Event(3) User Contributed Perl Documentation Event(3)NAMEEvent - Event loop processing
SYNOPSIS
use Event qw(loop unloop);
# initialize application
Event->flavor(attribute => value, ...);
my $ret = loop();
# and some callback will call
unloop('ok');
DESCRIPTION
ALERT: Marc Lehmann may have taken over the future of event loops in
Perl. Check out his libev library and EV Perl module. 25 Aug 2009
The Event module provide a central facility to watch for various types
of events and invoke a callback when these events occur. The idea is
to delay the handling of events so that they may be dispatched in
priority order when it is safe for callbacks to execute.
Events (in the ordinary sense of the word) are detected by watchers,
which reify them as events (in the special Event module sense). For
clarity, the former type of events may be called "source events", and
the latter "target events". Source events, such as signals arriving,
happen whether or not they are being watched. If a source event occurs
which a watcher is actively watching then the watcher generates a
corresponding target event. Target events are only created by
watchers. If several watchers are interested in the same source event
then each will generate their own target event. Hence, any particular
source event may result in zero, one, two, or any number of target
events: the same as the number of watchers which were actively watching
for it.
Target events are queued to be processed in priority order (priority
being determined by the creating watcher) and in FIFO order among
events of the same priority. Queued ("pending") events can, in some
cases, be cancelled before being processed. A queued event is
processed by being passed to the callback function (or method on a
particular object or class) which was specified to the watcher.
A watcher, once created, operates autonomously without the Event user
having to retain any reference to it. However, keeping a reference
makes it possible to modify most of the watcher's characteristics. A
watcher can be switched between active and inactive states. When
inactive, it does not generate target events.
Some types of source event are not reified as target events
immediately. Signals received, for example, are counted initially. The
counted signals are reified at certain execution points. Hence, signal
events may be processed out of order, and if handled carelessly, on the
wrong side of a state change in event handling. A useful way to view
this is that occurrence of the source event is not actually the arrival
of the signal but is triggered by the counting of the signal.
Reification can be forced when necessary. The schedule on which some
other events are created is non-obvious. This is especially the case
with watchers that watch for a condition rather than an event. In some
cases, target events are generated on a schedule that depends on the
operation of the event loop.
PERL API
Events (the occurrence of such) are noticed and queued by 'event
watchers'. The creation and configuration of event watchers is the
primary topic of the rest of this document.
The following functions control or interrogate the event loop as a
whole:
$result = loop([$timeout])
Will enter a loop that calls one_event() until unloop() is called.
The argument passed to unloop() is the return value of loop().
Loops can be nested.
unloop($result)
Make the inner-most loop() return with $result.
unloop_all($result)
Cause all pending loop()s to return immediately. This is not
implemented with "die". It is works as if "unloop($result)" were
called for all nested loops.
sweep([$max_prio])
Queue all pending events and dispatch any with priority strictly
less than $max_prio (the highest priority is 0). The default is to
process all events except idle events. (While idle events are
ignored by sweep, idle watchers are not ignored. If you want to
avoid triggering an idle watcher then set "max" to "undef" or
"stop()" it.)
one_event([$timeout])
If any events are outstanding then invoke the corresponding
callback of the highest priority event. If there are no events
available, block forever or until $timeout. Use of this API is not
recommended because it is not efficient and does not trap
exceptions. However, you might wish to understand how it works:
1. Queue asyncronous events (signals, etc). That is, previously
recorded events are reified.
2. If there are any events with priority 5 or less (see
StarvePrio) then service the next one and return.
3. Calculate the maximum wait time (minimum time till the next
timer expiration) and pass control to the poll/select system
call. Upon return, queue all pending events.
4. Queue asyncronous events again.
5. If there are any events then service the next one and return.
6. Service the next idle watcher.
StarvePrio is the priority level for which events are dispatched
during step 2. It cannot be changed without a recompile. In the
rare case that an event is always pending at step 2 then I/O
watchers will starve. However, this is highly unlikely since async
watchers should never queue events so rapidly.
all_watchers()
Returns a list of all watchers (including stopped watchers).
all_running()
Returns a list of all watchers with actively running callbacks.
