1 |
=head1 Introduction to AnyEvent |
2 |
|
3 |
This is a tutorial that will introduce you to the features of AnyEvent. |
4 |
|
5 |
The first part introduces the core AnyEvent module (after swamping you a |
6 |
bit in evangelism), which might already provide all you ever need. |
7 |
|
8 |
The second part focuses on network programming using sockets, for which |
9 |
AnyEvent offers a lot of support you can use. |
10 |
|
11 |
|
12 |
=head1 What is AnyEvent? |
13 |
|
14 |
If you don't care for the whys and want to see code, skip this section! |
15 |
|
16 |
AnyEvent is first of all just a framework to do event-based |
17 |
programming. Typically such frameworks are an all-or-nothing thing: If you |
18 |
use one such framework, you can't (easily, or even at all) use another in |
19 |
the same program. |
20 |
|
21 |
AnyEvent is different - it is a thin abstraction layer above all kinds |
22 |
of event loops. Its main purpose is to move the choice of the underlying |
23 |
framework (the event loop) from the module author to the program author |
24 |
using the module. |
25 |
|
26 |
That means you can write code that uses events to control what it |
27 |
does, without forcing other code in the same program to use the same |
28 |
underlying framework as you do - i.e. you can create a Perl module |
29 |
that is event-based using AnyEvent, and users of that module can still |
30 |
choose between using L<Gtk2>, L<Tk>, L<Event> or no event loop at |
31 |
all: AnyEvent comes with its own event loop implementation, so your |
32 |
code works regardless of other modules that might or might not be |
33 |
installed. The latter is important, as AnyEvent does not have any |
34 |
dependencies to other modules, which makes it easy to install, for |
35 |
example, when you lack a C compiler. |
36 |
|
37 |
A typical problem with Perl modules such as L<Net::IRC> is that they |
38 |
come with their own event loop: In L<Net::IRC>, the program who uses it |
39 |
needs to start the event loop of L<Net::IRC>. That means that one cannot |
40 |
integrate this module into a L<Gtk2> GUI for instance, as that module, |
41 |
too, enforces the use of its own event loop (namely L<Glib>). |
42 |
|
43 |
Another example is L<LWP>: it provides no event interface at all. It's a |
44 |
pure blocking HTTP (and FTP etc.) client library, which usually means that |
45 |
you either have to start a thread or have to fork for a HTTP request, or |
46 |
use L<Coro::LWP>, if you want to do something else while waiting for the |
47 |
request to finish. |
48 |
|
49 |
The motivation behind these designs is often that a module doesn't want to |
50 |
depend on some complicated XS-module (Net::IRC), or that it doesn't want |
51 |
to force the user to use some specific event loop at all (LWP). |
52 |
|
53 |
L<AnyEvent> solves this dilemma, by B<not> forcing module authors to either |
54 |
|
55 |
=over 4 |
56 |
|
57 |
=item write their own event loop (because guarantees to offer one |
58 |
everywhere - even on windows). |
59 |
|
60 |
=item choose one fixed event loop (because AnyEvent works with all |
61 |
important event loops available for Perl, and adding others is trivial). |
62 |
|
63 |
=back |
64 |
|
65 |
If the module author uses L<AnyEvent> for all his event needs (IO events, |
66 |
timers, signals, ...) then all other modules can just use his module and |
67 |
don't have to choose an event loop or adapt to his event loop. The choice |
68 |
of the event loop is ultimately made by the program author who uses all |
69 |
the modules and writes the main program. And even there he doesn't have to |
70 |
choose, he can just let L<AnyEvent> choose the best available event loop |
71 |
for him. |
72 |
|
73 |
Read more about this in the main documentation of the L<AnyEvent> module. |
74 |
|
75 |
|
76 |
=head1 Introduction to Event-Based Programming |
77 |
|
78 |
So what exactly is programming using events? It quite simply means that |
79 |
instead of your code actively waiting for something, such as the user |
80 |
entering something on STDIN: |
81 |
|
82 |
$| = 1; print "enter your name> "; |
83 |
|
84 |
my $name = <STDIN>; |
85 |
|
86 |
You instead tell your event framework to notify you in the event of some |
87 |
data being available on STDIN, by using a callback mechanism: |
