AnyEvent-MP/MP/Intro.pod

=head1 Message Passing for the Non-Blocked Mind

=head1 Introduction and Terminology

This is a tutorial about how to get the swing of the new L<AnyEvent::MP>
module. Which allows us to transparently pass messages to our own process
and to other processes on another or the same host.

What kind of messages? Well, basically a message here means a list of
Perl strings, numbers, hashes and arrays, mostly everything that can be
expressed as a L<JSON> text (as JSON is used by default in the protocol).

And next you might ask: between which entities are those messages
being "passed"? Effectively between I<nodes>: a nodes is basically a
process/program that use L<AnyEvent::MP> and can run either on the same or
different hosts.

To make this more managable, every node can contain any number of
I<ports>: Ports are ultimately the receivers of your messages.

In this tutorial I'll show you how to write a simple chat server based on
L<AnyEvent::MP>.

=head1 System Requirements

Before we can start we have to make sure some things work on your
system.

You should of course also make sure that L<AnyEvent> and L<AnyEvent::MP>
are installed. But how to do that is out of scope of this tutorial.

Then we have to setup a I<shared secret>. For two L<AnyEvent::MP> nodes to
be able to communicate with each other and authenticate each other it is
necessary to setup the same I<shared secret> for both of them. For testing
you can write a random string into the file C<.aemp-secret> in your home
directory:

   mcookie > ~/.aemp-secret

   # or something more predictable
   echo "secret123#4blabla_please_pick_your_own" > ~/.aemp-secret

Connections will only be successful when the nodes that want to connect
to each other have the same I<shared secret>. For more security, you can
put a self-signed SSL/TLS key/certificate pair into the file (or a normal
key/certificate and it's CA certificate).

B<If something does not work as expected, and for example tcpdump shows
that the connections are closed almost immediatly, you should make sure
that F<~/.aemp-secret> is the same on all hosts/user accounts that you try
to connect with each other!>

=head1 The Chat Client

OK, lets start by implementing the "frontend" of the client. We will
develop the client first and postpone the server for later, as the most
complex things actually happen in the client.

We will use L<AnyEvent::Handle> to do non-blocking IO read on standard
input (all of this code deals with actually handling user input, no
message passing yet):

   #!perl

   use AnyEvent;
   use AnyEvent::Handle;

   sub send_message {
      die "This is where we will send the messages to the server"
         . "in the next step of this tutorial.\n"
   }

   # make an AnyEvent condition variable for the 'quit' condition
   # (when we want to exit the client).
   my $quit_cv = AnyEvent->condvar;

   my $stdin_hdl = AnyEvent::Handle->new (
      fh       => *STDIN,
      on_error => sub { $quit_cv->send },
      on_read  => sub {
         my ($hdl) = @_;

         $hdl->push_read (line => sub {
            my ($hdl, $line) = @_;

            if ($line =~ /^\/quit/) { # /quit will end the client
               $quit_cv->send;
            } else {
               send_message ($line);
            }
         });
      }
   );

   $quit_cv->recv;

This is now a very basic client. Explaining explicitly what
L<AnyEvent::Handle> does or what a I<condvar> is all about is out of scope
of this document, please consult L<AnyEvent::Intro> or the manual pages
for L<AnyEvent> and L<AnyEvent::Handle>.

=head1 First Steps Into Messaging

To supply the C<send_message> function we now take a look at
L<AnyEvent::MP>. This is an example of how it might look like:

   ... # the use lines from the above snippet

   use AnyEvent::MP;

   sub send_message {
      my ($msg) = @_;

      snd $server_port, message => $msg;
   }

   ... # the rest of the above script

The C<snd> function is exported by L<AnyEvent::MP>, it stands for 'send
a message'. The first argument is the I<port> (a I<port> is something
that can receive messages, represented by a printable string) of the
server which will receive the message. How we get this port will be
explained in the next step.

The remaining arguments of C<snd> are C<message> and C<$msg>, the first
two elements of the I<message> (a I<message> in L<AnyEvent::MP> is a
simple list of values, which can be sent to a I<port>).

So all the function does is send the two values C<message> (a constant
string to tell the server what to expect) and the actual message string.