Watchers are returned in order of most recent to least recent.
all_idle()
Returns a list of all the idle watchers. If the event queue is
very busy, all the idle watchers will sit on the idle queue waiting
to run. However, be aware that if an idle watcher has the "max"
attribute set then it will queue a normal event when its "max" wait
time is exceeded.
queue_pending()
Examines asynchronous source events (timers & signals) and reifies
them as target events. "queue_pending()" is only called implicitly
by "sweep()" and "one_event()". Otherwise, "queue_pending()" is
not called implicitly.
NOTE: Signal watchers generate target events according to which
watchers are active at the time that "queue_pending()" is called
rather than according to the time the signal is received. This is
best explained by example. See the file "demo/queue_pending.t".
Event Watcher Constructors
All watchers are constructed in one of the following ways:
$w = Event->flavor( [attr1 => $value,]... );
$w = Event::flavor($Class, [attr1 => $value,]...);
$w = Event::flavor->new([attr1 => $value,]...);
Where flavor is substituted with the kind of watcher. Built-in types
include idle, io, signal, timer, and var.
New watchers (hopefully) have reasonable defaults and can also be
customized by passing extra attributes to the constructor. When
created, watcher objects are "started" and are waiting for events (see
"$event->start" below).
NetServer::Portal can display watchers in real-time, formatted
similarly to the popular "top" program. You may find this a useful
aide for debugging.
Shared Watcher Attributes
Watchers are configured with attributes (also known as properties).
For example:
$watcher->cb(\&some_code); # set callback
warn $event->w->desc.": ".$event->hits." events happened; Wow!";
All watchers support the following attributes: cb, cbtime, debug, desc,
prio, max_cb_tm, reentrant, and repeat. Watcher constructors accept
the preceding and additionally: async and nice. Moreover, watchers
also offer extra attributes according to their specialty.
Shared Watcher Methods
The following methods are available for all watchers:
$watcher->start
Activate the watcher. Watchers refuse to "start()" without
sufficient configuration information to generate events.
Constructors always invoke "start()" unless the "parked=>1" option
is requested. You will need to set the parked option if you
preallocate unconfigured watchers.
Note: If there are any unreified asynchronous events that are of
interest to the watcher, it will see these events even though they
happened before it was started. This affects signal watchers, but
there will only be existing unreified signal events if Event was
already handling the signal, which it would only do if there were
another active watcher for the same signal. If this situation
might occur, and it would be a problem for the new watcher to see
older events, call "queue_pending()" immediately before starting
the new watcher in order to reify any outstanding events. This
explaination may be more clear if read along with
"demo/queue_pending.t".
$watcher->again
This is the same as the "start" except if a watcher has special
repeat behavior. For example, repeating timers recalculate their
alarm time using the "interval" parameter.
$watcher->now
Cause the watcher to generate an event, even if it is stopped. The
callback may or may not run immediately depending upon the event's
priority. If you must unconditionally invoke the callback,
consider something like
$w->cb->($w);
$watcher->stop
Don't look for events any more. Running events are allowed to
complete but pending events are cancelled. Note that a stopped
watcher can be reactivated by calling the "start" or "again"
methods.
Watchers are stopped implicitly if their new configuration deprives
them of the ability to generate events. For instance:
my $io_watcher = Event->io(timeout => 1); # started
$io_watcher->timeout(undef); # stopped implicitly
$io_watcher->timeout(1); # still stopped
$io_watcher->start; # restarted
$watcher->cancel
Stop and destroy $watcher. Running events are allowed to complete
but pending events are cancelled. Cancelled watchers are no longer
valid except for read-only operations. For example, prio() can
return the watcher's priority, but start() will fail.
$watcher->is_cancelled
Reports whether the $watcher has been cancelled.
$watcher->is_active
Reports whether the $watcher has been started. The return value is
not affected by suspend.
$watcher->is_running
Zero if the callback is not running. Otherwise, the number of
levels that the callback has been entered. This can be greater
than one if a "reentrant" callback invokes "loop" (or "sweep", with
lesser probability).
