AnyEvent-MP/MP/Intro.pod

=head1 Message Passing for the Non-Blocked Mind

=head2 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 process on other 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 in 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"?
Basically between C<nodes>, which are basically your applications (as in
processes) that use L<AnyEvent::MP> that run either on the same or different
hosts.

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

=head2 System Requirements

Before we can start we have to make sure some things work on your
system. First of all the host C<localhost> should resolve to a local
IP address. Next you should be able to do TCP over that address.

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 a I<shared secret>. For testing you can write a
random string followed by a newline into the file C<.aemp-secret> in your
home directory:

   echo "secret123#4blabla_please_pick_your_own" > ~/.aemp-secret

Only if the nodes that want to connect to each other have the same I<shared
secret> connections will be successful.

B<If something does not work as expected, and for example tcpdump shows
that the connections are broken up early, you should make sure that ~/.aemp-secret
is the same on both hosts/user accounts you are connecting!>

=head2 The Chat Client

OK, lets start by implementing the "frontend" of the client. We will delay the
explanation and the code of the server until we finished the client, 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:

   #!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 $stdin_hdl = AnyEvent::Handle->new (
      fh      => \*STDIN,
      on_read => sub {
         my ($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>.

=head2 First Step Into Messaging

Now we take a look at L<AnyEvent::MP>. We need to know what to do in
C<send_message>. 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) of the server which will receive the message . How we get it
will be explained in the next step.  The next arguments of C<snd> are
C<message> and C<$msg> are the first two elements of the I<message> (a
I<message> in L<AnyEvent::MP> is a be a simple list of values, which can be
sent to I<ports>).

Thats all fine so far, but how do we get the C<$server_port>? Well, we will
need to get the unique I<port id> of the server's port where he 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 'well known 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 registered it
under the name C<chatter>.

BTW, these "well known 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 a name we have
to ask the server I<node> what I<port id> has the well known port with the
name C<chatter>.

Another new term, what is a I<node>: The messaging network that can be created with
L<AnyEvent::MP> consists of I<nodes>. A I<node> handles all the connection and
low level message sending logic for its application. The application in this
case is the server. Also every client has/is a I<node>.

=head2 Getting 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 $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 { ...

Now that was a lot of new stuff. In order to ask the server and receive an
answer we need to have a I<port> where we can receive the answer.
This is what C<create_port> will do for us, it just creates a new local
port and returns us an object (that will btw. stringify to the I<port id>),
that we can use to receive messages.

Next thing is the C<$server_node>. In order to refer to another node we need
some kind of string to reference it. The I<noderef> is basically a comma
separated list of C<host:port> pairs. We assume in this tutorial that the
server runs on your localhost at port 1299, this gives us the noderef
C<localhost:1299>.

Now you might ask what the C<#> at the end in C<$server_node> the above
example is about. Well, what I didn't tell you yet is that each I<node> has a
default I<port> to receive messages. The default port is the empty string
C<"">. The I<default port> of a I<node> also provides some special services for
us, for example resolving a well known port to a I<port id>.

Now to this line:

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

We send a message with first element being C<wkp> (standing for 'well known
port'). Then the well known 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 the result message: The
result message will have as it's first argument the string C<resolved> and
will be sent to C<$client_port> (the I<port id> of our own just created
port).

Next comes the receiver for this C<wkp> request.

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

This sets up a receiver on our own port for the result message with the first
argument being the string C<resolved>. Receivers can match the contents of
the messages before actually 'sending' it to the given callback.

B<Please note> that the given callback has to return either a true or a false
value for indicating whether it is B<done> (true value) or still wants to
B<continue> (false value) receiving messages.

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

We name the first element of the message C<$type> in this case. It's a common
idiom to code the 'type' of a message into it's first element, this allows for
simple matching.

The result message will contain the I<port id> of the well known port C<chatter>
as next element, and will be put in C<$chatter_port_id>.

Next we just assign C<$server_port> and return a 1 (a true value)
from the callback. It indicates that we are done and don't want to receive
further C<resolved> messages with this callback.