Thats all fine and simple so far, but where do we get the
C<$server_port>? Well, we need to get the unique I<port id> of the
server's port where it wants to receive all the incoming chat messages. A
I<port id> is unfortunately a very unique string, which we are unable to
know in advance. But L<AnyEvent::MP> supports the concept of 'registered
ports', which is basically a port on the server side registered under
a well known name.

For example, the server has a port for receiving chat messages with a
unique I<port id> and registers it under the name C<chatter>.

BTW, these "registered port names" should follow similar rules as Perl
identifiers, so you should prefix them with your package/module name to
make them unique, unless you use them in the main program.

As I<messages> can only be sent to a I<port id> and not just to some name
we have to ask the server node for the I<port id> of the port registered
as C<chatter>.

=head1 Finding The Chatter Port

Ok, lots of talk, now some code. Now we will actually get the
C<$server_port> from the backend:

   ...

   use AnyEvent::MP;

   my $server_node = "127.0.0.1:1299";

   my $client_port = port;

   snd $server_node, lookup => "chatter", $client_port, "resolved";

   my $resolved_cv = AnyEvent->condvar;
   my $server_port;

   # setup a receiver callback for the 'resolved' message:
   rcv $client_port, resolved => sub {
      my ($tag, $chatter_port_id) = @_;

      print "Resolved the server port 'chatter' to $chatter_port_id\n";
      $server_port = $chatter_port_id;

      $resolved_cv->send;
      1
   };

   # lets block the client until we have resolved the server port.
   $resolved_cv->recv;

   # now setup another receiver callback for the chat messages:
   rcv $client_port, message => sub {
      my ($tag, $msg) = @_;

      print "chat> $msg\n";
      0
   };

   # send a 'join' message to the server:
   snd $server_port, join => "$client_port";

   sub send_message { ...

Now that was a lot of new stuff:

First we define the C<$server_node>: In order to refer to another node
we need some kind of string to reference it - the node reference. The
I<noderef> is basically a comma separated list of C<address:port>
pairs. We assume in this tutorial that the server runs on C<127.0.0.1>
(localhost) on port 1299, which results in the noderef C<127.0.0.1:1299>.

Next, in order to receive a reply from the other node or the server we
need to have a I<port> that messages can be sent to. This is what the
C<port> function will do for us, it just creates a new local port and
returns it's I<port ID> that can then be used to receive messages.

When you look carefully, you will see that the first C<snd> uses the
C<$server_node> (a noderef) as destination port. Well, what I didn't
tell you yet is that each I<node> has a default I<port> to receive
messages. The ID of this port is the same as the noderef.

This I<default port> provides some special services for us, for example
resolving a registered name to a I<port id> (a-ha! finally!).

This is exactly what this line does:

   snd $server_node, lookup => "chatter", $client_port, "resolved";

This sends a message with first element being C<lookup>, followed by the
(hopefully) registered port name that we want to resolve to a I<port
id>: C<chatter>. And in order for the server node to be able to send us
back the resolved I<port ID> we have to tell it where to send it: The
result message will be sent to C<$client_port> (the I<port id> of the
port we just created), and will have the string C<resolved> as the first
element.

When the node receives this message, it will look up the name, gobble up
all the extra arguments we passed, append the resolved name, and send the
resulting list as a message.

Next we register a receiver for this C<lookup>-request.

   rcv $client_port, resolved => sub {
      my ($tag, $chatter_port_id) = @_;
      ...
      1
   };

This sets up a receiver on our own port for messages with the first
element being the string C<resolved>. Receivers can match the contents of
the messages before actually executing the specified callback.

B<Please note> that the every C<rcv> callback has to return either a true
or a false value, indicating whether it is B<successful>/B<done> (true) or
still wants to B<continue> (false) receiving messages.

In this case we tell the C<$client_port> to look into all the messages
it receives and look for the string C<resolved> in the first element of
the message. If it is found, the given callback will be called with the
message elements as arguments.

Using a string as the first element of the message is called I<tagging>
the message. It's common practise to code the 'type' of a message into
it's first element, as this allows for simple matching.

The result message will contain the I<port ID> of the well known port
C<chatter> as second element, which will be stored in C<$chatter_port_id>.

This port ID will then be stored in C<$server_port>, followed by calling
C<send> on $resolved_cv> so the program will continue.

The callback then returns a C<1> (a true value), to indicate that it has
done it's job and doesn't want to receive further C<resolved> messages.