$watcher->is_suspended
Reports whether the $watcher is suspended. Suspension is a
debugging feature; see the discussion of the "suspend" attribute
below.
$watcher->pending
In scalar context, returns a boolean indicating whether this
watcher has any events pending in the event queue. In array
context, returns a list of all the watcher's pending events.
Note that this does not check for unreified asynchronous events.
Call "queue_pending()" first if you want to see signals received
since the last operation of the event loop.
Watcher Types
idle
Extra attributes: min => $seconds, max => $seconds
Watches for the Event system to be idle, i.e., to have no events
pending. If the system is never idle, an event will be generated
at least every "max" seconds. While Event is idle, events will be
generated not more often than "min" seconds.
If neither "min" nor "max" is specified, the watcher defaults to
one-shot behaviour ("repeat" false), otherwise it defaults to
repeating. In either case, the default can be overidden by
specifying a "repeat" attribute. "min" defaults to 0.01, and "max"
defaults to infinity.
var Extra attributes: var => \$var, poll => 'rw'
Var watchers generate events when the given variable is read from
or written to, as specified by "poll". "poll" defaults to "w".
As perl is a concise language, it is often difficult to predict
when a variable will be read. For this reason, variable watchers
should poll only for writes unless you know what you are doing.
timer
Extra attributes: at => $time, after => $sec, interval => $sec,
hard => $bool
Generate events at particular times. The $time and $sec are in
seconds. Fractional seconds may be used if Time::HiRes is
available. "at" and "after" are mutually exclusive.
"at" or "after" specify the initial time that the event will
trigger. Subsequent timer events occur at intervals specified by
"interval" or "after" (in that order of preference) if either was
supplied. The timer defaults to one-shot behaviour if "interval"
was not specified, or repeating behaviour if "interval" was
specified; in either case this can be overridden by providing
"repeat" explicitly.
You need to know the time at the start of today if you are trying
to set timers to trigger at day relative times. You can find it
with:
use Time::Local;
my $TodaySeconds = int timelocal(0,0,0,(localtime)[3,4,5]);
This calculation may seem a little heavy weight. If you want to
use UTC rather than local time then you can use this instead:
my $TodaySeconds = time - time % 86400;
Beware that, due to lags in the event loop, the "interval" timeout
may already be in the past. If the "hard" flag is set, the event
will be queued for execution relative to the last time the callback
was invoked. However, if "hard" is false the new timeout will be
calculated relative to the current time. "hard" defaults to false.
io Extra attributes: fd => $fd, poll => 'rwe' [timeout => $seconds,
hard => $bool, timeout_cb => \&code]
The callback is invoked when the file descriptor, "fd", has data to
be read, written, or pending exceptions. "fd" can be a GLOB, an
IO::Handle object, or a file number (file descriptor). "poll"
defaults to "r".
Note that it is your option whether to have multiple watchers per
file handle or to use a single watcher for all event conditions.
If "timeout" is set, events are also generated regularly if no
actual I/O event occurs. If "timeout_cb" is set then timeouts use
this alternate callback instead of the main callback.
signal
Extra attribute: signal => $str
Generates events based on signal arrival. The events are not
actually generated immediately when the signal arrives: signals
received are counted and reified by "queue_pending()" or implicitly
by "one_event()". Several signals of the same type may be merged
into a single event. In such cases, the number of signals
represented by a single event is stored in the "hits" attribute.
PRIORITY
Priority is used to sort the event queue. Meaningful priorities range
from -1 to 6 inclusive. Lower numbers mean higher priority (-1 is the
highest priority and 6 is the lowest). If multiple events get queued,
the ones with the highest priority are serviced first. Events with
equal priority are serviced in first-in-first-out order.
use Event qw(PRIO_HIGH PRIO_NORMAL); # some constants
LEVELS: -1 0 1 2 3 4 5 6
----------------------+-------------+---------------
PRIO_HIGH PRIO_NORMAL
A negative priority causes the callback to be invoked immediately upon
event occurrence. Use this with caution. While it may seem
advantageous to use negative priorities, they bypass the whole point of
having an event queue.
