'misc';
        $event = timer_new(sub { print "Called after three seconds\n" });
        $event->add(3);
    }

    event_mainloop;

Note that the line following the undef() or the reassignment has to be within the "no warnings 'misc'"-block because this is the line where this warning is actually triggered and not the line with the undef() or reassignment itself.

EXCEPTION HANDLING

Some programs simply cannot afford to die. It is a possible that a callback is triggered and finds itself in a situation where it just cannot proceed. Think of a callback that is supposed to append to a file and in the meantime the disk has filled so that no space is left on the device.

It is now possible to provide exception handlers for these cases. The idea is that these exception handlers are called with the same arguments the callback was previously triggered with (plus the error message as second argument) which gives you the change to further investigate the cause of the failure and possibly take counter-measures.

You can register exception handlers per event using the except_handler() method. Furthermore, you can register one global exception handler that is going to be used for all events that don't have their own handler:

* event_register_except_handler( $function )

$function is a code-reference that will be called whenever the callback of an event dies:

    use Event::Lib;
    
    sub handler {
        my ($event, $exception, $type, @args) = @_;
        # error handling here
        ...
    }

    event_register_except_handler(\&handler);
    ...

If you don't call event_register_except_handler() Event::Lib will use its own basic default handler. This handler simply dies with the original error message.

Exceptions raised by event_add()

If the exception was raised by event_add(), then the event's exception handler is called. This is either the one registered with except_handler() on a per-event basis, the global one set via event_register_except_handler() or, if both of these was not done, the default handler.

In any case, the exception handler called from event_add() is called with slightly different arguments. This is in order to allow the handler to distinguish between the case where an exception was raised by an event-handler or where it was raised by event_add().

The first two arguments being the event in question and the error message are the same for both kind of exceptions. What differs is the third argument, $type. It will always be negative when event_add() triggered this exception.

In particular, the type of event $type will be for a...

... filehandle-event:

The negated type-flags with which the event was created. This means that for the following exception-handler and when "$e->add" failed:

    sub exception_handler {
        my ($e, $err, $evtype, @args) = @_;
        
        # ref($e) eq "Event::Lib::event"
        # $err =~ /^Couldn't add event at/
        # $evtype == -(EV_READ|EV_PERSIST)
        # @args == (1, 2, 3)
        # $! will contain the OS-level error
    }
        
    my $e = event_new(\*FH, EV_READ|EV_PERSIST, \&handler, 1 .. 3);
    ...
    $e->add;

... timer-event:

$type will be -EV_TIMEOUT:

    sub exception_handler {
        my ($e, $err, $evtype, @args) = @_;
        
        # ref($e) eq "Event::Lib::timer"
        # $err =~ /^Couldn't add event at/
        # $evtype == -EV_TIMEOUT
        # @args == (1, 2, 3)
    }
        
    my $e = timer_new(\&handler, 1 .. 3);
    ...
    $e->add;

... signal-event:

$type will be the negated signal number this event was supposed to handle:

    sub exception_handler {
        my ($e, $err, $evtype, @args);
        
        # ref($e) eq "Event::Lib::signal"
        # $err =~ /^Couldn't add event at/
        # $evtype == -SIGTERM
        # @args == (1, 2, 3)
    }
        
    my $e = signal_new(SIGTERM, \&handler, 1 .. 3);
    ...
    $e->add;

As a consequence, first have your exception-handler test the sign of $evtype. If it was negative, use "ref($e)" to extract the kind of event.

PRIORITIES

Events can be assigned a priority. The lower its assigned priority is, the earlier this event is processed. Using prioritized events in your programs requires two steps. The first one is to set the number of available priorities. Setting those should happen once in your script and before calling event_mainloop():

* event_priority_init( $priorities )

Sets the number of different events to $priorities.

Assigning a priority to each event then happens thusly:

* $event->set_priority( $priority )

Gives $event (which can be any of the three type of events) the priority $priority. Remember that a lower priority means the event is processed earlier!

Note: If your installed version of libevent does not yet contain priorities which happens for pre-1.0 versions, the above will become no-ops. Other than that, your scripts will remain functional.

FUNCTIONS FOR DEBUGGING, TRACING ET AL.

There are some functions that will aid you in finding problems in your program or even to assure you that your program is ok but there might be a bug in Event::Lib.

* event_log_level( $loglevel )

You can specify what kind of messages Event::Lib should dump to stderr by using thid function.