Now we continue to the rest of the client by calling C<send> on
C<$resolved_cv>.

First new step after this is setting up the chat message receiver callback.

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

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

We assume that all messages that are broadcast to all clients by the server
will contain the string C<message> as first element, and the actual message as
second element. The callback returns a false value this time, to indicate that
it wants to continue receiving messages.

Last but not least we actually tell the server to send us
the new chat messages from other clients. We do so by sending the
message type C<join> followed by our own I<port id>.

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

Then the server knows where to send all the new messages to.

=head2 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;

=head2 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.

=head2 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).

=head2 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:	1.7
Committed:	Tue Aug 4 07:58:53 2009 UTC (14 years, 10 months ago) by elmex
Branch:	MAIN
Changes since 1.6:	+51 -1 lines
Log Message:	enhanced the Intro a bit.
#	Content
1	=head1 Message Passing for the Non-Blocked Mind
2
3	=head2 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 and
7	to other process on other 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 in 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 being "passed"?
14	Basically between C<nodes>, which are basically your applications (as in
15	processes) that use L<AnyEvent::MP> that run either on the same or different
16	hosts.
17
18	In this tutorial I'll show you how to write a simple chat server based on
19	L<AnyEvent::MP>.
20
21	=head2 System Requirements
22
23	Before we can start we have to make sure some things work on your
24	system. First of all the host C<localhost> should resolve to a local
25	IP address. Next you should be able to do TCP over that address.
26
27	You should of course also make sure that L<AnyEvent> and L<AnyEvent::MP>
28	are installed. But how to do that is out of scope of this tutorial.
29
30	Then we have to setup a I<shared secret>. For two L<AnyEvent::MP> nodes
31	to be able to communicate with each other and authenticate each other
32	it is necessary to setup a I<shared secret>. For testing you can write a
33	random string followed by a newline into the file C<.aemp-secret> in your
34	home directory:
35
36	echo "secret123#4blabla_please_pick_your_own" > ~/.aemp-secret
37
38	Only if the nodes that want to connect to each other have the same I<shared
39	secret> connections will be successful.
40
41	B<If something does not work as expected, and for example tcpdump shows
42	that the connections are broken up early, you should make sure that ~/.aemp-secret
43	is the same on both hosts/user accounts you are connecting!>
44
45	=head2 The Chat Client
46
47	OK, lets start by implementing the "frontend" of the client. We will delay the
48	explanation and the code of the server until we finished the client, as the
49	most complex things actually happen in the client.
50
51	We will use L<AnyEvent::Handle> to do non-blocking IO read on standard input:
52
53	#!perl
54	use AnyEvent;
55	use AnyEvent::Handle;
56
57	sub send_message {
58	die "This is where we will send the messages to the server"
59	. "in the next step of this tutorial.\n"
60	}
61
62	# make an AnyEvent condition variable for the 'quit' condition
63	# (when we want to exit the client).
64	my $quit_cv = AnyEvent->condvar;
65
66	my $stdin_hdl = AnyEvent::Handle->new (
67	fh => \*STDIN,
68	on_read => sub {
69	my ($hdl) = @_;
70
71	$hdl->push_read (line => sub {
72	my ($hdl, $line) = @_;
73
74	if ($line =~ /^\/quit/) { # /quit will end the client
75	$quit_cv->send;
76
77	} else {
78	send_message ($line);
79	}
80	});
81	}
82	);
83
84	$quit_cv->recv;
85
86	This is now a very basic client. Explaining explicitly what
87	L<AnyEvent::Handle> does or what a I<condvar> is all about is out of scope
88	of this document, please consult L<AnyEvent::Intro> or the manual pages
89	for L<AnyEvent> and L<AnyEvent::Handle>.
90
91	=head2 First Step Into Messaging
92