Each watcher has a default priority, assigned by its constructor:
io PRIO_NORMAL
signal PRIO_HIGH
timer PRIO_NORMAL
var PRIO_NORMAL
Default priorities are stored in ${"Event::${type}::DefaultPriority"}.
If the default priority is not satisfactory for your purposes, the
constructor options "nice", "async", or "prio" can be used to adjust
it. "nice" specifies an offset from the default priority; "async"
forces the priority to -1; and "prio" assigns a given priority of your
choice. If more than one of these options are given then "prio"
overrides "async" overrides "nice".
WATCHER CONSTRUCTOR ATTRIBUTES
These options are only supported as constructor arguments.
after => $seconds
See the discussion of the timer watcher.
async => $bool
If $bool then the watcher priority is set to -1.
nice => $offset
Offset from the default priority.
parked => $yes
By default, watcher constructors automatically invoke the "start()"
method. If you don't want the watcher started then request
"parked=>1".
WATCHER ATTRIBUTES
at => $time
The expiration time in the same units as the system clock. For a
timer, "at" will usually be in the future.
cb => \&code
cb => [$class_or_object, $method_name]
The function or method to call when an event is dispatched. The
callback is invoked with $event as its only argument.
Perhaps you are wondering what happens if something goes wrong and
an untrapped "die" occurs within your callback? $Event::DIED is
just for this purpose. See the full description of "DIED" below.
cbtime => $time
When the callback was invoked most recently.
data => $anything
The "data()" method associates arbitrary data with a watcher.
This method is not intended for implementers of watchers. If you
are subclassing or implementing a watcher, consider the "private()"
method.
debug => $bool
Debugging can be activated globally or per watcher. When debugging
is enabled for a particular watcher, $Event::DebugLevel is treated
as two levels higher. Levels of 1, 2, 3, or 4 give progressively
more diagnostics on STDERR.
desc => $string
An identifying name. If this is not passed explicitly to the
constructor, it will be initialized with a string that attempts to
identify the location in the source code where the watcher was
constructed.
fd => $filehandle
This attribute can accept either a perl-esque filehandle or a
system call derived file descriptor number.
hard => $bool
Determines how repeating timers (or timeouts) are recalculated.
The timer is restarted either before or after the callback
depending on whether it is true or false, respectively. In long-
running callbacks this can make a significant difference.
interval => $seconds
How long between repeating timeouts. The "at" attribute is
recalculated using "interval" upon callback return.
max => $seconds
The maximum number of seconds to wait before triggering the
callback. Similar to a "timeout".
max_cb_tm => $seconds
The maximum number of seconds to spend in a callback. If a
callback uses more time then it is aborted. Defaults to 1 sec.
This feature is normally disabled. See Event::Stats.
min => $seconds
Enforce a minimum number of seconds between triggering events.
poll => $bits
Determines which kinds of events are of interest. This attribute
can be set with either strings or bit constants. The bit constants
are available via 'use Event::Watcher qw(R W E T);'.
string constant description
------ -------- ---------------
'r' R read
'w' W write
'e' E exception
't' T timeout
Thus, both of these statements enable interest in read:
$w->poll($w->poll . 'r');
$w->poll($w->poll | R);
A given type of watcher may support all or a subset of the
available events.
prio => $level
Changes the watcher's priority to the given level. Events
generated by a watcher usually inherit the priority of the watcher.
private => $anything
Use the "private()" method to associate arbitrary data with a
watcher. This method is intended for implementers of watchers or
watcher subclasses. Each caller's package accesses its own private
attribute.
reentrant => $bool
By default, callbacks are allowed to invoke "sweep" or "loop" which
in turn may invoke the same callback again recursively. This can
be useful but can also be confusing. Moreover, if you keep
reentering callbacks you will quickly run out of stack space.
Disable this feature per watcher by setting reentrant to false.
This will cause the watcher to be suspended during recursive calls
to "sweep" or "loop".
repeat => $bool
The repeat flag controls whether the callback should either be one-
shot or continue waiting for new events. The default setting
depends on the type of watcher. io, signal, and var default to
true.
signal => $str
The callback is invoked after the specified signal is received.