$loglevel is one of _EVENT_LOG_DEBUG, _EVENT_LOG_MSG, _EVENT_LOG_WARN, _EVENT_LOG_ERR and _EVENT_LOG_NONE and will instruct Event::Lib to only output messages of at least that severity. "_EVENT_LOG_NONE" will suppress any messages. Not calling this function is equivalent to doing

    event_log_level( _EVENT_LOG_ERR );

* $event->trace

This turns on tracing for $event. Tracing means that diagnostic messages are written to STDERR whenever something happens to this $event. This includes implicit action such as the destruction of an event or explicit things like calling add() or remove() or other methods on $event.

Returns $event so that you can easily plug it into your code:

    event_new(...)->trace->add;

Once an event is traced, there is as of now no way to untrace it again.

* Event::Lib::Debug::get_pending_events()

This function is only available when you built Event::Lib with "DEFINE=-DEVENT_LIB_DEBUG" (as an argument to perl Makefile.PL). Additionally, you have to run your program with the environment variable "EVENT_LIB_DEBUG_PENDING" set in order to get any output from this function. The environment has to be set before "use Event::Lib;":

    BEGIN {
        $ENV{ EVENT_LIB_DEBUG_PENDING } = 1;
    }

    use Event::Lib;

or by setting it in your shell. For the bash, this looks like:

    $ EVENT_LIB_DEBUG_PENDING=1 perl event_script.pl

This function will return a list of all currently still pending events. Each element of this list is a reference to an array, where the first element is the event object, the second the type of event ("EV_TIMEOUT", "EV_SIGNAL", "EV_READ" etc.) and the remaining elements the additional arguments this event was constructed with.

* Event::Lib::Debug::dump_pending_events()

Similar to the above, only that it will dump all currently pending events to STDERR with some additional information that might be of interest.

Again, this is only available when the module was build with "-DEVENT_LIB_DEBUG" and with the environment variable "EVENT_LIB_DEBUG_PENDING" set.

CONFIGURATION

Event::Lib can be told which kernel notification method not to use. This happens via the use of environment variables (there is no other way due to libevent). They have to be set in a BEGIN-block before you use() Event::Lib:

    BEGIN {
        $ENV{ $_ } = 1 for qw/EVENT_NOPOLL EVENT_NOEPOLL/;
    }

    use Event::Lib;

This will disable "poll" and "epoll" so it will use one of the remaining methods, which could be either "select", "devpoll" or "kqueue".

The variables that you may set are the following:

EVENT_NOPOLL
EVENT_NOSELECT
EVENT_NOEPOLL
EVENT_NODEVPOLL
EVENT_NOKQUEUE

If you set all of the above variables, it is a fatal error and you'll receive the message "event_init: no event mechanism available". There is one other variable available:

•

EVENT_LOG_LEVEL

This is the environment-variable version of set_log_level() intended to conveniently run your script more verbosely for debugging purpose. The lower this value is, the more informational output libevent produces on STDERR. "EVENT_LOG_LEVEL=0" means maximum debugging output whereas "EVENT_LOG_LEVEL=4" means no output at all:

    $ EVENT_LOG_LEVEL=0 perl your_script.pl

EXAMPLE: A SIMPLE TCP SERVER

Here's a reasonably complete example how to use this library to create a simple TCP server serving many clients at once. It makes use of all three kinds of events:

    use POSIX qw/SIGHUP/;
    use IO::Socket::INET;
    use Event::Lib;

    $| = 1;

    # Invoked when a new client connects to us
    sub handle_incoming {
        my $e = shift;
        my $h = $e->fh;
        
        my $client = $h->accept or die "Should not happen";
        $client->blocking(0);

        # set up a new event that watches the client socket
        my $event = event_new($client, EV_READ|EV_PERSIST, \&handle_client);
        $event->add;
    }

    # Invoked when the client's socket becomes readable
    sub handle_client {
        my $e = shift;
        my $h = $e->fh;
        printf "Handling %s:%s\n", $h->peerhost, $h->peerport;
        while (<$h>) {
            print "\t$_";
            if (/^quit$/) {
                # this client says goodbye
                close $h;
                $e->remove;
                last;
            }
        }
    }   
        
    # This just prints the number of
    # seconds elapsed
    my $secs;
    sub show_time {
        my $e = shift;
        print "\r", $secs++;
        $e->add;
    }

    # Do something when receiving SIGHUP
    sub sighup {
        my $e = shift;
        # a common thing to do would be
        # re-reading a config-file or so
        ...
    }

    # Create a listening socket
    my $server = IO::Socket::INET->new(
        LocalAddr   => 'localhost',
        LocalPort   => 9000,
        Proto       => 'tcp',
        ReuseAddr   => SO_REUSEADDR,
        Listen      => 1,
        Blocking    => 0,
    ) or die $@;

    my $main  = event_new($server, EV_READ|EV_PERSIST, \&handle_incoming);
    my $timer = timer_new(\&show_time);
    my $hup   = signal_new(SIGHUP, \&sighup);
   
    $_->add for $main, $timer, $hup;

    event_mainloop;

    __END__

You can test the above server with this little program of which you can start a few several simultaneous instances:

    use IO::Socket::INET;

    my $server = IO::Socket::INET->new( 
        Proto       => 'tcp',
        PeerAddr    => 'localhost',
        PeerPort    => 9000,
    ) or die $@;

    print $server "HI!\n";
    sleep 10;
    print $server "quit\n";

    __END__

OTHER EVENT MODULES

There are already a handful of similar modules on the CPAN. The two most prominent ones are Event and the venerable POE framework.