88 |
|
89 |
use AnyEvent; |
90 |
|
91 |
$| = 1; print "enter your name> "; |
92 |
|
93 |
my $name; |
94 |
|
95 |
my $wait_for_input = AnyEvent->io ( |
96 |
fh => \*STDIN, # which file handle to check |
97 |
poll => "r", # which event to wait for ("r"ead data) |
98 |
cb => sub { # what callback to execute |
99 |
$name = <STDIN>; # read it |
100 |
} |
101 |
); |
102 |
|
103 |
# do something else here |
104 |
|
105 |
Looks more complicated, and surely is, but the advantage of using events |
106 |
is that your program can do something else instead of waiting for |
107 |
input. Waiting as in the first example is also called "blocking" because |
108 |
you "block" your process from executing anything else while you do so. |
109 |
|
110 |
The second example avoids blocking, by only registering interest in a read |
111 |
event, which is fast and doesn't block your process. Only when read data |
112 |
is available will the callback be called, which can then proceed to read |
113 |
the data. |
114 |
|
115 |
The "interest" is represented by an object returned by C<< AnyEvent->io |
116 |
>> called a "watcher" object - called like that because it "watches" your |
117 |
file handle (or other event sources) for the event you are interested in. |
118 |
|
119 |
In the example above, we create an I/O watcher by calling the C<< |
120 |
AnyEvent->io >> method. Disinterest in some event is simply expressed by |
121 |
forgetting about the watcher, for example, by C<undef>'ing the variable it |
122 |
is stored in. AnyEvent will automatically clean up the watcher if it is no |
123 |
longer used, much like Perl closes your file handles if you no longer use |
124 |
them anywhere. |
125 |
|
126 |
=head2 Condition Variables |
127 |
|
128 |
However, the above is not a fully working program, and will not work |
129 |
as-is. The reason is that your callback will not be invoked out of the |
130 |
blue, you have to run the event loop. Also, event-based programs sometimes |
131 |
have to block, too, as when there simply is nothing else to do and |
132 |
everything waits for some events, it needs to block the process as well. |
133 |
|
134 |
In AnyEvent, this is done using condition variables. Condition variables |
135 |
are named "condition variables" because they represent a condition that is |
136 |
initially false and needs to be fulfilled. |
137 |
|
138 |
You can also call them "merge points", "sync points", "rendezvous ports" |
139 |
or even callbacks and many other things (and they are often called like |
140 |
this in other frameworks). The important point is that you can create them |
141 |
freely and later wait for them to become true. |
142 |
|
143 |
Condition variables have two sides - one side is the "producer" of the |
144 |
condition (whatever code detects the condition), the other side is the |
145 |
"consumer" (the code that waits for that condition). |
146 |
|
147 |
In our example in the previous section, the producer is the event callback |
148 |
and there is no consumer yet - let's change that now: |
149 |
|
150 |
use AnyEvent; |
151 |
|
152 |
$| = 1; print "enter your name> "; |
153 |
|
154 |
my $name; |
155 |
|
156 |
my $name_ready = AnyEvent->condvar; |
157 |
|
158 |
my $wait_for_input = AnyEvent->io ( |
159 |
fh => \*STDIN, |
160 |
poll => "r", |
161 |
cb => sub { |
162 |
$name = <STDIN>; |
163 |
$name_ready->send; |
164 |
} |
165 |
); |
166 |
|
167 |
# do something else here |
168 |
|
169 |
# now wait until the name is available: |
170 |
$name_ready->recv; |
171 |
|
172 |
undef $wait_for_input; # watche rno longer needed |
173 |
|
174 |
print "your name is $name\n"; |
175 |
|
176 |
This program creates an AnyEvent condvar by calling the C<< |
177 |
AnyEvent->condvar >> method. It then creates a watcher as usual, but |
178 |
inside the callback it C<send>'s the C<$name_ready> condition variable, |
179 |
which causes anybody waiting on it to continue. |
180 |
|
181 |
The "anybody" in this case is the code that follows, which calls C<< |
182 |
$name_ready->recv >>: The producer calls C<send>, the consumer calls |
183 |
C<recv>. |
184 |
|
185 |
If there is no C<$name> available yet, then the call to C<< |
186 |
$name_ready->recv >> will halt your program until the condition becomes |
187 |
true. |
188 |
|
189 |
