AnyEvent::Intro(3pm) User Contributed Perl Documentation AnyEvent::Intro(3pm)
NAME
AnyEvent::Intro - an introductory tutorial to AnyEvent
Introduction to AnyEvent
This is a tutorial that will introduce you to the features of AnyEvent.
The first part introduces the core AnyEvent module (after swamping you
a bit in evangelism), which might already provide all you ever need: If
you are only interested in AnyEvent's event handling capabilities, read
no further.
The second part focuses on network programming using sockets, for which
AnyEvent offers a lot of support you can use, and a lot of workarounds
around portability quirks.
What is AnyEvent?
If you don't care for the whys and want to see code, skip this section!
AnyEvent is first of all just a framework to do event-based
programming. Typically such frameworks are an all-or-nothing thing: If
you use one such framework, you can't (easily, or even at all) use
another in the same program.
AnyEvent is different - it is a thin abstraction layer on top of other
event loops, just like DBI is an abstraction of many different database
APIs. Its main purpose is to move the choice of the underlying
framework (the event loop) from the module author to the program author
using the module.
That means you can write code that uses events to control what it does,
without forcing other code in the same program to use the same
underlying framework as you do - i.e. you can create a Perl module that
is event-based using AnyEvent, and users of that module can still
choose between using Gtk2, Tk, Event (or run inside Irssi or rxvt-
unicode) or any other supported event loop. AnyEvent even comes with
its own pure-perl event loop implementation, so your code works
regardless of other modules that might or might not be installed. The
latter is important, as AnyEvent does not have any hard dependencies to
other modules, which makes it easy to install, for example, when you
lack a C compiler. No matter what environment, AnyEvent will just cope
with it.
A typical limitation of existing Perl modules such as Net::IRC is that
they come with their own event loop: In Net::IRC, a program which uses
it needs to start the event loop of Net::IRC. That means that one
cannot integrate this module into a Gtk2 GUI for instance, as that
module, too, enforces the use of its own event loop (namely Glib).
Another example is LWP: it provides no event interface at all. It's a
pure blocking HTTP (and FTP etc.) client library, which usually means
that you either have to start another process or have to fork for a
HTTP request, or use threads (e.g. Coro::LWP), if you want to do
something else while waiting for the request to finish.
The motivation behind these designs is often that a module doesn't want
to depend on some complicated XS-module (Net::IRC), or that it doesn't
want to force the user to use some specific event loop at all (LWP),
out of fear of severely limiting the usefulness of the module: If your
module requires Glib, it will not run in a Tk program.
AnyEvent solves this dilemma, by not forcing module authors to either:
- write their own event loop (because it guarantees the availability of
an event loop everywhere - even on windows with no extra modules
installed).
- choose one specific event loop (because AnyEvent works with most
event loops available for Perl).
If the module author uses AnyEvent for all his (or her) event needs (IO
events, timers, signals, ...) then all other modules can just use his
module and don't have to choose an event loop or adapt to his event
loop. The choice of the event loop is ultimately made by the program
author who uses all the modules and writes the main program. And even
there he doesn't have to choose, he can just let AnyEvent choose the
most efficient event loop available on the system.
Read more about this in the main documentation of the AnyEvent module.
Introduction to Event-Based Programming
So what exactly is programming using events? It quite simply means that
instead of your code actively waiting for something, such as the user
entering something on STDIN:
$| = 1; print "enter your name> ";
my $name = <STDIN>;
You instead tell your event framework to notify you in the event of
some data being available on STDIN, by using a callback mechanism:
use AnyEvent;
$| = 1; print "enter your name> ";
my $name;
my $wait_for_input = AnyEvent->io (
fh => \*STDIN, # which file handle to check
poll => "r", # which event to wait for ("r"ead data)
cb => sub { # what callback to execute
$name = <STDIN>; # read it
}
);
# do something else here
Looks more complicated, and surely is, but the advantage of using
events is that your program can do something else instead of waiting
for input (side note: combining AnyEvent with a thread package such as
Coro can recoup much of the simplicity, effectively getting the best of
two worlds).
Waiting as done in the first example is also called "blocking" the
process because you "block"/keep your process from executing anything
else while you do so.
The second example avoids blocking by only registering interest in a
read event, which is fast and doesn't block your process. The callback
will be called only when data is available and can be read without
blocking.
The "interest" is represented by an object returned by "AnyEvent->io"
called a "watcher" object - thus named because it "watches" your file
handle (or other event sources) for the event you are interested in.
In the example above, we create an I/O watcher by calling the
"AnyEvent->io" method. A lack of further interest in some event is
expressed by simply forgetting about its watcher, for example by
"undef"-ing the only variable it is stored in. AnyEvent will
automatically clean up the watcher if it is no longer used, much like
Perl closes your file handles if you no longer use them anywhere.
A short note on callbacks
A common issue that hits people is the problem of passing parameters to
callbacks. Programmers used to languages such as C or C++ are often
used to a style where one passes the address of a function (a function
reference) and some data value, e.g.:
sub callback {
my ($arg) = @_;
$arg->method;
}
my $arg = ...;
call_me_back_later \&callback, $arg;
This is clumsy, as the place where behaviour is specified (when the
callback is registered) is often far away from the place where
behaviour is implemented. It also doesn't use Perl syntax to invoke the
code. There is also an abstraction penalty to pay as one has to name
the callback, which often is unnecessary and leads to nonsensical or
duplicated names.