After this the chat message receiver callback is registered with the port:

   rcv $client_port, message => sub {
      my ($tag, $msg) = @_;

      print "chat> $msg\n";

      0
   };

We assume that all messages that are broadcast to the clients by the
server contain the string tag C<message> as first element, and the actual
message as second element. The callback returns a false value this time,
to indicate that it is not yet done and wants to receive further messages.

The last thing to do is to tell the server to send us new chat messages
from other clients. We do so by sending the message C<join> followed by
our own I<port ID>.

   # send the server a 'join' message:
   snd $server_port, join => $client_port;

This way the server knows where to send all the new messages to.

=head1 The Completed Client

This is the complete client script:

   #!perl

   use AnyEvent;
   use AnyEvent::Handle;
   use AnyEvent::MP;

   my $resolved_cv = AnyEvent->condvar;

   my $client_port = create_port;

   my $server_node = "localhost:1299#";

   snd $server_node, wkp => "chatter", "$client_port", "resolved";

   my $server_port;

   # setup a receiver callback for the 'resolved' message:
   $client_port->rcv (resolved => sub {
      my ($client_port, $type, $chatter_port_id) = @_;

      print "Resolved the server port 'chatter' to $chatter_port_id\n";
      $server_port = $chatter_port_id;

      $resolved_cv->send;
      1
   });

   # lets block the client until we resolved the server port.
   $resolved_cv->recv;

   # now setup another receiver callback for the chat messages:
   $client_port->rcv (message => sub {
      my ($client_port, $type, $msg) = @_;

      print "chat> $msg\n";
      0
   });

   # send the server a 'join' message:
   snd $server_port, join => "$client_port";

   sub send_message {
      my ($msg) = @_;

      snd $server_port, message => $msg;
   }

   # make an AnyEvent condition variable for the 'quit' condition
   # (when we want to exit the client).
   my $quit_cv = AnyEvent->condvar;

   my $stdin_hdl = AnyEvent::Handle->new (
      fh      => \*STDIN,
      on_read => sub {
         my ($hdl) = @_;

         $hdl->push_read (line => sub {
            my ($hdl, $line) = @_;

            if ($line =~ /^\/quit/) { # /quit will end the client
               $quit_cv->send;

            } else {
               send_message ($line);
            }
         });
      }
   );

   $quit_cv->recv;

=head1 The Server

Ok, now finally to the server. What do we need? Well, we need to setup
the well known port C<chatter> where all clients send their messages to.

Up and into code right now:

   #!perl

   use AnyEvent;
   use AnyEvent::MP;

   become_public "localhost:1299";

   my $chatter_port = create_port;
   $chatter_port->register ("chatter");

   my %client_ports;

   $chatter_port->rcv (join => sub {
      my ($chatter_port, $type, $client_port) = @_;

      print "got new client port: $client_port\n";

      $client_ports{$client_port} = 1;
      0
   });

   $chatter_port->rcv (message => sub {
      my ($chatter_port, $type, $msg) = @_;

      print "message> $msg\n";

      snd $_, message => $msg for keys %client_ports;
      0
   });

   AnyEvent->condvar->recv;

This is all. Looks much easier, doesn't it? I'll explain it only shortly, as
we had the discussion of the C<rcv> method in the client part of this tutorial
above.

First this:

   become_public "localhost:1299";

This will tell our I<node> to become a I<public> node, which means that it can
be contacted via TCP. The first argument should be the I<noderef> the server
wants to be reachable at. In this case it's the TCP port 1299 on localhost.

Next we basically setup two receivers, one for the C<join> messages and
another one for the actual messages of type C<messsage>.

In the C<join> message we get the client's port, which we just remember in the
C<%client_ports> hash. In the receiver for the message type C<message> we will
just iterate through all known C<%client_ports> and relay the message to them.

And thats it.

=head1 The Remaining Problems

The shown implementation still has some bugs. For instance: How does the
server know that the client isn't there anymore, and can cleanup the
C<%client_ports> hash? And also the chat messages have no originator,
so we don't know who actually sent the message (which would be quite useful
for human-to-human interaction: to know who the other one is :).

But aside from these issues I hope this tutorial got you the swing of
L<AnyEvent::MP> and explained some common idioms.

How to solve the reliability and C<%client_ports> cleanup problem will
be explained later in this tutorial (TODO).