The $str string should be something like 'INT' or 'QUIT'. Also see
the documentation for %SIG.
A given signal can be handled by %SIG or Event, but not both at the
same time. Event handles the signal as long as there is at least
one active watcher. If all watchers for the signal are cancelled or
stopped then Event sets the signal handler to SIG_DFL.
suspend => $bool
Stop looking for events. Running events are allowed to complete,
but queued events are cancelled.
Suspend is for debugging. If you suspend all watchers in an
application then you can examine the complete state unchanged for
as long as you like without worrying about timer expirations. If
you actually wish to stop a watcher then use the "stop()" method.
timeout => $seconds
The number of seconds before a watcher times out.
timeout_cb => \&code
timeout_cb => [$class_or_object, $method_name]
This is an optional attribute for use when it is desired that
timeouts be serviced in a separate code path than normal events.
When this attribute is unset, timeouts are serviced by "cb".
var => $ref
A reference to the variable being watched.
EVENT ATTRIBUTES
got => $bits
"got" is available in the callback of watchers with "poll". "got"
is in the same format as "poll" except that it gives what kind of
event actually happened. In contrast, "poll" is just an indication
of interest.
hits => $int
Signals in quick succession can be clumped into a single event.
The number of signals clumped together is indicated by this
attribute. This is always one for event types which don't clump.
prio => $level
Be aware that this priority can differ from the watcher's priority.
For instance, the watcher's priority may have changed since the
event was generated. Moreover, the C extension API offers the
freedom to queue events of arbitrary priority.
w => $watcher
This method return the event's watcher. It is read-only.
Customization and Exceptions
· $Event::DebugLevel
Enables progressively more debugging output. Meaningful levels
range from 1 (least output) to 5 (most output). Also see "debug".
· $Event::DIED
When "loop" or "sweep" is called, an exception context is
established for the duration of event processing. If an exception
is detected then $Event::DIED is invoked. The default hook uses
"warn" to output the exception. After the DIED handler completes,
event processing continues as if nothing happened.
If you'd like more detailed output you can install the verbose
handler:
$Event::DIED = \&Event::verbose_exception_handler;
Or you can write your own. The handler is invoked like this:
$Event::DIED->($event, $@);
If you do not want to continue looping after an error, you can do
something like this:
$Event::DIED = sub {
Event::verbose_exception_handler(@_);
Event::unloop_all();
};
· Event->add_hooks(key => sub { ... }, ...);
The bulk of Event's implementation is in C for maximum performance.
The "add_hooks" method allows insertion of perl code at key points
in the optimized event processing core. While flexible, this can
hurt performance *significantly*. If you want customization *and*
performance, please see the C API.
Currently support hooks are detailed as follows:
hook purpose
------------- ----------------------------------------------
prepare returns minimum time to block (timeable)
check assess state after normal return from select/poll
asynccheck check for signals, etc
callback invoked before each event callback
C APIEvent also has a direct API for callbacks written exclusively in C.
See Event::MakeMaker.
WHAT ABOUT THREADS?
Event loops and threads are two different solutions to the same
problem: asynchronous processing. Event loops have been around since
the beginning of computing. They are well understood and proven to be
a good solution for many applications.
While event loops make use of basic operating system services, the bulk
of their implementation is usually outside the kernel. While an event
loop may appear to do many things in parallel, it does not, even on
multiprocessor hardware. Actions are always dispatched sequentially.
This implies that long running callbacks must be avoided because
otherwise event processing is halted.
Event loops work well when actions are short and to the point. Long-
running tasks must be broken into short steps and scheduled for
execution. Some sort of a state machine is usually required. While a
big, complex application server is usually simpler to implement in a
multithreaded fashion, a web browser can easily get by without threads.
Consider a JPEG file download and render. When some new bytes are
available they are sorted to the right place on the screen. Only a
little state must be kept to keep track of how much has been rendered
and to process subsequent incoming bytes.
Threads can either substitute for an event loop or complement it.