In Perl, one can specify behaviour much more directly by using
closures. Closures are code blocks that take a reference to the
enclosing scope(s) when they are created. This means lexical variables
in scope when a closure is created can be used inside the closure:
my $arg = ...;
call_me_back_later sub { $arg->method };
Under most circumstances, closures are faster, use fewer resources and
result in much clearer code than the traditional approach. Faster,
because parameter passing and storing them in local variables in Perl
is relatively slow. Fewer resources, because closures take references
to existing variables without having to create new ones, and clearer
code because it is immediately obvious that the second example calls
the "method" method when the callback is invoked.
Apart from these, the strongest argument for using closures with
AnyEvent is that AnyEvent does not allow passing parameters to the
callback, so closures are the only way to achieve that in most cases
:->
A little hint to catch mistakes
AnyEvent does not check the parameters you pass in, at least not by
default. to enable checking, simply start your program with
"AE_STRICT=1" in the environment, or put "use AnyEvent::Strict" near
the top of your program:
AE_STRICT=1 perl myprogram
You can find more info on this and additional debugging aids later in
this introduction.
Condition Variables
Back to the I/O watcher example: The code is not yet a fully working
program, and will not work as-is. The reason is that your callback will
not be invoked out of the blue; you have to run the event loop first.
Also, event-based programs need to block sometimes too, such as when
there is nothing to do, and everything is waiting for new events to
arrive.
In AnyEvent, this is done using condition variables. Condition
variables are named "condition variables" because they represent a
condition that is initially false and needs to be fulfilled.
You can also call them "merge points", "sync points", "rendezvous
ports" or even callbacks and many other things (and they are often
called these names in other frameworks). The important point is that
you can create them freely and later wait for them to become true.
Condition variables have two sides - one side is the "producer" of the
condition (whatever code detects and flags the condition), the other
side is the "consumer" (the code that waits for that condition).
In our example in the previous section, the producer is the event
callback and there is no consumer yet - let's change that right now:
use AnyEvent;
$| = 1; print "enter your name> ";
my $name;
my $name_ready = AnyEvent->condvar;
my $wait_for_input = AnyEvent->io (
fh => \*STDIN,
poll => "r",
cb => sub {
$name = <STDIN>;
$name_ready->send;
}
);
# do something else here
# now wait until the name is available:
$name_ready->recv;
undef $wait_for_input; # watcher no longer needed
print "your name is $name\n";
This program creates an AnyEvent condvar by calling the
"AnyEvent->condvar" method. It then creates a watcher as usual, but
inside the callback it "send"s the $name_ready condition variable,
which causes whoever is waiting on it to continue.
The "whoever" in this case is the code that follows, which calls
"$name_ready->recv": The producer calls "send", the consumer calls
"recv".
If there is no $name available yet, then the call to
"$name_ready->recv" will halt your program until the condition becomes
true.
As the names "send" and "recv" imply, you can actually send and receive
data using this, for example, the above code could also be written like
this, without an extra variable to store the name in:
use AnyEvent;
$| = 1; print "enter your name> ";
my $name_ready = AnyEvent->condvar;
my $wait_for_input = AnyEvent->io (
fh => \*STDIN, poll => "r",
cb => sub { $name_ready->send (scalar <STDIN>) }
);
# do something else here
# now wait and fetch the name
my $name = $name_ready->recv;
undef $wait_for_input; # watcher no longer needed
print "your name is $name\n";
You can pass any number of arguments to "send", and every subsequent
call to "recv" will return them.
The "main loop"
Most event-based frameworks have something called a "main loop" or
"event loop run function" or something similar.
Just like in "recv" AnyEvent, these functions need to be called
eventually so that your event loop has a chance of actually looking for
the events you are interested in.
For example, in a Gtk2 program, the above example could also be written
like this:
use Gtk2 -init;
use AnyEvent;
############################################
# create a window and some label
my $window = new Gtk2::Window "toplevel";
$window->add (my $label = new Gtk2::Label "soon replaced by name");
$window->show_all;
############################################
# do our AnyEvent stuff
$| = 1; print "enter your name> ";
my $wait_for_input = AnyEvent->io (
fh => \*STDIN, poll => "r",
cb => sub {
# set the label
$label->set_text (scalar <STDIN>);
print "enter another name> ";
}
);
############################################
# Now enter Gtk2's event loop
main Gtk2;
No condition variable anywhere in sight - instead, we just read a line
from STDIN and replace the text in the label. In fact, since nobody
"undef"s $wait_for_input you can enter multiple lines.
Instead of waiting for a condition variable, the program enters the
Gtk2 main loop by calling "Gtk2->main", which will block the program
and wait for events to arrive.
This also shows that AnyEvent is quite flexible - you didn't have to do
anything to make the AnyEvent watcher use Gtk2 (actually Glib) - it
just worked.
Admittedly, the example is a bit silly - who would want to read names
from standard input in a Gtk+ application? But imagine that instead of
doing that, you make an HTTP request in the background and display its
results. In fact, with event-based programming you can make many HTTP
requests in parallel in your program and still provide feedback to the
user and stay interactive.