=head1 Inside The Protocol

Now, for the interested parties, let me explain some details about the protocol
that L<AnyEvent::MP> nodes use to communicate to each other. If you are not
interested you can skip this section.

Usually TCP is used for communication. Each I<node>, if configured to be a
I<public> node with the C<become_public> function will listen on the configured
TCP port (default is usually 4040).

If now one I<node> wants to send a message to another I<node> it will connect
to the host and port given in the I<port id>.

Then some handshaking occurs to check whether both I<nodes> have the same
I<shared secret> configured. Optionally even TLS can be enabled (about how to
do this exactly please consult the L<AnyEvent::MP> man pages, just a hint: It
should be enough to put the private key and (self signed) certificate in the
C<~/.aemp-secret> file of all nodes).

Now the serialized (usually L<JSON> is used for this, but it is also possible
to use other serialization formats, like L<Storable>) messages are sent over
the wire using a simple framing protocol.

=head1 SEE ALSO

L<AnyEvent>

L<AnyEvent::Handle>

L<AnyEvent::MP>

=head1 AUTHOR

  Robin Redeker <elmex@ta-sa.org>

Revision:	1.9
Committed:	Tue Aug 4 20:47:39 2009 UTC (14 years, 10 months ago) by root
Branch:	MAIN
Changes since 1.8:	+109 -104 lines
Log Message:	* empty log message *
#	Content
1	=head1 Message Passing for the Non-Blocked Mind
2
3	=head1 Introduction and Terminology
4
5	This is a tutorial about how to get the swing of the new L<AnyEvent::MP>
6	module. Which allows us to transparently pass messages to our own process
7	and to other processes on another or the same host.
8
9	What kind of messages? Well, basically a message here means a list of
10	Perl strings, numbers, hashes and arrays, mostly everything that can be
11	expressed as a L<JSON> text (as JSON is used by default in the protocol).
12
13	And next you might ask: between which entities are those messages
14	being "passed"? Effectively between I<nodes>: a nodes is basically a
15	process/program that use L<AnyEvent::MP> and can run either on the same or
16	different hosts.
17
18	To make this more managable, every node can contain any number of
19	I<ports>: Ports are ultimately the receivers of your messages.
20
21	In this tutorial I'll show you how to write a simple chat server based on
22	L<AnyEvent::MP>.
23
24	=head1 System Requirements
25
26	Before we can start we have to make sure some things work on your
27	system.
28
29	You should of course also make sure that L<AnyEvent> and L<AnyEvent::MP>
30	are installed. But how to do that is out of scope of this tutorial.
31
32	Then we have to setup a I<shared secret>. For two L<AnyEvent::MP> nodes to
33	be able to communicate with each other and authenticate each other it is
34	necessary to setup the same I<shared secret> for both of them. For testing
35	you can write a random string into the file C<.aemp-secret> in your home
36	directory:
37
38	mcookie > ~/.aemp-secret
39
40	# or something more predictable
41	echo "secret123#4blabla_please_pick_your_own" > ~/.aemp-secret
42
43	Connections will only be successful when the nodes that want to connect
44	to each other have the same I<shared secret>. For more security, you can
45	put a self-signed SSL/TLS key/certificate pair into the file (or a normal
46	key/certificate and it's CA certificate).
47
48	B<If something does not work as expected, and for example tcpdump shows
49	that the connections are closed almost immediatly, you should make sure
50	that F<~/.aemp-secret> is the same on all hosts/user accounts that you try
51	to connect with each other!>
52
53	=head1 The Chat Client
54
55	OK, lets start by implementing the "frontend" of the client. We will
56	develop the client first and postpone the server for later, as the most
57	complex things actually happen in the client.
58
59	We will use L<AnyEvent::Handle> to do non-blocking IO read on standard
60	input (all of this code deals with actually handling user input, no
61	message passing yet):
62
63	#!perl
64
65	use AnyEvent;
66	use AnyEvent::Handle;
67
68	sub send_message {
69	die "This is where we will send the messages to the server"
70	. "in the next step of this tutorial.\n"
71	}
72
73	# make an AnyEvent condition variable for the 'quit' condition
74	# (when we want to exit the client).
75	my $quit_cv = AnyEvent->condvar;
76
77	my $stdin_hdl = AnyEvent::Handle->new (
78	fh => *STDIN,
79	on_error => sub { $quit_cv->send },