93	Now we take a look at L<AnyEvent::MP>. We need to know what to do in
94	C<send_message>. This is an example of how it might look like:
95
96	... # the use lines from the above snippet
97
98	use AnyEvent::MP;
99
100	sub send_message {
101	my ($msg) = @_;
102
103	snd $server_port, message => $msg;
104	}
105
106	... # the rest of the above script
107
108	The C<snd> function is exported by L<AnyEvent::MP>, it stands for 'send a
109	message'. The first argument is the I<port> (a I<port> is something that can
110	receive messages) of the server which will receive the message . How we get it
111	will be explained in the next step. The next arguments of C<snd> are
112	C<message> and C<$msg> are the first two elements of the I<message> (a
113	I<message> in L<AnyEvent::MP> is a be a simple list of values, which can be
114	sent to I<ports>).
115
116	Thats all fine so far, but how do we get the C<$server_port>? Well, we will
117	need to get the unique I<port id> of the server's port where he wants to
118	receive all the incoming chat messages. A I<port id> is unfortunately a very
119	unique string, which we are unable to know in advance. But L<AnyEvent::MP>
120	supports the concept of 'well known ports', which is basically a port on the
121	server side registered under a well known name. For example, the server has a
122	port for receiving chat messages with a unique I<port id> and registered it
123	under the name C<chatter>.
124
125	BTW, these "well known port names" should follow similar rules as Perl
126	identifiers, so you should prefix them with your package/module name to
127	make them unique, unless you use them in the main program.
128
129	As I<messages> can only be sent to a I<port id> and not just to a name we have
130	to ask the server I<node> what I<port id> has the well known port with the
131	name C<chatter>.
132
133	Another new term, what is a I<node>: The messaging network that can be created with
134	L<AnyEvent::MP> consists of I<nodes>. A I<node> handles all the connection and
135	low level message sending logic for its application. The application in this
136	case is the server. Also every client has/is a I<node>.
137
138	=head2 Getting The Chatter Port
139
140	Ok, lots of talk, now some code. Now we will actually get the C<$server_port>
141	from the backend:
142
143	...
144
145	use AnyEvent::MP;
146
147	my $resolved_cv = AnyEvent->condvar;
148
149	my $client_port = create_port;
150
151	my $server_node = "localhost:1299#";
152
153	snd $server_node, wkp => "chatter", "$client_port", "resolved";
154
155	my $server_port;
156
157	# setup a receiver callback for the 'resolved' message:
158	$client_port->rcv (resolved => sub {
159	my ($client_port, $type, $chatter_port_id) = @_;
160
161	print "Resolved the server port 'chatter' to $chatter_port_id\n";
162	$server_port = $chatter_port_id;
163
164	$resolved_cv->send;
165	1
166	});
167
168	# lets block the client until we resolved the server port.
169	$resolved_cv->recv;
170
171	# now setup another receiver callback for the chat messages:
172	$client_port->rcv (message => sub {
173	my ($client_port, $type, $msg) = @_;
174
175	print "chat> $msg\n";
176	0
177	});
178
179	# send the server a 'join' message:
180	snd $server_port, join => "$client_port";
181
182	sub send_message { ...
183
184	Now that was a lot of new stuff. In order to ask the server and receive an
185	answer we need to have a I<port> where we can receive the answer.
186	This is what C<create_port> will do for us, it just creates a new local
187	port and returns us an object (that will btw. stringify to the I<port id>),
188	that we can use to receive messages.
189
190	Next thing is the C<$server_node>. In order to refer to another node we need
191	some kind of string to reference it. The I<noderef> is basically a comma
192	separated list of C<host:port> pairs. We assume in this tutorial that the
193	server runs on your localhost at port 1299, this gives us the noderef
194	C<localhost:1299>.
195
196	Now you might ask what the C<#> at the end in C<$server_node> the above
197	example is about. Well, what I didn't tell you yet is that each I<node> has a
198	default I<port> to receive messages. The default port is the empty string
199	C<"">. The I<default port> of a I<node> also provides some special services for
