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 $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 {
      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_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;

=head1 The Server

Ok, we finally come to the server.

The server of course also needs to set up a port, and in addition needs to
I<register> it, so the clients can find it.

Again, let's jump directly into the code:

   #!perl

   use AnyEvent;
   use AnyEvent::MP;

   become_public "127.0.0.1:1299";

   my $chatter_port = port;

   reg $chatter_port, "chatter";

   my %client_ports;

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

         print "got new client port: $client_port\n";
         $client_ports{$client_port} = 1;

         0
      },
      message => sub {
         my ($tag, $msg) = @_;

         print "message> $msg\n";

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

         0
      };

   AnyEvent->condvar->recv;

That is all. Looks much simpler than the client, doesn't it?
   
Let's quickly look over it, as C<rcv> has already been discussed in the
client part of this tutorial above.

First this:

   become_public "127.0.0.1:1299";

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
C<127.0.0.1>.

Next we set up two receivers, one for the C<join> messages and another one
for the actual messages of type C<messsage>. This is done with a single
call to C<rcv>, which allows multiple C<< match => $callback >> pairs.

In the C<join> callback we receive the client port, which is simply
remembered in the C<%client_ports> hash. In the C<message> callback we
just iterate through all known C<%client_ports> and relay the message to
them.

That concludes the server.

=head1 The Remaining Problems

The implementation as shown still has some bugs. For instance: How does
the server know that the client isn't there anymore, so it can clean up
the C<%client_ports> hash? 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 showed you the basics 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 4040).

If then 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> know the
I<shared secret>. Optionally, TLS can be enabled (about how to do this
exactly please consult the L<AnyEvent::MP> man page, 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).

After the handshake, messages will be exchanged using a serialiser
(usually L<JSON> is used for this, but it is also possible to use other
serialization formats such as L<Storable>).

