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.

The easiest way is to use the F<aemp> utility:

   aemp gensecret

This creates the F<$HOME/.perl-anyevent-mp> config file and generates a
random shared secret. You can copy this file to any other system and then
communicate with it. You cna also select your own shared secret (F<aemp
setsecret>) and for increased security requirements you can even create a
TLS certificate (F<aemp gencert>).

Connections will only be successful when the nodes that want to connect to
each other have the same I<shared secret> (or successfully verify the TLS
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<~/.perl-anyevent-mp> 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:	1.12
Committed:	Mon Aug 10 19:23:43 2009 UTC (14 years, 9 months ago) by root
Branch:	MAIN
CVS Tags:	rel-0_6
Changes since 1.11:	+17 -15 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.
35
36	The easiest way is to use the F<aemp> utility:
37
38	aemp gensecret
39
40	This creates the F<$HOME/.perl-anyevent-mp> config file and generates a
41	random shared secret. You can copy this file to any other system and then
42	communicate with it. You cna also select your own shared secret (F<aemp
43	setsecret>) and for increased security requirements you can even create a
44	TLS certificate (F<aemp gencert>).
45
46	Connections will only be successful when the nodes that want to connect to
47	each other have the same I<shared secret> (or successfully verify the TLS
48	certificate).
49
50	B<If something does not work as expected, and for example tcpdump shows
51	that the connections are closed almost immediatly, you should make sure
52	that F<~/.perl-anyevent-mp> is the same on all hosts/user accounts that
53	you try to connect with each other!>
54
55	=head1 The Chat Client
56
57	OK, lets start by implementing the "frontend" of the client. We will
58	develop the client first and postpone the server for later, as the most
59	complex things actually happen in the client.
60
61	We will use L<AnyEvent::Handle> to do non-blocking IO read on standard
62	input (all of this code deals with actually handling user input, no
63	message passing yet):
64
65	#!perl
66
67	use AnyEvent;
68	use AnyEvent::Handle;
69
70	sub send_message {
71	die "This is where we will send the messages to the server"
72	. "in the next step of this tutorial.\n"
73	}
74
75	# make an AnyEvent condition variable for the 'quit' condition
76	# (when we want to exit the client).
77	my $quit_cv = AnyEvent->condvar;
78
79	my $stdin_hdl = AnyEvent::Handle->new (
80	fh => *STDIN,
81	on_error => sub { $quit_cv->send },
82	on_read => sub {
83	my ($hdl) = @_;
84
85	$hdl->push_read (line => sub {
86	my ($hdl, $line) = @_;
87
88	if ($line =~ /^\/quit/) { # /quit will end the client
89	$quit_cv->send;
90	} else {
91	send_message ($line);
92	}
93	});
94	}
95	);
96
97	$quit_cv->recv;
98
99	This is now a very basic client. Explaining explicitly what
100	L<AnyEvent::Handle> does or what a I<condvar> is all about is out of scope
101	of this document, please consult L<AnyEvent::Intro> or the manual pages
102	for L<AnyEvent> and L<AnyEvent::Handle>.
103
104	=head1 First Steps Into Messaging
105
106	To supply the C<send_message> function we now take a look at
107	L<AnyEvent::MP>. This is an example of how it might look like:
108
109	... # the use lines from the above snippet
110
111	use AnyEvent::MP;
112
113	sub send_message {
114	my ($msg) = @_;
115
116	snd $server_port, message => $msg;
117	}
118
119	... # the rest of the above script
120
121	The C<snd> function is exported by L<AnyEvent::MP>, it stands for 'send
122	a message'. The first argument is the I<port> (a I<port> is something
123	that can receive messages, represented by a printable string) of the
124	server which will receive the message. How we get this port will be
125	explained in the next step.
126
127	The remaining arguments of C<snd> are C<message> and C<$msg>, the first
128	two elements of the I<message> (a I<message> in L<AnyEvent::MP> is a
129	simple list of values, which can be sent to a I<port>).