And in the next part you will see how to do just that - by implementing
an HTTP request, on our own, with the utility modules AnyEvent comes
with.
Before that, however, let's briefly look at how you would write your
program using only AnyEvent, without ever calling some other event
loop's run function.
In the example using condition variables, we used those to start
waiting for events, and in fact, condition variables are the solution:
my $quit_program = AnyEvent->condvar;
# create AnyEvent watchers (or not) here
$quit_program->recv;
If any of your watcher callbacks decide to quit (this is often called
an "unloop" in other frameworks), they can just call
"$quit_program->send". Of course, they could also decide not to and
call "exit" instead, or they could decide never to quit (e.g. in a
long-running daemon program).
If you don't need some clean quit functionality and just want to run
the event loop, you can do this:
AnyEvent->condvar->recv;
And this is, in fact, the closest to the idea of a main loop run
function that AnyEvent offers.
Timers and other event sources
So far, we have used only I/O watchers. These are useful mainly to find
out whether a socket has data to read, or space to write more data. On
sane operating systems this also works for console windows/terminals
(typically on standard input), serial lines, all sorts of other
devices, basically almost everything that has a file descriptor but
isn't a file itself. (As usual, "sane" excludes windows - on that
platform you would need different functions for all of these,
complicating code immensely - think "socket only" on windows).
However, I/O is not everything - the second most important event source
is the clock. For example when doing an HTTP request you might want to
time out when the server doesn't answer within some predefined amount
of time.
In AnyEvent, timer event watchers are created by calling the
"AnyEvent->timer" method:
use AnyEvent;
my $cv = AnyEvent->condvar;
my $wait_one_and_a_half_seconds = AnyEvent->timer (
after => 1.5, # after how many seconds to invoke the cb?
cb => sub { # the callback to invoke
$cv->send;
},
);
# can do something else here
# now wait till our time has come
$cv->recv;
Unlike I/O watchers, timers are only interested in the amount of
seconds they have to wait. When (at least) that amount of time has
passed, AnyEvent will invoke your callback.
Unlike I/O watchers, which will call your callback as many times as
there is data available, timers are normally one-shot: after they have
"fired" once and invoked your callback, they are dead and no longer do
anything.
To get a repeating timer, such as a timer firing roughly once per
second, you can specify an "interval" parameter:
my $once_per_second = AnyEvent->timer (
after => 0, # first invoke ASAP
interval => 1, # then invoke every second
cb => sub { # the callback to invoke
$cv->send;
},
);
More esoteric sources
AnyEvent also has some other, more esoteric event sources you can tap
into: signal, child and idle watchers.
Signal watchers can be used to wait for "signal events", which means
your process was sent a signal (such as "SIGTERM" or "SIGUSR1").
Child-process watchers wait for a child process to exit. They are
useful when you fork a separate process and need to know when it exits,
but you do not want to wait for that by blocking.
Idle watchers invoke their callback when the event loop has handled all
outstanding events, polled for new events and didn't find any, i.e.,
when your process is otherwise idle. They are useful if you want to do
some non-trivial data processing that can be done when your program
doesn't have anything better to do.
All these watcher types are described in detail in the main AnyEvent
manual page.
Sometimes you also need to know what the current time is:
"AnyEvent->now" returns the time the event toolkit uses to schedule
relative timers, and is usually what you want. It is often cached
(which means it can be a bit outdated). In that case, you can use the
more costly "AnyEvent->time" method which will ask your operating
system for the current time, which is slower, but also more up to date.
Network programming and AnyEvent
So far you have seen how to register event watchers and handle events.
This is a great foundation to write network clients and servers, and
might be all that your module (or program) ever requires, but writing
your own I/O buffering again and again becomes tedious, not to mention
that it attracts errors.
While the core AnyEvent module is still small and self-contained, the
distribution comes with some very useful utility modules such as
AnyEvent::Handle, AnyEvent::DNS and AnyEvent::Socket. These can make
your life as a non-blocking network programmer a lot easier.
Here is a quick overview of these three modules:
AnyEvent::DNS
This module allows fully asynchronous DNS resolution. It is used mainly
by AnyEvent::Socket to resolve hostnames and service ports for you, but
is a great way to do other DNS resolution tasks, such as reverse
lookups of IP addresses for log files.
AnyEvent::Handle
This module handles non-blocking IO on (socket-, pipe- etc.) file
handles in an event based manner. It provides a wrapper object around
your file handle that provides queueing and buffering of incoming and
outgoing data for you.
It also implements the most common data formats, such as text lines, or
fixed and variable-width data blocks.
AnyEvent::Socket
This module provides you with functions that handle socket creation and
IP address magic. The two main functions are "tcp_connect" and
"tcp_server". The former will connect a (streaming) socket to an
internet host for you and the later will make a server socket for you,
to accept connections.
This module also comes with transparent IPv6 support, this means: If
you write your programs with this module, you will be IPv6 ready
without doing anything special.
It also works around a lot of portability quirks (especially on the
windows platform), which makes it even easier to write your programs in
a portable way (did you know that windows uses different error codes
for all socket functions and that Perl does not know about these? That
"Unknown error 10022" (which is "WSAEINVAL") can mean that our
"connect" call was successful? That unsuccessful TCP connects might
never be reported back to your program? That "WSAEINPROGRESS" means
your "connect" call was ignored instead of being in progress?