80	on_read => sub {
81	my ($hdl) = @_;
82
83	$hdl->push_read (line => sub {
84	my ($hdl, $line) = @_;
85
86	if ($line =~ /^\/quit/) { # /quit will end the client
87	$quit_cv->send;
88	} else {
89	send_message ($line);
90	}
91	});
92	}
93	);
94
95	$quit_cv->recv;
96
97	This is now a very basic client. Explaining explicitly what
98	L<AnyEvent::Handle> does or what a I<condvar> is all about is out of scope
99	of this document, please consult L<AnyEvent::Intro> or the manual pages
100	for L<AnyEvent> and L<AnyEvent::Handle>.
101
102	=head1 First Steps Into Messaging
103
104	To supply the C<send_message> function we now take a look at
105	L<AnyEvent::MP>. This is an example of how it might look like:
106
107	... # the use lines from the above snippet
108
109	use AnyEvent::MP;
110
111	sub send_message {
112	my ($msg) = @_;
113
114	snd $server_port, message => $msg;
115	}
116
117	... # the rest of the above script
118
119	The C<snd> function is exported by L<AnyEvent::MP>, it stands for 'send
120	a message'. The first argument is the I<port> (a I<port> is something
121	that can receive messages, represented by a printable string) of the
122	server which will receive the message. How we get this port will be
123	explained in the next step.
124
125	The remaining arguments of C<snd> are C<message> and C<$msg>, the first
126	two elements of the I<message> (a I<message> in L<AnyEvent::MP> is a
127	simple list of values, which can be sent to a I<port>).
128
129	So all the function does is send the two values C<message> (a constant
130	string to tell the server what to expect) and the actual message string.
131
132	Thats all fine and simple so far, but where do we get the
133	C<$server_port>? Well, we need to get the unique I<port id> of the
134	server's port where it wants to receive all the incoming chat messages. A
135	I<port id> is unfortunately a very unique string, which we are unable to
136	know in advance. But L<AnyEvent::MP> supports the concept of 'registered
137	ports', which is basically a port on the server side registered under
138	a well known name.
139
140	For example, the server has a port for receiving chat messages with a
141	unique I<port id> and registers it under the name C<chatter>.
142
143	BTW, these "registered port names" should follow similar rules as Perl
144	identifiers, so you should prefix them with your package/module name to
145	make them unique, unless you use them in the main program.
146
147	As I<messages> can only be sent to a I<port id> and not just to some name
148	we have to ask the server node for the I<port id> of the port registered
149	as C<chatter>.
150
151	=head1 Finding The Chatter Port
152
153	Ok, lots of talk, now some code. Now we will actually get the
154	C<$server_port> from the backend:
155
156	...
157
158	use AnyEvent::MP;
159
160	my $server_node = "127.0.0.1:1299";
161
162	my $client_port = port;
163
164	snd $server_node, lookup => "chatter", $client_port, "resolved";
165
166	my $resolved_cv = AnyEvent->condvar;
167	my $server_port;
168
169	# setup a receiver callback for the 'resolved' message:
170	rcv $client_port, resolved => sub {
171	my ($tag, $chatter_port_id) = @_;
172
173	print "Resolved the server port 'chatter' to $chatter_port_id\n";
174	$server_port = $chatter_port_id;
175
176	$resolved_cv->send;
177	1
178	};
179
180	# lets block the client until we have resolved the server port.
181	$resolved_cv->recv;
182
183	# now setup another receiver callback for the chat messages:
184	rcv $client_port, message => sub {
185	my ($tag, $msg) = @_;
186
187	print "chat> $msg\n";
188	0
189	};
190
191	# send a 'join' message to the server:
192	snd $server_port, join => "$client_port";
193
194	sub send_message { ...
195
196	Now that was a lot of new stuff:
197
198	First we define the C<$server_node>: In order to refer to another node
199	we need some kind of string to reference it - the node reference. The
200	I<noderef> is basically a comma separated list of C<address:port>
201	pairs. We assume in this tutorial that the server runs on C<127.0.0.1>
202	(localhost) on port 1299, which results in the noderef C<127.0.0.1:1299>.
203
204	Next, in order to receive a reply from the other node or the server we
205	need to have a I<port> that messages can be sent to. This is what the
206	C<port> function will do for us, it just creates a new local port and
207	returns it's I<port ID> that can then be used to receive messages.
208
209	When you look carefully, you will see that the first C<snd> uses the