As the names C<send> and C<recv> imply, you can actually send and receive |
190 |
data using this, for example, the above code could also be written like |
191 |
this, without an extra variable to store the name in: |
192 |
|
193 |
use AnyEvent; |
194 |
|
195 |
$| = 1; print "enter your name> "; |
196 |
|
197 |
my $name_ready = AnyEvent->condvar; |
198 |
|
199 |
my $wait_for_input = AnyEvent->io ( |
200 |
fh => \*STDIN, poll => "r", |
201 |
cb => sub { $name_ready->send (scalar = <STDIN>) } |
202 |
); |
203 |
|
204 |
# do something else here |
205 |
|
206 |
# now wait and fetch the name |
207 |
my $name = $name_ready->recv; |
208 |
|
209 |
undef $wait_for_input; # watche rno longer needed |
210 |
|
211 |
print "your name is $name\n"; |
212 |
|
213 |
You can pass any number of arguments to C<send>, and everybody call to |
214 |
C<recv> will return them. |
215 |
|
216 |
=head2 The "main loop" |
217 |
|
218 |
Most event-based frameworks have something called a "main loop" or "event |
219 |
loop run function" or something similar. |
220 |
|
221 |
Just like in C<recv> AnyEvent, these functions need to be called |
222 |
eventually so that your event loop has a chance of actually looking for |
223 |
those events you are interested in. |
224 |
|
225 |
For example, in a L<Gtk2> program, the above example could also be written |
226 |
like this: |
227 |
|
228 |
use Gtk2 -init; |
229 |
use AnyEvent; |
230 |
|
231 |
############################################ |
232 |
# create a window and some label |
233 |
|
234 |
my $window = new Gtk2::Window "toplevel"; |
235 |
$window->add (my $label = new Gtk2::Label "soon replaced by name"); |
236 |
|
237 |
$window->show_all; |
238 |
|
239 |
############################################ |
240 |
# do our AnyEvent stuff |
241 |
|
242 |
$| = 1; print "enter your name> "; |
243 |
|
244 |
my $name_ready = AnyEvent->condvar; |
245 |
|
246 |
my $wait_for_input = AnyEvent->io ( |
247 |
fh => \*STDIN, poll => "r", |
248 |
cb => sub { |
249 |
# set the label |
250 |
$label->set_text (scalar <STDIN>); |
251 |
print "enter another name> "; |
252 |
} |
253 |
); |
254 |
|
255 |
############################################ |
256 |
# Now enter Gtk2's event loop |
257 |
|
258 |
main Gtk2; |
259 |
|
260 |
No condition variable anywhere in sight - instead, we just read a line |
261 |
from STDIN and replace the text in the label. In fact, since nobody |
262 |
C<undef>'s C<$wait_for_input> you can enter multiple lines. |
263 |
|
264 |
Instead of waiting for a condition variable, the program enters the Gtk2 |
265 |
main loop by calling C<< Gtk2->main >>, which will block the program and |
266 |
wait for events to arrive. |
267 |
|
268 |
This also shows that AnyEvent is quite flexible - you didn't have anything |
269 |
to do to make the AnyEvent watcher use Gtk2 (actually Glib) - it just |
270 |
worked. |
271 |
|
272 |
Admittedly, the example is a bit silly - who would want to read names |
273 |
form standard input in a Gtk+ application. But imagine that instead of |
274 |
doing that, you would make a HTTP request in the background and display |
275 |
it's results. In fact, with event-based programming you can make many |
276 |
http-requests in parallel in your program and still provide feedback to |
277 |
the user and stay interactive. |
278 |
|
279 |
In the next part you will see how to do just that - by implementing an |
280 |
HTTP request, on our own, with the utility modules AnyEvent comes with. |
281 |
|
282 |
Before that, however, let's briefly look at how you would write your |
283 |
program with using only AnyEvent, without ever calling some other event |
284 |
loop's run function. |
285 |
|
286 |
In the example using condition variables, we used that, and in fact, this |
287 |
is the solution: |
288 |
|
289 |
my $quit_program = AnyEvent->condvar; |
290 |
|
291 |
# create AnyEvent watchers (or not) here |
292 |
|
293 |
$quit_program->recv; |
294 |
|
295 |
If any of your watcher callbacks decide to quit, they can simply call |
296 |
C<< $quit_program->send >>. Of course, they could also decide not to and |
297 |
simply call C<exit> instead, or they could decide not to quit, ever (e.g. |
298 |
in a long-running daemon program). |
299 |
|
300 |
In that case, you can simply use: |
301 |
|
302 |
AnyEvent->condvar->recv; |