AnyEvent::Socket works around all of these Windows/Perl bugs for you).
Implementing a parallel finger client with non-blocking connects and
AnyEvent::Socket
The finger protocol is one of the simplest protocols in use on the
internet. Or in use in the past, as almost nobody uses it anymore.
It works by connecting to the finger port on another host, writing a
single line with a user name and then reading the finger response, as
specified by that user. OK, RFC 1288 specifies a vastly more complex
protocol, but it basically boils down to this:
# telnet freebsd.org finger
Trying 8.8.178.135...
Connected to freebsd.org (8.8.178.135).
Escape character is '^]'.
larry
Login: lile Name: Larry Lile
Directory: /home/lile Shell: /usr/local/bin/bash
No Mail.
Mail forwarded to: lile@stdio.com
No Plan.
So let's write a little AnyEvent function that makes a finger request:
use AnyEvent;
use AnyEvent::Socket;
sub finger($$) {
my ($user, $host) = @_;
# use a condvar to return results
my $cv = AnyEvent->condvar;
# first, connect to the host
tcp_connect $host, "finger", sub {
# the callback receives the socket handle - or nothing
my ($fh) = @_
or return $cv->send;
# now write the username
syswrite $fh, "$user\015\012";
my $response;
# register a read watcher
my $read_watcher; $read_watcher = AnyEvent->io (
fh => $fh,
poll => "r",
cb => sub {
my $len = sysread $fh, $response, 1024, length $response;
if ($len <= 0) {
# we are done, or an error occurred, lets ignore the latter
undef $read_watcher; # no longer interested
$cv->send ($response); # send results
}
},
);
};
# pass $cv to the caller
$cv
}
That's a mouthful! Let's dissect this function a bit, first the overall
function and execution flow:
sub finger($$) {
my ($user, $host) = @_;
# use a condvar to return results
my $cv = AnyEvent->condvar;
# first, connect to the host
tcp_connect $host, "finger", sub {
...
};
$cv
}
This isn't too complicated, just a function with two parameters that
creates a condition variable $cv, initiates a TCP connect to $host, and
returns $cv. The caller can use the returned $cv to receive the finger
response, but one could equally well pass a third argument, a callback,
to the function.
Since we are programming event'ish, we do not wait for the connect to
finish - it could block the program for a minute or longer!
Instead, we pass "tcp_connect" a callback to invoke when the connect is
done. The callback is called with the socket handle as its first
argument if the connect succeeds, and no arguments otherwise. The
important point is that it will always be called as soon as the outcome
of the TCP connect is known.
This style of programming is also called "continuation style": the
"continuation" is simply the way the program continues - normally at
the next line after some statement (the exception is loops or things
like "return"). When we are interested in events, however, we instead
specify the "continuation" of our program by passing a closure, which
makes that closure the "continuation" of the program.
The "tcp_connect" call is like saying "return now, and when the
connection is established or the attempt failed, continue there".
Now let's look at the callback/closure in more detail:
# the callback receives the socket handle - or nothing
my ($fh) = @_
or return $cv->send;
The first thing the callback does is to save the socket handle in $fh.
When there was an error (no arguments), then our instinct as expert
Perl programmers would tell us to "die":
my ($fh) = @_
or die "$host: $!";
While this would give good feedback to the user (if he happens to watch
standard error), our program would probably stop working here, as we
never report the results to anybody, certainly not the caller of our
"finger" function, and most event loops continue even after a "die"!
This is why we instead "return", but also call "$cv->send" without any
arguments to signal to the condvar consumer that something bad has
happened. The return value of "$cv->send" is irrelevant, as is the
return value of our callback. The "return" statement is used for the
side effect of, well, returning immediately from the callback.
Checking for errors and handling them this way is very common, which is
why this compact idiom is so handy.
As the next step in the finger protocol, we send the username to the
finger daemon on the other side of our connection (the kernel.org
finger service doesn't actually wait for a username, but the net is
running out of finger servers fast):
syswrite $fh, "$user\015\012";
Note that this isn't 100% clean socket programming - the socket could,
for whatever reasons, not accept our data. When writing a small amount
of data like in this example it doesn't matter, as a socket buffer is
almost always big enough for a mere "username", but for real-world
cases you might need to implement some kind of write buffering - or use
AnyEvent::Handle, which handles these matters for you, as shown in the
next section.
What we do have to do is implement our own read buffer - the response
data could arrive late or in multiple chunks, and we cannot just wait
for it (event-based programming, you know?).
To do that, we register a read watcher on the socket which waits for
data:
my $read_watcher; $read_watcher = AnyEvent->io (
fh => $fh,
poll => "r",
There is a trick here, however: the read watcher isn't stored in a
global variable, but in a local one - if the callback returns, it would
normally destroy the variable and its contents, which would in turn
unregister our watcher.
To avoid that, we refer to the watcher variable in the watcher
callback. This means that, when the "tcp_connect" callback returns,
perl thinks (quite correctly) that the read watcher is still in use -
namely inside the inner callback - and thus keeps it alive even if
nothing else in the program refers to it anymore (it is much like Baron
Münchhausen keeping himself from dying by pulling himself out of a
swamp).
The trick, however, is that instead of:
my $read_watcher = AnyEvent->io (...