Event

In its functionality it's quite close to Event::Lib with some additional features not present in this module (you can watch variables, for example). Interface-wise, it's quite a bit heavier while Event::Lib gets away with just a handful of functions and methods. On the other hand, it has been around for years and so you may expect Event to be rock-stable.

The one main advantage of Event::Lib appears to be in its innards. The underlying libevent is capable of employing not just the "poll" and "select" notification mechanisms but also other and possibly better performing ones such as "kqueue", "devpoll" and "epoll" where available.

POE

POE is definitely more than the above. It's really a threading environment in disguise. Purely event-based techniques have limitations, most notably that an event-handler blocks all other pending events until it is done with its work. It's therefore not possible to write a parallel link-checker only with Event or Event::Lib. You still need threads or fork(2) for that.

That's where POE enters the scene. It is truely capable of running jobs in parallel. Such jobs are usually encapsulated in "POE::Component" objects of which already quite a few premade ones exist on the CPAN.

This power comes at a price. POE has a somewhat steep learning-curve and forces you to think in POE concepts. For medium- and large-sized applications, this doesn't have to be a bad thing. Once grokked, it's easy to add more components to your project, so it's almost infinitely extensible.

Stem

Stem is a very close rival to POE and they are nose-to-nose when it comes to features. However, Stem's design is a lot easier to understand and to adapt to your need, mostly because it doesn't come up with its own methodology and terminology. It is very well thought out without being over-designed.

It's easy and straight-forward to do simple event-looping (it currently comes with its own well-conceived event loop; additionally it can make use of Event when available). So called Stem cells can be easily plugged together to build big applications where these cells can run in parallel, both in an asynchronous or synchronized fashion.

It's main drawback (as of now) is its lack of documentation. However, It's been written in a clean way so its source can often serve as a drop-in replacement for the lack of documentation.

Conclusion

Use the right tools for your job. Event::Lib and Event are good for writing servers that serve many clients at once, or in general: Anything that requires you to watch resources and do some work when something interesting happens with those resources. Once the work needed to be carried out per event gets too complex, you may still use "fork".

Or you use Stem or POE. You get the watching and notifying capabilities alright, but also the power to do things in parallel without creating threads or child processes manually.

EXPORT

This modules exports by default the following functions:

    event_init
    event_log_level
    event_priority_init
    event_register_except_handler
    event_fork
    
    event_new
    timer_new
    signal_new

    event_add
    
    event_mainloop
    event_one_loop
    event_one_nbloop

plus the following constants:

    EV_PERSIST
    EV_READ
    EV_SIGNAL
    EV_TIMEOUT
    EV_WRITE
    _EVENT_LOG_DEBUG
    _EVENT_LOG_MSG
    _EVENT_LOG_WARN
    _EVENT_LOG_ERR
    _EVENT_LOG_NONE

BUGS

This library is not thread-safe.

The module has turned out to be quite stable under stress-situations handling many thousands simultaneous connections with a very decent performance which it owes to the underlying libevent. However, event-based applications can reach a stupendous complexity and it is not possible to foresee every kind of conceivable scenario.

If you therefore find a bug (a crash, a memory leak, inconsistencies or omissions in this documentation, or just about anything else), don't hesitate to contact me. See "AUTHOR" further below for details.

TO-DO

Thread-safety is high on the list. Recent libevent has thread-support which will make this fairly easy.

Not all of libevent's public interface is implemented. The buffered events are still missing. They will be added once I grok what they are for.

THANKS

This module wouldn't be in its current state without the patient and professional help of MailChannels Corporation (http://www.mailchannels.com). Over the course of five months, Stas Bekman, Ken Simpson and Mike Smith exchanged hundreds of emails with me, pointing out the many glitches that were in the module and coming up with test-cases that made it possible for me to fix all these issues.

VERSION

This is version 1.03.

AUTHOR

Tassilo von Parseval, <tassilo.von.parseval@rwth-aachen.de>

COPYRIGHT AND LICENSE

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself, either Perl version 5.8.4 or, at your option, any later version of Perl 5 you may have available.