303 |
|
304 |
And this is, in fact, closest to the idea of a main loop run function that |
305 |
AnyEvent offers. |
306 |
|
307 |
=head2 Timers and other event sources |
308 |
|
309 |
So far, we have only used I/O watchers. These are useful mainly to find |
310 |
out whether a Socket has data to read, or space to write more data. On sane |
311 |
operating systems this also works for console windows/terminals (typically |
312 |
on standard input), serial lines, all sorts of other devices, basically |
313 |
almost everything that has a file descriptor but isn't a file itself. (As |
314 |
usual, "sane" excludes windows - on that platform you would need different |
315 |
functions for all of these, complicating code immensely - think "socket |
316 |
only" on windows). |
317 |
|
318 |
However, I/O is not everything - the second most important event source is |
319 |
the clock. For example when doing an HTTP request you might want to time |
320 |
out when the server doesn't answer within some predefined amount of time. |
321 |
|
322 |
In AnyEvent, timer event watchers are created by calling the C<< |
323 |
AnyEvent->timer >> method: |
324 |
|
325 |
use AnyEvent; |
326 |
|
327 |
my $cv = AnyEvent->condvar; |
328 |
|
329 |
my $wait_one_and_a_half_seconds = AnyEvent->timer ( |
330 |
after => 1.5, # after how many seconds to invoke the cb? |
331 |
cb => sub { # the callback to invoke |
332 |
$cv->send; |
333 |
}, |
334 |
); |
335 |
|
336 |
# can do something else here |
337 |
|
338 |
# now wait till our time has come |
339 |
$cv->recv; |
340 |
|
341 |
Unlike I/O watchers, timers are only interested in the amount of seconds |
342 |
they have to wait. When that amount of time has passed, AnyEvent will |
343 |
invoke your callback. |
344 |
|
345 |
Unlike I/O watchers, which will call your callback as many times as there |
346 |
is data available, timers are one-shot: after they have "fired" once and |
347 |
invoked your callback, they are dead and no longer do anything. |
348 |
|
349 |
To get a repeating timer, such as a timer firing roughly once per second, |
350 |
you have to recreate it: |
351 |
|
352 |
use AnyEvent; |
353 |
|
354 |
my $time_watcher; |
355 |
|
356 |
sub once_per_second { |
357 |
print "tick\n"; |
358 |
|
359 |
# (re-)create the watcher |
360 |
$time_watcher = AnyEvent->timer ( |
361 |
after => 1, |
362 |
cb => \&once_per_second, |
363 |
); |
364 |
} |
365 |
|
366 |
# now start the timer |
367 |
once_per_second; |
368 |
|
369 |
Having to recreate your timer is a restriction put on AnyEvent that is |
370 |
present in most event libraries it uses. It is so annoying that some |
371 |
future version might work around this limitation, but right now, it's the |
372 |
only way to do repeating timers. |
373 |
|
374 |
Fortunately most timers aren't really repeating but specify timeouts of |
375 |
some sort. |
376 |
|
377 |
=head3 More esoteric sources |
378 |
|
379 |
AnyEvent also has some other, more esoteric event sources you can tap |
380 |
into: signal and child watchers. |
381 |
|
382 |
Signal watchers can be used to wait for "signal events", which simply |
383 |
means your process got send a signal (such as C<SIGTERM> or C<SIGUSR1>). |
384 |
|
385 |
Process watchers wait for a child process to exit. They are useful when |
386 |
you fork a separate process and need to know when it exits, but you do not |
387 |
wait for that by blocking. |
388 |
|
389 |
Both watcher types are described in detail in the main L<AnyEvent> manual |
390 |
page. |
391 |
|
392 |
|
393 |
=head1 Network programming and AnyEvent |
394 |
|
395 |
So far you have seen how to register event watchers and handle events. |
396 |
|
397 |
This is a great foundation to write network clients and servers, and might be |
398 |
all that your module (or program) ever requires, but writing your own I/O |
399 |
buffering again and again becomes tedious, not to mention that it attracts |
400 |
errors. |
401 |
|
402 |
While the core L<AnyEvent> module is still small and self-contained, |
403 |
the distribution comes with some very useful utility modules such as |
404 |
L<AnyEvent::Handle>, L<AnyEvent::DNS> and L<AnyEvent::Socket>. These can |
405 |
make your life as non-blocking network programmer a lot easier. |
406 |
|
407 |