The program does:
my $read_watcher; $read_watcher = AnyEvent->io (...
The reason for this is a quirk in the way Perl works: variable names
declared with "my" are only visible in the next statement. If the whole
"AnyEvent->io" call, including the callback, would be done in a single
statement, the callback could not refer to the $read_watcher variable
to "undef"ine it, so it is done in two statements.
Whether you'd want to format it like this is of course a matter of
style. This way emphasizes that the declaration and assignment really
are one logical statement.
The callback itself calls "sysread" for as many times as necessary,
until "sysread" returns either an error or end-of-file:
cb => sub {
my $len = sysread $fh, $response, 1024, length $response;
if ($len <= 0) {
Note that "sysread" has the ability to append data it reads to a scalar
if we specify an offset, a feature which we make use of in this
example.
When "sysread" indicates we are done, the callback "undef"ines the
watcher and then "send"s the response data to the condition variable.
All this has the following effects:
Undefining the watcher destroys it, as our callback was the only one
still having a reference to it. When the watcher gets destroyed, it
destroys the callback, which in turn means the $fh handle is no longer
used, so that gets destroyed as well. The result is that all resources
will be nicely cleaned up by perl for us.
Using the finger client
Now, we could probably write the same finger client in a simpler way if
we used "IO::Socket::INET", ignored the problem of multiple hosts and
ignored IPv6 and a few other things that "tcp_connect" handles for us.
But the main advantage is that we can not only run this finger function
in the background, we even can run multiple sessions in parallel, like
this:
my $f1 = finger "kuriyama", "freebsd.org";
my $f2 = finger "icculus?listarchives=1", "icculus.org";
my $f3 = finger "mikachu", "icculus.org";
print "kuriyama's gpg key\n" , $f1->recv, "\n";
print "icculus' plan archive\n" , $f2->recv, "\n";
print "mikachu's plan zomgn\n" , $f3->recv, "\n";
It doesn't look like it, but in fact all three requests run in
parallel. The code waits for the first finger request to finish first,
but that doesn't keep it from executing them parallel: when the first
"recv" call sees that the data isn't ready yet, it serves events for
all three requests automatically, until the first request has finished.
The second "recv" call might either find the data is already there, or
it will continue handling events until that is the case, and so on.
By taking advantage of network latencies, which allows us to serve
other requests and events while we wait for an event on one socket, the
overall time to do these three requests will be greatly reduced,
typically all three are done in the same time as the slowest of the
three requests.
By the way, you do not actually have to wait in the "recv" method on an
AnyEvent condition variable - after all, waiting is evil - you can also
register a callback:
$f1->cb (sub {
my $response = shift->recv;
# ...
});
The callback will be invoked only when "send" is called. In fact,
instead of returning a condition variable you could also pass a third
parameter to your finger function, the callback to invoke with the
response:
sub finger($$$) {
my ($user, $host, $cb) = @_;
How you implement it is a matter of taste - if you expect your function
to be used mainly in an event-based program you would normally prefer
to pass a callback directly. If you write a module and expect your
users to use it "synchronously" often (for example, a simple http-get
script would not really care much for events), then you would use a
condition variable and tell them "simply "->recv" the data".
Problems with the implementation and how to fix them
To make this example more real-world-ready, we would not only implement
some write buffering (for the paranoid, or maybe denial-of-service
aware security expert), but we would also have to handle timeouts and
maybe protocol errors.
Doing this quickly gets unwieldy, which is why we introduce
AnyEvent::Handle in the next section, which takes care of all these
details for you and lets you concentrate on the actual protocol.
Implementing simple HTTP and HTTPS GET requests with AnyEvent::Handle
The AnyEvent::Handle module has been hyped quite a bit in this document
so far, so let's see what it really offers.
As finger is such a simple protocol, let's try something slightly more
complicated: HTTP/1.0.
An HTTP GET request works by sending a single request line that
indicates what you want the server to do and the URI you want to act it
on, followed by as many "header" lines ("Header: data", same as e-mail
headers) as required for the request, followed by an empty line.
The response is formatted very similarly, first a line with the
response status, then again as many header lines as required, then an
empty line, followed by any data that the server might send.
Again, let's try it out with "telnet" (I condensed the output a bit -
if you want to see the full response, do it yourself).
# telnet www.google.com 80
Trying 209.85.135.99...
Connected to www.google.com (209.85.135.99).
Escape character is '^]'.
GET /test HTTP/1.0
HTTP/1.0 404 Not Found
Date: Mon, 02 Jun 2008 07:05:54 GMT
Content-Type: text/html; charset=UTF-8
<html><head>
[...]
Connection closed by foreign host.
The "GET ..." and the empty line were entered manually, the rest of the
telnet output is google's response, in this case a "404 not found" one.