130
131	So all the function does is send the two values C<message> (a constant
132	string to tell the server what to expect) and the actual message string.
133
134	Thats all fine and simple so far, but where do we get the
135	C<$server_port>? Well, we need to get the unique I<port id> of the
136	server's port where it wants to receive all the incoming chat messages. A
137	I<port id> is unfortunately a very unique string, which we are unable to
138	know in advance. But L<AnyEvent::MP> supports the concept of 'registered
139	ports', which is basically a port on the server side registered under
140	a well known name.
141
142	For example, the server has a port for receiving chat messages with a
143	unique I<port id> and registers it under the name C<chatter>.
144
145	BTW, these "registered port names" should follow similar rules as Perl
146	identifiers, so you should prefix them with your package/module name to
147	make them unique, unless you use them in the main program.
148
149	As I<messages> can only be sent to a I<port id> and not just to some name
150	we have to ask the server node for the I<port id> of the port registered
151	as C<chatter>.
152
153	=head1 Finding The Chatter Port
154
155	Ok, lots of talk, now some code. Now we will actually get the
156	C<$server_port> from the backend:
157
158	...
159
160	use AnyEvent::MP;
161
162	my $server_node = "127.0.0.1:1299";
163
164	my $client_port = port;
165
166	snd $server_node, lookup => "chatter", $client_port, "resolved";
167
168	my $resolved_cv = AnyEvent->condvar;
169	my $server_port;
170
171	# setup a receiver callback for the 'resolved' message:
172	rcv $client_port, resolved => sub {
173	my ($tag, $chatter_port_id) = @_;
174
175	print "Resolved the server port 'chatter' to $chatter_port_id\n";
176	$server_port = $chatter_port_id;
177
178	$resolved_cv->send;
179	1
180	};
181
182	# lets block the client until we have resolved the server port.
183	$resolved_cv->recv;
184
185	# now setup another receiver callback for the chat messages:
186	rcv $client_port, message => sub {
187	my ($tag, $msg) = @_;
188
189	print "chat> $msg\n";
190	0
191	};
192
193	# send a 'join' message to the server:
194	snd $server_port, join => "$client_port";
195
196	sub send_message { ...
197
198	Now that was a lot of new stuff:
199
200	First we define the C<$server_node>: In order to refer to another node
201	we need some kind of string to reference it - the node reference. The
202	I<noderef> is basically a comma separated list of C<address:port>
203	pairs. We assume in this tutorial that the server runs on C<127.0.0.1>
204	(localhost) on port 1299, which results in the noderef C<127.0.0.1:1299>.
205
206	Next, in order to receive a reply from the other node or the server we
207	need to have a I<port> that messages can be sent to. This is what the
208	C<port> function will do for us, it just creates a new local port and
209	returns it's I<port ID> that can then be used to receive messages.
210
211	When you look carefully, you will see that the first C<snd> uses the
212	C<$server_node> (a noderef) as destination port. Well, what I didn't
213	tell you yet is that each I<node> has a default I<port> to receive
214	messages. The ID of this port is the same as the noderef.
215
216	This I<default port> provides some special services for us, for example
217	resolving a registered name to a I<port id> (a-ha! finally!).
218
219	This is exactly what this line does:
220
221	snd $server_node, lookup => "chatter", $client_port, "resolved";
222
223	This sends a message with first element being C<lookup>, followed by the
224	(hopefully) registered port name that we want to resolve to a I<port
225	id>: C<chatter>. And in order for the server node to be able to send us
226	back the resolved I<port ID> we have to tell it where to send it: The
227	result message will be sent to C<$client_port> (the I<port id> of the
228	port we just created), and will have the string C<resolved> as the first
229	element.
230
231	When the node receives this message, it will look up the name, gobble up
232	all the extra arguments we passed, append the resolved name, and send the
233	resulting list as a message.