Here is a quick overview over these three modules: |
408 |
|
409 |
=head2 L<AnyEvent::DNS> |
410 |
|
411 |
This module allows fully asynchronous DNS resolution. It is used mainly by |
412 |
L<AnyEvent::Socket> to resolve hostnames and service ports for you, but is |
413 |
a great way to do other DNS resolution tasks, such as reverse lookups of |
414 |
IP addresses for log files. |
415 |
|
416 |
=head2 L<AnyEvent::Handle> |
417 |
|
418 |
This module handles non-blocking IO on file handles in an event based |
419 |
manner. It provides a wrapper object around your file handle that provides |
420 |
queueing and buffering of incoming and outgoing data for you. |
421 |
|
422 |
It also implements the most common data formats, such as text lines, or |
423 |
fixed and variable-width data blocks. |
424 |
|
425 |
=head2 L<AnyEvent::Socket> |
426 |
|
427 |
This module provides you with functions that handle socket creation |
428 |
and IP address magic. The two main functions are C<tcp_connect> and |
429 |
C<tcp_server>. The former will connect a (streaming) socket to an internet |
430 |
host for you and the later will make a server socket for you, to accept |
431 |
connections. |
432 |
|
433 |
This module also comes with transparent IPv6 support, this means: If you |
434 |
write your programs with this module, you will be IPv6 ready without doing |
435 |
anything special. |
436 |
|
437 |
It also works around a lot of portability quirks (especially on the |
438 |
windows platform), which makes it even easier to write your programs in a |
439 |
portable way (did you know that windows uses different error codes for all |
440 |
socket functions and that Perl does not know about these? That "Unknown |
441 |
error 10022" (which is C<WSAEINVAL>) can mean that our C<connect> call was |
442 |
successful? That unsuccessful TCP connects might never be reported back |
443 |
to your program? That C<WSAEINPROGRESS> means your C<connect> call was |
444 |
ignored instead of being in progress? AnyEvent::Socket works around all of |
445 |
these Windows/Perl bugs for you). |
446 |
|
447 |
=head2 First experiments with non-blocking connects: a parallel finger |
448 |
client. |
449 |
|
450 |
The finger protocol is one of the simplest protocols in use on the |
451 |
internet. Or in use in the past, as almost nobody uses it anymore. |
452 |
|
453 |
It works by connecting to the finger port on another host, writing a |
454 |
single line with a user name and then reading the finger response, as |
455 |
specified by that user. OK, RFC 1288 specifies a vastly more complex |
456 |
protocol, but it basically boils down to this: |
457 |
|
458 |
# telnet idsoftware.com finger |
459 |
Trying 192.246.40.37... |
460 |
Connected to idsoftware.com (192.246.40.37). |
461 |
Escape character is '^]'. |
462 |
johnc |
463 |
Welcome to id Software's Finger Service V1.5! |
464 |
|
465 |
[...] |
466 |
Now on the web: |
467 |
[...] |
468 |
|
469 |
Connection closed by foreign host. |
470 |
|
471 |
Yeah, I<was> used indeed, but at least the finger daemon still works, so |
472 |
let's write a little AnyEvent function that makes a finger request: |
473 |
|
474 |
use AnyEvent; |
475 |
use AnyEvent::Socket; |
476 |
|
477 |
sub finger($$) { |
478 |
my ($user, $host) = @_; |
479 |
|
480 |
# use a condvar to return results |
481 |
my $cv = AnyEvent->condvar; |
482 |
|
483 |
# first, connect to the host |
484 |
tcp_connect $host, "finger", sub { |
485 |
# the callback receives the socket handle - or nothing |
486 |
my ($fh) = @_ |
487 |
or return $cv->send; |
488 |
|
489 |
# now write the username |
490 |
syswrite $fh, "$user\015\012"; |
491 |
|
492 |
my $response; |
493 |
|
494 |
# register a read watcher |
495 |
my $read_watcher; $read_watcher = AnyEvent->io ( |
496 |
fh => $fh, |
497 |
poll => "r", |
498 |
cb => sub { |
499 |
my $len = sysread $fh, $response, 1024, length $response; |
500 |
|
501 |
if ($len <= 0) { |
502 |
# we are done, or an error occured, lets ignore the latter |
503 |
undef $read_watcher; # no longer interested |
504 |
$cv->send ($response); # send results |
505 |
} |
506 |
}, |
507 |
); |
508 |
}; |
509 |
|
510 |
# pass $cv to the caller |
511 |
$cv |
512 |
} |
513 |
|
514 |
That's a mouthful! Let's dissect this function a bit, first the overall function: |
515 |
|