200	us, for example resolving a well known port to a I<port id>.
201
202	Now to this line:
203
204	snd $server_node, wkp => "chatter", "$client_port", "resolved";
205
206	We send a message with first element being C<wkp> (standing for 'well known
207	port'). Then the well known port name that we want to resolve to a I<port id>:
208	C<chatter>. And in order for the server node to be able to send us back the
209	resolved I<port id> we have to tell it where to send the result message: The
210	result message will have as it's first argument the string C<resolved> and
211	will be sent to C<$client_port> (the I<port id> of our own just created
212	port).
213
214	Next comes the receiver for this C<wkp> request.
215
216	$client_port->rcv (resolved => sub {
217	my ($client_port, $type, $chatter_port_id) = @_;
218	...
219	1
220	});
221
222	This sets up a receiver on our own port for the result message with the first
223	argument being the string C<resolved>. Receivers can match the contents of
224	the messages before actually 'sending' it to the given callback.
225
226	B<Please note> that the given callback has to return either a true or a false
227	value for indicating whether it is B<done> (true value) or still wants to
228	B<continue> (false value) receiving messages.
229
230	In this case we tell the C<$client_port> to look into the received messages and
231	look for the string C<resolved> in the first element of the message. If it is
232	found, the given callback will be called with the C<$client_port> as first
233	argument, and the message as the remaining arguments.
234
235	We name the first element of the message C<$type> in this case. It's a common
236	idiom to code the 'type' of a message into it's first element, this allows for
237	simple matching.
238
239	The result message will contain the I<port id> of the well known port C<chatter>
240	as next element, and will be put in C<$chatter_port_id>.
241
242	Next we just assign C<$server_port> and return a 1 (a true value)
243	from the callback. It indicates that we are done and don't want to receive
244	further C<resolved> messages with this callback.
245
246	Now we continue to the rest of the client by calling C<send> on
247	C<$resolved_cv>.
248
249	First new step after this is setting up the chat message receiver callback.
250
251	$client_port->rcv (message => sub {
252	my ($client_port, $type, $msg) = @_;
253
254	print "chat> $msg\n";
255	0
256	});
257
258	We assume that all messages that are broadcast to all clients by the server
259	will contain the string C<message> as first element, and the actual message as
260	second element. The callback returns a false value this time, to indicate that
261	it wants to continue receiving messages.
262
263	Last but not least we actually tell the server to send us
264	the new chat messages from other clients. We do so by sending the
265	message type C<join> followed by our own I<port id>.
266
267	# send the server a 'join' message:
268	snd $server_port, join => "$client_port";
269
270	Then the server knows where to send all the new messages to.
271
272	=head2 The Completed Client
273
274	This is the complete client script:
275
276	#!perl
277
278	use AnyEvent;
279	use AnyEvent::Handle;
280	use AnyEvent::MP;
281
282	my $resolved_cv = AnyEvent->condvar;
283
284	my $client_port = create_port;
285
286	my $server_node = "localhost:1299#";
287
288	snd $server_node, wkp => "chatter", "$client_port", "resolved";
289
290	my $server_port;
291
292	# setup a receiver callback for the 'resolved' message:
293	$client_port->rcv (resolved => sub {
294	my ($client_port, $type, $chatter_port_id) = @_;
295
296	print "Resolved the server port 'chatter' to $chatter_port_id\n";
297	$server_port = $chatter_port_id;
298
299	$resolved_cv->send;
300	1
301	});
302
303	# lets block the client until we resolved the server port.
304	$resolved_cv->recv;
305
306	# now setup another receiver callback for the chat messages:
307	$client_port->rcv (message => sub {
308	my ($client_port, $type, $msg) = @_;
309
310	print "chat> $msg\n";
311	0
312	});
313
314	# send the server a 'join' message:
315	snd $server_port, join => "$client_port";
316
317	sub send_message {
318	my ($msg) = @_;
319
320	snd $server_port, message => $msg;