So, here is how you would do it with "AnyEvent::Handle":
sub http_get {
my ($host, $uri, $cb) = @_;
# store results here
my ($response, $header, $body);
my $handle; $handle = new AnyEvent::Handle
connect => [$host => 'http'],
on_error => sub {
$cb->("HTTP/1.0 500 $!");
$handle->destroy; # explicitly destroy handle
},
on_eof => sub {
$cb->($response, $header, $body);
$handle->destroy; # explicitly destroy handle
};
$handle->push_write ("GET $uri HTTP/1.0\015\012\015\012");
# now fetch response status line
$handle->push_read (line => sub {
my ($handle, $line) = @_;
$response = $line;
});
# then the headers
$handle->push_read (line => "\015\012\015\012", sub {
my ($handle, $line) = @_;
$header = $line;
});
# and finally handle any remaining data as body
$handle->on_read (sub {
$body .= $_[0]->rbuf;
$_[0]->rbuf = "";
});
}
And now let's go through it step by step. First, as usual, the overall
"http_get" function structure:
sub http_get {
my ($host, $uri, $cb) = @_;
# store results here
my ($response, $header, $body);
my $handle; $handle = new AnyEvent::Handle
... create handle object
... push data to write
... push what to expect to read queue
}
Unlike in the finger example, this time the caller has to pass a
callback to "http_get". Also, instead of passing a URL as one would
expect, the caller has to provide the hostname and URI - normally you
would use the "URI" module to parse a URL and separate it into those
parts, but that is left to the inspired reader :)
Since everything else is left to the caller, all "http_get" does is
initiate the connection by creating the AnyEvent::Handle object (which
calls "tcp_connect" for us) and leave everything else to its callback.
The handle object is created, unsurprisingly, by calling the "new"
method of AnyEvent::Handle:
my $handle; $handle = new AnyEvent::Handle
connect => [$host => 'http'],
on_error => sub {
$cb->("HTTP/1.0 500 $!");
$handle->destroy; # explicitly destroy handle
},
on_eof => sub {
$cb->($response, $header, $body);
$handle->destroy; # explicitly destroy handle
};
The "connect" argument tells AnyEvent::Handle to call "tcp_connect" for
the specified host and service/port.
The "on_error" callback will be called on any unexpected error, such as
a refused connection, or unexpected end-of-file while reading headers.
Instead of having an extra mechanism to signal errors, connection
errors are signalled by crafting a special "response status line", like
this:
HTTP/1.0 500 Connection refused
This means the caller cannot distinguish (easily) between locally-
generated errors and server errors, but it simplifies error handling
for the caller a lot.
The error callback also destroys the handle explicitly, because we are
not interested in continuing after any errors. In AnyEvent::Handle
callbacks you have to call "destroy" explicitly to destroy a handle.
Outside of those callbacks you can just forget the object reference and
it will be automatically cleaned up.
Last but not least, we set an "on_eof" callback that is called when the
other side indicates it has stopped writing data, which we will use to
gracefully shut down the handle and report the results. This callback
is only called when the read queue is empty - if the read queue expects
some data and the handle gets an EOF from the other side this will be
an error - after all, you did expect more to come.
If you wanted to write a server using AnyEvent::Handle, you would use
"tcp_accept" and then create the AnyEvent::Handle with the "fh"
argument.
The write queue
The next line sends the actual request:
$handle->push_write ("GET $uri HTTP/1.0\015\012\015\012");
No headers will be sent (this is fine for simple requests), so the
whole request is just a single line followed by an empty line to signal
the end of the headers to the server.
The more interesting question is why the method is called "push_write"
and not just write. The reason is that you can always add some write
data without blocking, and to do this, AnyEvent::Handle needs some
write queue internally - and "push_write" pushes some data onto the end
of that queue, just like Perl's "push" pushes data onto the end of an
array.
The deeper reason is that at some point in the future, there might be
"unshift_write" as well, and in any case, we will shortly meet
"push_read" and "unshift_read", and it's usually easiest to remember if
all those functions have some symmetry in their name. So "push" is used
as the opposite of "unshift" in AnyEvent::Handle, not as the opposite
of "pull" - just like in Perl.
Note that we call "push_write" right after creating the
AnyEvent::Handle object, before it has had time to actually connect to
the server. This is fine, pushing the read and write requests will
queue them in the handle object until the connection has been
established. Alternatively, we could do this "on demand" in the
"on_connect" callback.
If "push_write" is called with more than one argument, then you can do
formatted I/O. For example, this would JSON-encode your data before
pushing it to the write queue:
$handle->push_write (json => [1, 2, 3]);
This pretty much summarises the write queue, there is little else to
it.
Reading the response is far more interesting, because it involves the
more powerful and complex read queue:
The read queue
The response consists of three parts: a single line with the response
status, a single paragraph of headers ended by an empty line, and the
request body, which is the remaining data on the connection.
For the first two, we push two read requests onto the read queue:
# now fetch response status line
$handle->push_read (line => sub {
my ($handle, $line) = @_;
$response = $line;
});
# then the headers
$handle->push_read (line => "\015\012\015\012", sub {
my ($handle, $line) = @_;
$header = $line;
});
While one can just push a single callback to parse all the data on the
queue, formatted I/O really comes to our aid here, since there is a
ready-made "read line" read type. The first read expects a single line,
ended by "\015\012" (the standard end-of-line marker in internet
protocols).
The second "line" is actually a single paragraph - instead of reading
it line by line we tell "push_read" that the end-of-line marker is
really "\015\012\015\012", which is an empty line. The result is that
the whole header paragraph will be treated as a single line and read.
The word "line" is interpreted very freely, much like Perl itself does
it.