516 |
sub finger($$) { |
517 |
my ($user, $host) = @_; |
518 |
|
519 |
# use a condvar to return results |
520 |
my $cv = AnyEvent->condvar; |
521 |
|
522 |
# first, connect to the host |
523 |
tcp_connect $host, "finger", sub { |
524 |
... |
525 |
}; |
526 |
|
527 |
$cv |
528 |
} |
529 |
|
530 |
This isn't too complicated, just a function with two parameters, which |
531 |
creates a condition variable, returns it, and while it does that, |
532 |
initiates a TCP connect to C<$host>. The condition variable |
533 |
will be used by the caller to receive the finger response. |
534 |
|
535 |
Since we are event-based programmers, we do not wait for the connect to |
536 |
finish - it could block your program for a minute or longer! Instead, |
537 |
we pass the callback it should invoke when the connect is done to |
538 |
C<tcp_connect>. If it is successful, our callback gets called with the |
539 |
socket handle as first argument, otherwise, nothing will be passed to our |
540 |
callback. |
541 |
|
542 |
Let's look at our callback in more detail: |
543 |
|
544 |
# the callback gets the socket handle - or nothing |
545 |
my ($fh) = @_ |
546 |
or return $cv->send; |
547 |
|
548 |
The first thing the callback does is indeed save the socket handle in |
549 |
C<$fh>. When there was an error (no arguments), then our instinct as |
550 |
expert Perl programmers would tell us to die: |
551 |
|
552 |
my ($fh) = @_ |
553 |
or die "$host: $!"; |
554 |
|
555 |
While this would give good feedback to the user, our program would |
556 |
probably freeze here, as we never report the results to anybody, certainly |
557 |
not the caller of our C<finger> function! |
558 |
|
559 |
This is why we instead return, but also call C<< $cv->send >> without any |
560 |
arguments to signal to our consumer that something bad has happened. The |
561 |
return value of C<< $cv->send >> is irrelevant, as is the return value of |
562 |
our callback. The return statement is simply used for the side effect of, |
563 |
well, returning immediately from the callback. |
564 |
|
565 |
As the next step in the finger protocol, we send the username to the |
566 |
finger daemon on the other side of our connection: |
567 |
|
568 |
syswrite $fh, "$user\015\012"; |
569 |
|
570 |
Note that this isn't 100% clean - the socket could, for whatever reasons, |
571 |
not accept our data. When writing a small amount of data like in this |
572 |
example it doesn't matter, but for real-world cases you might need to |
573 |
implement some kind of write buffering - or use L<AnyEvent::Handle>, which |
574 |
handles these matters for you. |
575 |
|
576 |
What we do have to do is to implement our own read buffer - the response |
577 |
data could arrive late or in multiple chunks, and we cannot just wait for |
578 |
it (event-based programming, you know?). |
579 |
|
580 |
To do that, we register a read watcher on the socket which waits for data: |
581 |
|
582 |
my $read_watcher; $read_watcher = AnyEvent->io ( |
583 |
fh => $fh, |
584 |
poll => "r", |
585 |
|
586 |
There is a trick here, however: the read watcher isn't stored in a global |
587 |
variable, but in a local one - if the callback returns, it would normally |
588 |
destroy the variable and its contents, which would in turn unregister our |
589 |
watcher. |
590 |
|
591 |
To avoid that, we C<undef>ine the variable in the watcher callback. This |
592 |
means that, when the C<tcp_connect> callback returns, that perl thinks |
593 |
(quite correctly) that the read watcher is still in use - namely in the |
594 |
callback. |
595 |
|
596 |
The callback itself calls C<sysread> for as many times as necessary, until |
597 |
C<sysread> returns an error or end-of-file: |
598 |
|
599 |
cb => sub { |
600 |
my $len = sysread $fh, $response, 1024, length $response; |
601 |
|
602 |
if ($len <= 0) { |
603 |
|
604 |
Note that C<sysread> has the ability to append data it reads to a scalar, |
605 |
which is what we make good use of in this example. |
606 |
|
607 |
When C<sysread> indicates we are done, the callback C<undef>ines |
608 |
the watcher and then C<send>'s the response data to the condition |
609 |
variable. All this has the following effects: |
610 |
|
611 |
Undefining the watcher destroys it, as our callback was the only one still |