Threads are similar to processes in that the operating system manages
task switching for you. However, the difference is that all threads
share the same address space. This is good and bad. Higher
performance can be achieved but since data is shared between threads,
extreme care must be taken to access or modify global data. The
operating system can switch threads at any moment or can execute
multiple threads simultaneously. I hope this sounds dangerous! It is!
Threads can introduce maddeningly complicated and hard to debug
synchronization problems.
Threads are like rocket fuel. They are essential when you really need
them but most applications would be better off with a simple event
loop. Even if threads are genuinely needed, consider confining them to
the parts of an application where truly scalable performance is really
worth the difficulty of a multithreaded implementation. For example,
most GUIs applications do not need threads and most scientific compute
intensive problems can be isolated from event dispatching. On the
other hand, high performance transaction servers generally do mandate a
truly multithreaded approach.
Another consideration is that threads are not quite as widely available
as event loops. While a few forward-thinking operating systems have
offered threads since the beginning, their addition to many popular
operating systems is much more recent and some still offer no threads
support. If portability is a requirement, one must check that threads
support is available and also carefully test a particular threads
implementation to see whether it supports the features you need. It is
likely that all platforms will have a solid implementation soon but at
this point in history it is best to double check.
Many suggestions by Mark Mielke <Mark.Mielke.markm@nt.com>
WHAT ABOUT NON-PREEMPTIVE THREADS?
The Java language is oriented to use non-preemptive threads, yet even
Java uses an event-loop for Swing (AFAIK). That is one of the reasons I
don't use Java for network-centric applications. My belief is that the
benefit of multi-threading is the gain in performance on SMP hardware.
In my view, non-preemptive threads (java green-threads) are usually
poor design. I find them harder to work with, harder to debug, and
slower for a rather marginal gain in readability. I really like working
with a state machine. I find it leads to more stable and better code.
It also has the benefit of abstracting away how concurrency is
achieved.
Contributed by artur@vogon-solutions.com, 12 Jul 1999.
BUGS
No support for epoll, or better, libevent.
The scope of events is pretty strange compared to most other perl
objects. I'm not sure if this is a bug or a feature (OK, probably it
was a mistake). We'll probably want to re-work things for Perl6.
The meaning of $io->timeout(0) might change. Use "undef" to unset the
timeout.
There seems to be some sort of bug in the global destruction phase:
Attempt to free unreferenced scalar during global destruction.
Use of uninitialized value during global destruction.
Explicit blessing to '' (assuming package main) during global
destruction.
THE FUTURE
Even if this module does not end up being the One and True Event Loop,
the author will insure that it is source compatible with its successor,
or arrange for gradual migration. Back in the early days, the Event
programming API was changing at every release. Care was taken to allow
the old API to continue to work, and the transition was eased by
printing out lots of warnings about the new usage. So you shouldn't
sit on your hands in anticipation of the One and True Event Loop. Just
start coding!
ALSO SEE
· Useful and Fun
Time::HiRes, NetServer::Portal, Time::Warp
· Message Passing
COPE, IPC::LDT, Event-tcp
· GUI
While Tk does not yet support Event, PerlQt does.
· C API
Inline
SUPPORT
If you have insights or complaints then please subscribe to the mailing
list! Send email to:
perl-loop-subscribe@perl.org
AUTHOR
Joshua N. Pritikin <jpritikin@pobox.com>
ACKNOWLEDGMENT
Initial 0.01 implementation by Graham Barr <gbarr@pobox.com>. Other
contributors include at least those lists below and folks mentioned in
the ChangeLog.
Gisle Aas <gisle@aas.no>
Uri Guttman <uri@sysarch.com>
Nick Ing-Simmons <nick@ni-s.u-net.com> (Tk)
Sarathy <gsar@engin.umich.edu>
Jochen Stenzel <perl@jochen-stenzel.de>
COPYRIGHT
Copyright AX 1997 Joshua Nathaniel Pritikin & Graham Barr
Copyright AX 1998, 1999, 2000, 2001, 2002, 2003, 2004 Joshua Nathaniel
Pritikin
All rights reserved. This program is free software; you can
redistribute it and/or modify it under the same terms as Perl itself.
perl v5.14.2 2012-01-15 Event(3)