Note that push read requests are pushed immediately after creating the
handle object - since AnyEvent::Handle provides a queue we can push as
many requests as we want, and AnyEvent::Handle will handle them in
order.
There is, however, no read type for "the remaining data". For that, we
install our own "on_read" callback:
# and finally handle any remaining data as body
$handle->on_read (sub {
$body .= $_[0]->rbuf;
$_[0]->rbuf = "";
});
This callback is invoked every time data arrives and the read queue is
empty - which in this example will only be the case when both response
and header have been read. The "on_read" callback could actually have
been specified when constructing the object, but doing it this way
preserves logical ordering.
The read callback adds the current read buffer to its $body variable
and, most importantly, empties the buffer by assigning the empty string
to it.
Given these instructions, AnyEvent::Handle will handle incoming data -
if all goes well, the callback will be invoked with the response data;
if not, it will get an error.
In general, you can implement pipelining (a semi-advanced feature of
many protocols) very easily with AnyEvent::Handle: If you have a
protocol with a request/response structure, your request
methods/functions will all look like this (simplified):
sub request {
# send the request to the server
$handle->push_write (...);
# push some response handlers
$handle->push_read (...);
}
This means you can queue as many requests as you want, and while
AnyEvent::Handle goes through its read queue to handle the response
data, the other side can work on the next request - queueing the
request just appends some data to the write queue and installs a
handler to be called later.
You might ask yourself how to handle decisions you can only make after
you have received some data (such as handling a short error response or
a long and differently-formatted response). The answer to this problem
is "unshift_read", which we will introduce together with an example in
the coming sections.
Using "http_get"
Finally, here is how you would use "http_get":
http_get "www.google.com", "/", sub {
my ($response, $header, $body) = @_;
print
$response, "\n",
$body;
};
And of course, you can run as many of these requests in parallel as you
want (and your memory supports).
HTTPS
Now, as promised, let's implement the same thing for HTTPS, or more
correctly, let's change our "http_get" function into a function that
speaks HTTPS instead.
HTTPS is a standard TLS connection (Transport Layer Security is the
official name for what most people refer to as "SSL") that contains
standard HTTP protocol exchanges. The only other difference to HTTP is
that by default it uses port 443 instead of port 80.
To implement these two differences we need two tiny changes, first, in
the "connect" parameter, we replace "http" by "https" to connect to the
https port:
connect => [$host => 'https'],
The other change deals with TLS, which is something AnyEvent::Handle
does for us if the Net::SSLeay module is available. To enable TLS with
AnyEvent::Handle, we pass an additional "tls" parameter to the call to
"AnyEvent::Handle::new":
tls => "connect",
Specifying "tls" enables TLS, and the argument specifies whether
AnyEvent::Handle is the server side ("accept") or the client side
("connect") for the TLS connection, as unlike TCP, there is a clear
server/client relationship in TLS.
That's all.
Of course, all this should be handled transparently by "http_get" after
parsing the URL. If you need this, see the part about exercising your
inspiration earlier in this document. You could also use the
AnyEvent::HTTP module from CPAN, which implements all this and works
around a lot of quirks for you too.
The read queue - revisited
HTTP always uses the same structure in its responses, but many
protocols require parsing responses differently depending on the
response itself.
For example, in SMTP, you normally get a single response line:
220 mail.example.net Neverusesendmail 8.8.8 <mailme@example.net>
But SMTP also supports multi-line responses:
220-mail.example.net Neverusesendmail 8.8.8 <mailme@example.net>
220-hey guys
220 my response is longer than yours
To handle this, we need "unshift_read". As the name (we hope) implies,
"unshift_read" will not append your read request to the end of the read
queue, but will prepend it to the queue instead.
This is useful in the situation above: Just push your response-line
read request when sending the SMTP command, and when handling it, you
look at the line to see if more is to come, and "unshift_read" another
reader callback if required, like this:
my $response; # response lines end up in here
my $read_response; $read_response = sub {
my ($handle, $line) = @_;
$response .= "$line\n";
# check for continuation lines ("-" as 4th character")
if ($line =~ /^...-/) {
# if yes, then unshift another line read
$handle->unshift_read (line => $read_response);
} else {
# otherwise we are done
# free callback
undef $read_response;
print "we are don reading: $response\n";
}
};
$handle->push_read (line => $read_response);
This recipe can be used for all similar parsing problems, for example
in NNTP, the response code to some commands indicates that more data
will be sent:
$handle->push_write ("article 42");
# read response line
$handle->push_read (line => sub {
my ($handle, $status) = @_;
# article data following?
if ($status =~ /^2/) {
# yes, read article body
$handle->unshift_read (line => "\012.\015\012", sub {
my ($handle, $body) = @_;
$finish->($status, $body);
});
} else {
# some error occurred, no article data
$finish->($status);
}
}
Your own read queue handler
Sometimes your protocol doesn't play nice, and uses lines or chunks of
data not formatted in a way handled out of the box by AnyEvent::Handle.
In this case you have to implement your own read parser.
To make up a contorted example, imagine you are looking for an even
number of characters followed by a colon (":"). Also imagine that
AnyEvent::Handle has no "regex" read type which could be used, so you'd
have to do it manually.
To implement a read handler for this, you would "push_read" (or
"unshift_read") a single code reference.