321	}
322
323	# make an AnyEvent condition variable for the 'quit' condition
324	# (when we want to exit the client).
325	my $quit_cv = AnyEvent->condvar;
326
327	my $stdin_hdl = AnyEvent::Handle->new (
328	fh => \*STDIN,
329	on_read => sub {
330	my ($hdl) = @_;
331
332	$hdl->push_read (line => sub {
333	my ($hdl, $line) = @_;
334
335	if ($line =~ /^\/quit/) { # /quit will end the client
336	$quit_cv->send;
337
338	} else {
339	send_message ($line);
340	}
341	});
342	}
343	);
344
345	$quit_cv->recv;
346
347	=head2 The Server
348
349	Ok, now finally to the server. What do we need? Well, we need to setup
350	the well known port C<chatter> where all clients send their messages to.
351
352	Up and into code right now:
353
354	#!perl
355
356	use AnyEvent;
357	use AnyEvent::MP;
358
359	become_public "localhost:1299";
360
361	my $chatter_port = create_port;
362	$chatter_port->register ("chatter");
363
364	my %client_ports;
365
366	$chatter_port->rcv (join => sub {
367	my ($chatter_port, $type, $client_port) = @_;
368
369	print "got new client port: $client_port\n";
370
371	$client_ports{$client_port} = 1;
372	0
373	});
374
375	$chatter_port->rcv (message => sub {
376	my ($chatter_port, $type, $msg) = @_;
377
378	print "message> $msg\n";
379
380	snd $_, message => $msg for keys %client_ports;
381	0
382	});
383
384	AnyEvent->condvar->recv;
385
386	This is all. Looks much easier, doesn't it? I'll explain it only shortly, as
387	we had the discussion of the C<rcv> method in the client part of this tutorial
388	above.
389
390	First this:
391
392	become_public "localhost:1299";
393
394	This will tell our I<node> to become a I<public> node, which means that it can
395	be contacted via TCP. The first argument should be the I<noderef> the server
396	wants to be reachable at. In this case it's the TCP port 1299 on localhost.
397
398	Next we basically setup two receivers, one for the C<join> messages and
399	another one for the actual messages of type C<messsage>.
400
401	In the C<join> message we get the client's port, which we just remember in the
402	C<%client_ports> hash. In the receiver for the message type C<message> we will
403	just iterate through all known C<%client_ports> and relay the message to them.
404
405	And thats it.
406
407	=head2 The Remaining Problems
408
409	The shown implementation still has some bugs. For instance: How does the
410	server know that the client isn't there anymore, and can cleanup the
411	C<%client_ports> hash? And also the chat messages have no originator,
412	so we don't know who actually sent the message (which would be quite useful
413	for human-to-human interaction: to know who the other one is :).
414
415	But aside from these issues I hope this tutorial got you the swing of
416	L<AnyEvent::MP> and explained some common idioms.
417
418	How to solve the reliability and C<%client_ports> cleanup problem will
419	be explained later in this tutorial (TODO).
420
421	=head2 Inside The Protocol
422
423	Now, for the interested parties, let me explain some details about the protocol
424	that L<AnyEvent::MP> nodes use to communicate to each other. If you are not
425	interested you can skip this section.
426
427	Usually TCP is used for communication. Each I<node>, if configured to be a
428	I<public> node with the C<become_public> function will listen on the configured
429	TCP port (default is usually 4040).
430
431	If now one I<node> wants to send a message to another I<node> it will connect
432	to the host and port given in the I<port id>.
433
434	Then some handshaking occurs to check whether both I<nodes> have the same
435	I<shared secret> configured. Optionally even TLS can be enabled (about how to
436	do this exactly please consult the L<AnyEvent::MP> man pages, just a hint: It
437	should be enough to put the private key and (self signed) certificate in the
438	C<~/.aemp-secret> file of all nodes).
439
440	Now the serialized (usually L<JSON> is used for this, but it is also possible
441	to use other serialization formats, like L<Storable>) messages are sent over
442	the wire using a simple framing protocol.
443
444	=head1 SEE ALSO
445
446	L<AnyEvent>
447
448	L<AnyEvent::Handle>
449
450	L<AnyEvent::MP>
451
452	=head1 AUTHOR
453
454	Robin Redeker <elmex@ta-sa.org>
455