234
235	Next we register a receiver for this C<lookup>-request.
236
237	rcv $client_port, resolved => sub {
238	my ($tag, $chatter_port_id) = @_;
239	...
240	1
241	};
242
243	This sets up a receiver on our own port for messages with the first
244	element being the string C<resolved>. Receivers can match the contents of
245	the messages before actually executing the specified callback.
246
247	B<Please note> that the every C<rcv> callback has to return either a true
248	or a false value, indicating whether it is B<successful>/B<done> (true) or
249	still wants to B<continue> (false) receiving messages.
250
251	In this case we tell the C<$client_port> to look into all the messages
252	it receives and look for the string C<resolved> in the first element of
253	the message. If it is found, the given callback will be called with the
254	message elements as arguments.
255
256	Using a string as the first element of the message is called I<tagging>
257	the message. It's common practise to code the 'type' of a message into
258	it's first element, as this allows for simple matching.
259
260	The result message will contain the I<port ID> of the well known port
261	C<chatter> as second element, which will be stored in C<$chatter_port_id>.
262
263	This port ID will then be stored in C<$server_port>, followed by calling
264	C<send> on $resolved_cv> so the program will continue.
265
266	The callback then returns a C<1> (a true value), to indicate that it has
267	done it's job and doesn't want to receive further C<resolved> messages.
268
269	After this the chat message receiver callback is registered with the port:
270
271	rcv $client_port, message => sub {
272	my ($tag, $msg) = @_;
273
274	print "chat> $msg\n";
275
276	0
277	};
278
279	We assume that all messages that are broadcast to the clients by the
280	server contain the string tag C<message> as first element, and the actual
281	message as second element. The callback returns a false value this time,
282	to indicate that it is not yet done and wants to receive further messages.
283
284	The last thing to do is to tell the server to send us new chat messages
285	from other clients. We do so by sending the message C<join> followed by
286	our own I<port ID>.
287
288	# send the server a 'join' message:
289	snd $server_port, join => $client_port;
290
291	This way the server knows where to send all the new messages to.
292
293	=head1 The Completed Client
294
295	This is the complete client script:
296
297	#!perl
298
299	use AnyEvent;
300	use AnyEvent::Handle;
301	use AnyEvent::MP;
302
303	my $server_node = "127.0.0.1:1299";
304
305	my $client_port = port;
306
307	snd $server_node, lookup => "chatter", $client_port, "resolved";
308
309	my $resolved_cv = AnyEvent->condvar;
310	my $server_port;
311
312	# setup a receiver callback for the 'resolved' message:
313	rcv $client_port, resolved => sub {
314	my ($tag, $chatter_port_id) = @_;
315
316	print "Resolved the server port 'chatter' to $chatter_port_id\n";
317	$server_port = $chatter_port_id;
318
319	$resolved_cv->send;
320	1
321	};
322
323	# lets block the client until we have resolved the server port.
324	$resolved_cv->recv;
325
326	# now setup another receiver callback for the chat messages:
327	rcv $client_port, message => sub {
328	my ($tag, $msg) = @_;
329
330	print "chat> $msg\n";
331	0
332	};
333
334	# send a 'join' message to the server:
335	snd $server_port, join => "$client_port";
336
337	sub send_message {
338	my ($msg) = @_;
339
340	snd $server_port, message => $msg;
341	}
342
343	# make an AnyEvent condition variable for the 'quit' condition
344	# (when we want to exit the client).