This code reference will then be called each time there is (new) data
available in the read buffer, and is expected to either successfully
eat/consume some of that data (and return true) or to return false to
indicate that it wants to be called again.
If the code reference returns true, then it will be removed from the
read queue (because it has parsed/consumed whatever it was supposed to
consume), otherwise it stays in the front of it.
The example above could be coded like this:
$handle->push_read (sub {
my ($handle) = @_;
# check for even number of characters + ":"
# and remove the data if a match is found.
# if not, return false (actually nothing)
$handle->{rbuf} =~ s/^( (?:..)* ) ://x
or return;
# we got some data in $1, pass it to whoever wants it
$finish->($1);
# and return true to indicate we are done
1
});
Debugging aids
Now that you have seen how to use AnyEvent, here's what to use when you
don't use it correctly, or simply hit a bug somewhere and want to debug
it:
Enable strict argument checking during development
AnyEvent does not, by default, do any argument checking. This can
lead to strange and unexpected results especially if you are just
trying to find your way with AnyEvent.
AnyEvent supports a special "strict" mode - off by default - which
does very strict argument checking, at the expense of slowing down
your program. During development, however, this mode is very useful
because it quickly catches the msot common errors.
You can enable this strict mode either by having an environment
variable "AE_STRICT" with a true value in your environment:
AE_STRICT=1 perl myprog
Or you can write "use AnyEvent::Strict" in your program, which has
the same effect (do not do this in production, however).
Increase verbosity, configure logging
AnyEvent, by default, only logs critical messages. If something
doesn't work, maybe there was a warning about it that you didn't
see because it was suppressed.
So during development it is recommended to push up the logging
level to at least warn level (5):
AE_VERBOSE=5 perl myprog
Other levels that might be helpful are debug (8) or even trace (9).
AnyEvent's logging is quite versatile - the AnyEvent::Log manpage
has all the details.
Watcher wrapping, tracing, the shell
For even more debugging, you can enable watcher wrapping:
AE_DEBUG_WRAP=2 perl myprog
This will have the effect of wrapping every watcher into a special
object that stores a backtrace of when it was created, stores a
backtrace when an exception occurs during watcher execution, and
stores a lot of other information. If that slows down your program
too much, then "AE_DEBUG_WRAP=1" avoids the costly backtraces.
Here is an example of what of information is stored:
59148536 DC::DB:472(Server::run)>io>DC::DB::Server::fh_read
type: io watcher
args: poll r fh GLOB(0x35283f0)
created: 2011-09-01 23:13:46.597336 +0200 (1314911626.59734)
file: ./blib/lib/Deliantra/Client/private/DC/DB.pm
line: 472
subname: DC::DB::Server::run
context:
tracing: enabled
cb: CODE(0x2d1fb98) (DC::DB::Server::fh_read)
invoked: 0 times
created
(eval 25) line 6 AnyEvent::Debug::Wrap::__ANON__('AnyEvent','fh',GLOB(0x35283f0),'poll','r','cb',CODE(0x2d1fb98)=DC::DB::Server::fh_read)
DC::DB line 472 AE::io(GLOB(0x35283f0),'0',CODE(0x2d1fb98)=DC::DB::Server::fh_read)
bin/deliantra line 2776 DC::DB::Server::run()
bin/deliantra line 2941 main::main()
There are many ways to get at this data - see the AnyEvent::Debug
and AnyEvent::Log manpages for more details.
The most interesting and interactive way is to create a debug
shell, for example by setting "AE_DEBUG_SHELL":
AE_DEBUG_WRAP=2 AE_DEBUG_SHELL=$HOME/myshell ./myprog
# while myprog is running:
socat readline $HOME/myshell
Note that anybody who can access $HOME/myshell can make your
program do anything he or she wants, so if you are not the only
user on your machine, better put it into a secure location ($HOME
might not be secure enough).
If you don't have "socat" (a shame!) and care even less about
security, you can also use TCP and "telnet":
AE_DEBUG_WRAP=2 AE_DEBUG_SHELL=127.0.0.1:1234 ./myprog
telnet 127.0.0.1 1234
The debug shell can enable and disable tracing of watcher
invocations, can display the trace output, give you a list of
watchers and lets you investigate watchers in detail.
This concludes our little tutorial.
Where to go from here?
This introduction should have explained the key concepts of AnyEvent -
event watchers and condition variables, AnyEvent::Socket - basic
networking utilities, and AnyEvent::Handle - a nice wrapper around
sockets.
You could either start coding stuff right away, look at those manual
pages for the gory details, or roam CPAN for other AnyEvent modules
(such as AnyEvent::IRC or AnyEvent::HTTP) to see more code examples (or
simply to use them).
If you need a protocol that doesn't have an implementation using
AnyEvent, remember that you can mix AnyEvent with one other event
framework, such as POE, so you can always use AnyEvent for your own
tasks plus modules of one other event framework to fill any gaps.
And last not least, you could also look at Coro, especially
Coro::AnyEvent, to see how you can turn event-based programming from
callback style back to the usual imperative style (also called
"inversion of control" - AnyEvent calls you, but Coro lets you call
AnyEvent).
Authors
Robin Redeker "<elmex at ta-sa.org>", Marc Lehmann
<schmorp@schmorp.de>.
perl v5.36.0 2022-10-20 AnyEvent::Intro(3pm)
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