612 |
having a reference to it. When the watcher gets destroyed, it destroys the |
613 |
callback, which in turn means the C<$fh> handle is no longer used, so that |
614 |
gets destroyed as well. The result is that all resources will be nicely |
615 |
cleaned up by perl for us. |
616 |
|
617 |
=head3 Using the finger client |
618 |
|
619 |
Now, we could probably write the same finger client in a simpler way if |
620 |
we used C<IO::Socket::INET>, ignored the problem of multiple hosts and |
621 |
ignored IPv6 and a few other things that C<tcp_connect> handles for us. |
622 |
|
623 |
But the main advantage is that we can not only run this finger function in |
624 |
the background, we even can run multiple sessions in parallel, like this: |
625 |
|
626 |
my $f1 = finger "trouble", "noc.dfn.de"; # check for trouble tickets |
627 |
my $f2 = finger "1736" , "noc.dfn.de"; # fetch ticket 1736 |
628 |
my $f3 = finger "johnc", "idsoftware.com"; # finger john |
629 |
|
630 |
print "trouble tickets:\n", $f1->recv, "\n"; |
631 |
print "trouble ticket #1736:\n", $f2->recv, "\n"; |
632 |
print "john carmacks finger file: ", $f3->recv, "\n"; |
633 |
|
634 |
It doesn't look like it, but in fact all three requests run in |
635 |
parallel. The code waits for the first finger request to finish first, but |
636 |
that doesn't keep it from executing in parallel, because when the first |
637 |
C<recv> call sees that the data isn't ready yet, it serves events for all |
638 |
three requests automatically. |
639 |
|
640 |
By taking advantage of network latencies, which allows us to serve other |
641 |
requests and events while we wait for an event on one socket, the overall |
642 |
time to do these three requests will be greatly reduces, typically all |
643 |
three are done in the same time as the slowest of them would use. |
644 |
|
645 |
By the way, you do not actually have to wait in the C<recv> method on an |
646 |
AnyEvent condition variable, you can also register a callback: |
647 |
|
648 |
$cv->cb (sub { |
649 |
my $response = shift->recv; |
650 |
# ... |
651 |
}); |
652 |
|
653 |
The callback will only be invoked when C<send> was called. In fact, |
654 |
instead of returning a condition variable you could also pass a third |
655 |
parameter to your finger function, the callback to invoke with the |
656 |
response: |
657 |
|
658 |
sub finger($$$) { |
659 |
my ($user, $host, $cb) = @_; |
660 |
|
661 |
What you use is a matter of taste - if you expect your function to be |
662 |
used mainly in an event-based program you would normally prefer to pass a |
663 |
callback directly. |
664 |
|
665 |
=head3 Criticism and fix |
666 |
|
667 |
To make this example more real-world-ready, we would not only implement |
668 |
some write buffering (for the paranoid), but we would also have to handle |
669 |
timeouts and maybe protocol errors. |
670 |
|
671 |
This quickly gets unwieldy, which is why we introduce L<AnyEvent::Handle> |
672 |
in the next section, which takes care of all these details for us. |
673 |
|
674 |
|
675 |
=head2 First experiments with AnyEvent::Handle |
676 |
|
677 |
Now let's start with something simple: a program that reads from standard |
678 |
input in a non-blocking way, that is, in a way that lets your program do |
679 |
other things while it is waiting for input. |
680 |
|
681 |
First, the full program listing: |
682 |
|
683 |
#!/usr/bin/perl |
684 |
|
685 |
use AnyEvent; |
686 |
use AnyEvent::Handle; |
687 |
|
688 |
my $end_prog = AnyEvent->condvar; |
689 |
|
690 |
my $handle = |
691 |
AnyEvent::Handle->new ( |
692 |
fh => \*STDIN, |
693 |
on_eof => sub { |
694 |
print "received EOF, exiting...\n"; |
695 |
$end_prog->broadcast; |
696 |
}, |
697 |
on_error => sub { |
698 |
print "error while reading from STDIN: $!\n"; |
699 |
$end_prog->broadcast; |
700 |
} |
701 |
); |
702 |
|
703 |
$handle->push_read (sub { |
704 |
my ($handle) = @_; |
705 |
|
706 |
if ($handle->rbuf =~ s/^.*?\bend\b.*$//s) { |
707 |
print "got 'end', existing...\n"; |
708 |
$end_prog->broadcast; |
709 |
return 1 |
710 |
} |
711 |
|
712 |
0 |
713 |
}); |
714 |
|
715 |
$end_prog->recv; |
716 |
|
717 |
That's a mouthful, so let's go through it step by step: |
718 |
|
719 |
#!/usr/bin/perl |
720 |
|
721 |
use AnyEvent; |
722 |
use AnyEvent::Handle; |