210	C<$server_node> (a noderef) as destination port. Well, what I didn't
211	tell you yet is that each I<node> has a default I<port> to receive
212	messages. The ID of this port is the same as the noderef.
213
214	This I<default port> provides some special services for us, for example
215	resolving a registered name to a I<port id> (a-ha! finally!).
216
217	This is exactly what this line does:
218
219	snd $server_node, lookup => "chatter", $client_port, "resolved";
220
221	This sends a message with first element being C<lookup>, followed by the
222	(hopefully) registered port name that we want to resolve to a I<port
223	id>: C<chatter>. And in order for the server node to be able to send us
224	back the resolved I<port ID> we have to tell it where to send it: The
225	result message will be sent to C<$client_port> (the I<port id> of the
226	port we just created), and will have the string C<resolved> as the first
227	element.
228
229	When the node receives this message, it will look up the name, gobble up
230	all the extra arguments we passed, append the resolved name, and send the
231	resulting list as a message.
232
233	Next we register a receiver for this C<lookup>-request.
234
235	rcv $client_port, resolved => sub {
236	my ($tag, $chatter_port_id) = @_;
237	...
238	1
239	};
240
241	This sets up a receiver on our own port for messages with the first
242	element being the string C<resolved>. Receivers can match the contents of
243	the messages before actually executing the specified callback.
244
245	B<Please note> that the every C<rcv> callback has to return either a true
246	or a false value, indicating whether it is B<successful>/B<done> (true) or
247	still wants to B<continue> (false) receiving messages.
248
249	In this case we tell the C<$client_port> to look into all the messages
250	it receives and look for the string C<resolved> in the first element of
251	the message. If it is found, the given callback will be called with the
252	message elements as arguments.
253
254	Using a string as the first element of the message is called I<tagging>
255	the message. It's common practise to code the 'type' of a message into
256	it's first element, as this allows for simple matching.
257
258	The result message will contain the I<port ID> of the well known port
259	C<chatter> as second element, which will be stored in C<$chatter_port_id>.
260
261	This port ID will then be stored in C<$server_port>, followed by calling
262	C<send> on $resolved_cv> so the program will continue.
263
264	The callback then returns a C<1> (a true value), to indicate that it has
265	done it's job and doesn't want to receive further C<resolved> messages.
266
267	After this the chat message receiver callback is registered with the port:
268
269	rcv $client_port, message => sub {
270	my ($tag, $msg) = @_;
271
272	print "chat> $msg\n";
273
274	0
275	};
276
277	We assume that all messages that are broadcast to the clients by the
278	server contain the string tag C<message> as first element, and the actual
279	message as second element. The callback returns a false value this time,
280	to indicate that it is not yet done and wants to receive further messages.
281
282	The last thing to do is to tell the server to send us new chat messages
283	from other clients. We do so by sending the message C<join> followed by
284	our own I<port ID>.
285
286	# send the server a 'join' message:
287	snd $server_port, join => $client_port;
288
289	This way the server knows where to send all the new messages to.
290
291	=head1 The Completed Client
292
293	This is the complete client script:
294
295	#!perl
296
297	use AnyEvent;
298	use AnyEvent::Handle;
299	use AnyEvent::MP;
300
301	my $resolved_cv = AnyEvent->condvar;
302
303	my $client_port = create_port;
304
305	my $server_node = "localhost:1299#";
306
307	snd $server_node, wkp => "chatter", "$client_port", "resolved";
308
309	my $server_port;
310
311	# setup a receiver callback for the 'resolved' message:
312	$client_port->rcv (resolved => sub {
313	my ($client_port, $type, $chatter_port_id) = @_;
314
315	print "Resolved the server port 'chatter' to $chatter_port_id\n";
316	$server_port = $chatter_port_id;
317
318	$resolved_cv->send;
319	1
320	});
321
322	# lets block the client until we resolved the server port.
323	$resolved_cv->recv;
324
325	# now setup another receiver callback for the chat messages:
326	$client_port->rcv (message => sub {
327	my ($client_port, $type, $msg) = @_;