345	my $quit_cv = AnyEvent->condvar;
346
347	my $stdin_hdl = AnyEvent::Handle->new (
348	fh => *STDIN,
349	on_error => sub { $quit_cv->send },
350	on_read => sub {
351	my ($hdl) = @_;
352
353	$hdl->push_read (line => sub {
354	my ($hdl, $line) = @_;
355
356	if ($line =~ /^\/quit/) { # /quit will end the client
357	$quit_cv->send;
358	} else {
359	send_message ($line);
360	}
361	});
362	}
363	);
364
365	$quit_cv->recv;
366
367	=head1 The Server
368
369	Ok, we finally come to the server.
370
371	The server of course also needs to set up a port, and in addition needs to
372	I<register> it, so the clients can find it.
373
374	Again, let's jump directly into the code:
375
376	#!perl
377
378	use AnyEvent;
379	use AnyEvent::MP;
380
381	become_public "127.0.0.1:1299";
382
383	my $chatter_port = port;
384
385	reg $chatter_port, "chatter";
386
387	my %client_ports;
388
389	rcv $chatter_port,
390	join => sub {
391	my ($tag, $client_port) = @_;
392
393	print "got new client port: $client_port\n";
394	$client_ports{$client_port} = 1;
395
396	0
397	},
398	message => sub {
399	my ($tag, $msg) = @_;
400
401	print "message> $msg\n";
402
403	snd $_, message => $msg
404	for keys %client_ports;
405
406	0
407	};
408
409	AnyEvent->condvar->recv;
410
411	That is all. Looks much simpler than the client, doesn't it?
412
413	Let's quickly look over it, as C<rcv> has already been discussed in the
414	client part of this tutorial above.
415
416	First this:
417
418	become_public "127.0.0.1:1299";
419
420	This will tell our I<node> to become a I<public> node, which means that it
421	can be contacted via TCP. The first argument should be the I<noderef> the
422	server wants to be reachable at. In this case it's the TCP port 1299 on
423	C<127.0.0.1>.
424
425	Next we set up two receivers, one for the C<join> messages and another one
426	for the actual messages of type C<messsage>. This is done with a single
427	call to C<rcv>, which allows multiple C<< match => $callback >> pairs.
428
429	In the C<join> callback we receive the client port, which is simply
430	remembered in the C<%client_ports> hash. In the C<message> callback we
431	just iterate through all known C<%client_ports> and relay the message to
432	them.
433
434	That concludes the server.
435
436	=head1 The Remaining Problems
437
438	The implementation as shown still has some bugs. For instance: How does
439	the server know that the client isn't there anymore, so it can clean up
440	the C<%client_ports> hash? Also, the chat messages have no originator, so
441	we don't know who actually sent the message (which would be quite useful
442	for human-to-human interaction: to know who the other one is :).
443
444	But aside from these issues I hope this tutorial showed you the basics of
445	L<AnyEvent::MP> and explained some common idioms.
446
447	How to solve the reliability and C<%client_ports> cleanup problem will
448	be explained later in this tutorial (TODO).
449
450	=head1 Inside The Protocol
451
452	Now, for the interested parties, let me explain some details about the protocol
453	that L<AnyEvent::MP> nodes use to communicate to each other. If you are not
454	interested you can skip this section.
455
456	Usually TCP is used for communication. Each I<node>, if configured to be a
457	I<public> node with the C<become_public> function will listen on the configured
458	TCP port (default is 4040).
459
460	If then one I<node> wants to send a message to another I<node> it will
461	connect to the host and port given in the I<port ID>.
462
463	Then some handshaking occurs to check whether both I<nodes> know the
464	I<shared secret>. Optionally, TLS can be enabled (about how to do this
465	exactly please consult the L<AnyEvent::MP> man page, just a hint: It
466	should be enough to put the private key and (self signed) certificate in
467	the C<~/.aemp-secret> file of all nodes).
468
469	After the handshake, messages will be exchanged using a serialiser
470	(usually L<JSON> is used for this, but it is also possible to use other
471	serialization formats such as L<Storable>).
472
473	=head1 SEE ALSO
474
475	L<AnyEvent>
476
477	L<AnyEvent::Handle>
478
479	L<AnyEvent::MP>
480
481	=head1 AUTHOR
482
483	Robin Redeker <elmex@ta-sa.org>
484