723 |
|
724 |
Nothing unexpected here, just load AnyEvent for the event functionality |
725 |
and AnyEvent::Handle for your file handling needs. |
726 |
|
727 |
my $end_prog = AnyEvent->condvar; |
728 |
|
729 |
Here the program creates a so-called 'condition variable': Condition |
730 |
variables are a great way to signal the completion of some event, or to |
731 |
state that some condition became true (thus the name). |
732 |
|
733 |
This condition variable represents the condition that the program wants to |
734 |
terminate. Later in the program, we will 'recv' that condition (call the |
735 |
C<recv> method on it), which will wait until the condition gets signalled |
736 |
(which is done by calling the C<send> method on it). |
737 |
|
738 |
The next step is to create the handle object: |
739 |
|
740 |
my $handle = |
741 |
AnyEvent::Handle->new ( |
742 |
fh => \*STDIN, |
743 |
on_eof => sub { |
744 |
print "received EOF, exiting...\n"; |
745 |
$end_prog->broadcast; |
746 |
}, |
747 |
|
748 |
This handle object will read from standard input. Setting the C<on_eof> |
749 |
callback should be done for every file handle, as that is a condition that |
750 |
we always need to check for when working with file handles, to prevent |
751 |
reading or writing to a closed file handle, or getting stuck indefinitely |
752 |
in case of an error. |
753 |
|
754 |
Speaking of errors: |
755 |
|
756 |
on_error => sub { |
757 |
print "error while reading from STDIN: $!\n"; |
758 |
$end_prog->broadcast; |
759 |
} |
760 |
); |
761 |
|
762 |
The C<on_error> callback is also not required, but we set it here in case |
763 |
any error happens when we read from the file handle. It is usually a good |
764 |
idea to set this callback and at least print some diagnostic message: Even |
765 |
in our small example an error can happen. More on this later... |
766 |
|
767 |
$handle->push_read (sub { |
768 |
|
769 |
Next we push a general read callback on the read queue, which |
770 |
will wait until we have received all the data we wanted to |
771 |
receive. L<AnyEvent::Handle> has two queues per file handle, a read and a |
772 |
write queue. The write queue queues pending data that waits to be written |
773 |
to the file handle. And the read queue queues reading callbacks. For more |
774 |
details see the documentation L<AnyEvent::Handle> about the READ QUEUE and |
775 |
WRITE QUEUE. |
776 |
|
777 |
my ($handle) = @_; |
778 |
|
779 |
if ($handle->rbuf =~ s/^.*?\bend\b.*$//s) { |
780 |
print "got 'end', existing...\n"; |
781 |
$end_prog->broadcast; |
782 |
return 1 |
783 |
} |
784 |
|
785 |
0 |
786 |
}); |
787 |
|
788 |
The actual callback waits until the word 'end' has been seen in the data |
789 |
received on standard input. Once we encounter the stop word 'end' we |
790 |
remove everything from the read buffer and call the condition variable |
791 |
we setup earlier, that signals our 'end of program' condition. And the |
792 |
callback returns with a true value, that signals we are done with reading |
793 |
all the data we were interested in (all data until the word 'end' has been |
794 |
seen). |
795 |
|
796 |
In all other cases, when the stop word has not been seen yet, we just |
797 |
return a false value, to indicate that we are not finished yet. |
798 |
|
799 |
The C<rbuf> method returns our read buffer, that we can directly modify as |
800 |
lvalue. Alternatively we also could have written: |
801 |
|
802 |
if ($handle->{rbuf} =~ s/^.*?\bend\b.*$//s) { |
803 |
|
804 |
The last line will wait for the condition that our program wants to exit: |
805 |
|
806 |
$end_prog->recv; |
807 |
|
808 |
The call to C<recv> will setup an event loop for us and wait for IO, timer |
809 |
or signal events and will handle them until the condition gets sent (by |
810 |
calling its C<send> method). |
811 |
|
812 |
The key points to learn from this example are: |
813 |
|
814 |
=over 4 |
815 |
|
816 |
=item * Condition variables are used to start an event loop. |
817 |
|
818 |
=item * How to registering some basic callbacks on AnyEvent::Handle's. |
819 |
|
820 |
=item * How to process data in the read buffer. |
821 |
|
822 |
=back |
823 |
|
824 |
=head1 AUTHORS |
825 |
|
826 |
Robin Redeker C<< <elmex at ta-sa.org> >>, Marc Lehmann <schmorp@schmorp.de>. |
827 |
|