=head1 SEE ALSO

L<AnyEvent>

L<AnyEvent::Handle>

L<AnyEvent::MP>

=head1 AUTHOR

  Robin Redeker <elmex@ta-sa.org>

Revision:	1.11
Committed:	Tue Aug 4 21:06:47 2009 UTC (14 years, 10 months ago) by root
Branch:	MAIN
CVS Tags:	rel-0_1, rel-0_4
Changes since 1.10:	+1 -1 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 $server_node = "127.0.0.1:1299";
302
303	my $client_port = port;
304
305	snd $server_node, lookup => "chatter", $client_port, "resolved";
306
307	my $resolved_cv = AnyEvent->condvar;
308	my $server_port;
309
310	# setup a receiver callback for the 'resolved' message:
311	rcv $client_port, resolved => sub {
312	my ($tag, $chatter_port_id) = @_;
313
314	print "Resolved the server port 'chatter' to $chatter_port_id\n";
315	$server_port = $chatter_port_id;
316
317	$resolved_cv->send;
318	1
319	};
320
321	# lets block the client until we have resolved the server port.
322	$resolved_cv->recv;
323
324	# now setup another receiver callback for the chat messages:
325	rcv $client_port, message => sub {
326	my ($tag, $msg) = @_;
327
328	print "chat> $msg\n";
329	0
330	};
331
332	# send a 'join' message to the server:
333	snd $server_port, join => "$client_port";
334
335	sub send_message {
336	my ($msg) = @_;
337
338	snd $server_port, message => $msg;
339	}
340
341	# make an AnyEvent condition variable for the 'quit' condition
342	# (when we want to exit the client).
343	my $quit_cv = AnyEvent->condvar;
344
345	my $stdin_hdl = AnyEvent::Handle->new (
346	fh => *STDIN,
347	on_error => sub { $quit_cv->send },
348	on_read => sub {
349	my ($hdl) = @_;
350
351	$hdl->push_read (line => sub {
352	my ($hdl, $line) = @_;
353
354	if ($line =~ /^\/quit/) { # /quit will end the client
355	$quit_cv->send;
356	} else {
357	send_message ($line);
358	}
359	});
360	}
361	);
362
363	$quit_cv->recv;
364
365	=head1 The Server
366
367	Ok, we finally come to the server.
368
369	The server of course also needs to set up a port, and in addition needs to
370	I<register> it, so the clients can find it.
371
372	Again, let's jump directly into the code:
373
374	#!perl
375
376	use AnyEvent;
377	use AnyEvent::MP;
378
379	become_public "127.0.0.1:1299";
380
381	my $chatter_port = port;
382
383	reg $chatter_port, "chatter";
384
385	my %client_ports;
386
387	rcv $chatter_port,
388	join => sub {
389	my ($tag, $client_port) = @_;
390
391	print "got new client port: $client_port\n";
392	$client_ports{$client_port} = 1;
393
394	0
395	},
396	message => sub {
397	my ($tag, $msg) = @_;
398
399	print "message> $msg\n";
400
401	snd $_, message => $msg
402	for keys %client_ports;
403
404	0
405	};
406
407	AnyEvent->condvar->recv;
408
409	That is all. Looks much simpler than the client, doesn't it?
410
411	Let's quickly look over it, as C<rcv> has already been discussed in the
412	client part of this tutorial above.
413
414	First this:
415
416	become_public "127.0.0.1:1299";
417
418	This will tell our I<node> to become a I<public> node, which means that it
419	can be contacted via TCP. The first argument should be the I<noderef> the
420	server wants to be reachable at. In this case it's the TCP port 1299 on
421	C<127.0.0.1>.
422
423	Next we set up two receivers, one for the C<join> messages and another one
424	for the actual messages of type C<messsage>. This is done with a single
425	call to C<rcv>, which allows multiple C<< match => $callback >> pairs.
426
427	In the C<join> callback we receive the client port, which is simply
428	remembered in the C<%client_ports> hash. In the C<message> callback we
429	just iterate through all known C<%client_ports> and relay the message to
430	them.
431
432	That concludes the server.
433
434	=head1 The Remaining Problems
435
436	The implementation as shown still has some bugs. For instance: How does
437	the server know that the client isn't there anymore, so it can clean up
438	the C<%client_ports> hash? Also, the chat messages have no originator, so
439	we don't know who actually sent the message (which would be quite useful
440	for human-to-human interaction: to know who the other one is :).
441
442	But aside from these issues I hope this tutorial showed you the basics of
443	L<AnyEvent::MP> and explained some common idioms.
444
445	How to solve the reliability and C<%client_ports> cleanup problem will
446	be explained later in this tutorial (TODO).
447
448	=head1 Inside The Protocol
449
450	Now, for the interested parties, let me explain some details about the protocol
451	that L<AnyEvent::MP> nodes use to communicate to each other. If you are not
452	interested you can skip this section.
453
454	Usually TCP is used for communication. Each I<node>, if configured to be a
455	I<public> node with the C<become_public> function will listen on the configured
456	TCP port (default is 4040).
457
458	If then one I<node> wants to send a message to another I<node> it will
459	connect to the host and port given in the I<port ID>.
460
461	Then some handshaking occurs to check whether both I<nodes> know the
462	I<shared secret>. Optionally, TLS can be enabled (about how to do this
463	exactly please consult the L<AnyEvent::MP> man page, just a hint: It
464	should be enough to put the private key and (self signed) certificate in
465	the C<~/.aemp-secret> file of all nodes).
466
467	After the handshake, messages will be exchanged using a serialiser
468	(usually L<JSON> is used for this, but it is also possible to use other
469	serialization formats such as L<Storable>).
470
471	=head1 SEE ALSO
472
473	L<AnyEvent>
474
475	L<AnyEvent::Handle>
476
477	L<AnyEvent::MP>
478
479	=head1 AUTHOR
480
481	Robin Redeker <elmex@ta-sa.org>
482