328
329	print "chat> $msg\n";
330	0
331	});
332
333	# send the server a 'join' message:
334	snd $server_port, join => "$client_port";
335
336	sub send_message {
337	my ($msg) = @_;
338
339	snd $server_port, message => $msg;
340	}
341
342	# make an AnyEvent condition variable for the 'quit' condition
343	# (when we want to exit the client).
344	my $quit_cv = AnyEvent->condvar;
345
346	my $stdin_hdl = AnyEvent::Handle->new (
347	fh => \*STDIN,
348	on_read => sub {
349	my ($hdl) = @_;
350
351	$hdl->push_read (line => sub {
352	my ($hdl, $line) = @_;
353
354	if ($line =~ /^\/quit/) { # /quit will end the client
355	$quit_cv->send;
356
357	} else {
358	send_message ($line);
359	}
360	});
361	}
362	);
363
364	$quit_cv->recv;
365
366	=head1 The Server
367
368	Ok, now finally to the server. What do we need? Well, we need to setup
369	the well known port C<chatter> where all clients send their messages to.
370
371	Up and into code right now:
372
373	#!perl
374
375	use AnyEvent;
376	use AnyEvent::MP;
377
378	become_public "localhost:1299";
379
380	my $chatter_port = create_port;
381	$chatter_port->register ("chatter");
382
383	my %client_ports;
384
385	$chatter_port->rcv (join => sub {
386	my ($chatter_port, $type, $client_port) = @_;
387
388	print "got new client port: $client_port\n";
389
390	$client_ports{$client_port} = 1;
391	0
392	});
393
394	$chatter_port->rcv (message => sub {
395	my ($chatter_port, $type, $msg) = @_;
396
397	print "message> $msg\n";
398
399	snd $_, message => $msg for keys %client_ports;
400	0
401	});
402
403	AnyEvent->condvar->recv;
404
405	This is all. Looks much easier, doesn't it? I'll explain it only shortly, as
406	we had the discussion of the C<rcv> method in the client part of this tutorial
407	above.
408
409	First this:
410
411	become_public "localhost:1299";
412
413	This will tell our I<node> to become a I<public> node, which means that it can
414	be contacted via TCP. The first argument should be the I<noderef> the server
415	wants to be reachable at. In this case it's the TCP port 1299 on localhost.
416
417	Next we basically setup two receivers, one for the C<join> messages and
418	another one for the actual messages of type C<messsage>.
419
420	In the C<join> message we get the client's port, which we just remember in the
421	C<%client_ports> hash. In the receiver for the message type C<message> we will
422	just iterate through all known C<%client_ports> and relay the message to them.
423
424	And thats it.
425
426	=head1 The Remaining Problems
427
428	The shown implementation still has some bugs. For instance: How does the
429	server know that the client isn't there anymore, and can cleanup the
430	C<%client_ports> hash? And also the chat messages have no originator,
431	so we don't know who actually sent the message (which would be quite useful
432	for human-to-human interaction: to know who the other one is :).
433
434	But aside from these issues I hope this tutorial got you the swing of
435	L<AnyEvent::MP> and explained some common idioms.
436
437	How to solve the reliability and C<%client_ports> cleanup problem will
438	be explained later in this tutorial (TODO).
439
440	=head1 Inside The Protocol
441
442	Now, for the interested parties, let me explain some details about the protocol
443	that L<AnyEvent::MP> nodes use to communicate to each other. If you are not
444	interested you can skip this section.
445
446	Usually TCP is used for communication. Each I<node>, if configured to be a
447	I<public> node with the C<become_public> function will listen on the configured
448	TCP port (default is usually 4040).
449
450	If now one I<node> wants to send a message to another I<node> it will connect
451	to the host and port given in the I<port id>.
452
453	Then some handshaking occurs to check whether both I<nodes> have the same
454	I<shared secret> configured. Optionally even TLS can be enabled (about how to
455	do this exactly please consult the L<AnyEvent::MP> man pages, just a hint: It
456	should be enough to put the private key and (self signed) certificate in the
457	C<~/.aemp-secret> file of all nodes).
458
459	Now the serialized (usually L<JSON> is used for this, but it is also possible
460	to use other serialization formats, like L<Storable>) messages are sent over
461	the wire using a simple framing protocol.
462
463	=head1 SEE ALSO
464
465	L<AnyEvent>
466
467	L<AnyEvent::Handle>
468
469	L<AnyEvent::MP>
470
471	=head1 AUTHOR
472
473	Robin Redeker <elmex@ta-sa.org>
474