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, anythin 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>. This example is used because it nicely shows how to
organise a simple application, but keep in mind that every node trusts any
other, so this chat cannot be used to implement a real chat server and
client system, but it can be used to implement a distributed chat system
for example.

=head1 System Requirements and System Setup

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

You should of course first 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 (use use TLS
certificates).

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

   aemp gensecret

This creates a 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 can also select your own shared secret (F<aemp
setsecret>) and for increased security requirements you can even create
a TLS certificate (F<aemp gencert>), causing connections to not just be
authenticated, but also to be encrypted.

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 of the other side).

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<initialise_node> 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:	1.14
Committed:	Fri Aug 14 03:27:59 2009 UTC (14 years, 9 months ago) by root
Branch:	MAIN
Changes since 1.13:	+18 -13 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, anythin 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
22	on L<AnyEvent::MP>. This example is used because it nicely shows how to
23	organise a simple application, but keep in mind that every node trusts any
24	other, so this chat cannot be used to implement a real chat server and
25	client system, but it can be used to implement a distributed chat system
26	for example.
27
28	=head1 System Requirements and System Setup
29
30	Before we can start we have to make sure some things work on your
31	system.
32
33	You should of course first make sure that L<AnyEvent> and L<AnyEvent::MP>
34	are installed. But how to do that is out of scope of this tutorial.
35
36	Then we have to setup a I<shared secret>: for two L<AnyEvent::MP> nodes to
37	be able to communicate with each other and authenticate each other it is
38	necessary to setup the same I<shared secret> for both of them (use use TLS
39	certificates).
40
41	The easiest way is to set this up is to use the F<aemp> utility:
42
43	aemp gensecret
44
45	This creates a F<$HOME/.perl-anyevent-mp> config file and generates a
46	random shared secret. You can copy this file to any other system and then
47	communicate with it. You can also select your own shared secret (F<aemp
48	setsecret>) and for increased security requirements you can even create
49	a TLS certificate (F<aemp gencert>), causing connections to not just be
50	authenticated, but also to be encrypted.
51
52	Connections will only be successful when the nodes that want to connect to
53	each other have the same I<shared secret> (or successfully verify the TLS
54	certificate of the other side).
55
56	B<If something does not work as expected, and for example tcpdump shows
57	that the connections are closed almost immediatly, you should make sure
58	that F<~/.perl-anyevent-mp> is the same on all hosts/user accounts that
59	you try to connect with each other!>
60
61	=head1 The Chat Client
62
63	OK, lets start by implementing the "frontend" of the client. We will
64	develop the client first and postpone the server for later, as the most
65	complex things actually happen in the client.
66
67	We will use L<AnyEvent::Handle> to do non-blocking IO read on standard
68	input (all of this code deals with actually handling user input, no
69	message passing yet):
70
71	#!perl
72
73	use AnyEvent;
74	use AnyEvent::Handle;
75
76	sub send_message {
77	die "This is where we will send the messages to the server"
78	. "in the next step of this tutorial.\n"
79	}
80
81	# make an AnyEvent condition variable for the 'quit' condition
82	# (when we want to exit the client).
83	my $quit_cv = AnyEvent->condvar;
84
85	my $stdin_hdl = AnyEvent::Handle->new (
86	fh => *STDIN,
87	on_error => sub { $quit_cv->send },
88	on_read => sub {
89	my ($hdl) = @_;
90
91	$hdl->push_read (line => sub {
92	my ($hdl, $line) = @_;
93
94	if ($line =~ /^\/quit/) { # /quit will end the client
95	$quit_cv->send;
96	} else {
97	send_message ($line);
98	}
99	});
100	}
101	);
102
103	$quit_cv->recv;
104
105	This is now a very basic client. Explaining explicitly what
106	L<AnyEvent::Handle> does or what a I<condvar> is all about is out of scope
107	of this document, please consult L<AnyEvent::Intro> or the manual pages
108	for L<AnyEvent> and L<AnyEvent::Handle>.
109
110	=head1 First Steps Into Messaging
111
112	To supply the C<send_message> function we now take a look at
113	L<AnyEvent::MP>. This is an example of how it might look like:
114
115	... # the use lines from the above snippet
116
117	use AnyEvent::MP;
118
119	sub send_message {
120	my ($msg) = @_;
121
122	snd $server_port, message => $msg;
123	}
124
125	... # the rest of the above script
126
127	The C<snd> function is exported by L<AnyEvent::MP>, it stands for 'send
128	a message'. The first argument is the I<port> (a I<port> is something
129	that can receive messages, represented by a printable string) of the
130	server which will receive the message. How we get this port will be
131	explained in the next step.
132
133	The remaining arguments of C<snd> are C<message> and C<$msg>, the first
134	two elements of the I<message> (a I<message> in L<AnyEvent::MP> is a
135	simple list of values, which can be sent to a I<port>).
136
137	So all the function does is send the two values C<message> (a constant
138	string to tell the server what to expect) and the actual message string.
139
140	Thats all fine and simple so far, but where do we get the
141	C<$server_port>? Well, we need to get the unique I<port id> of the
142	server's port where it wants to receive all the incoming chat messages. A
143	I<port id> is unfortunately a very unique string, which we are unable to
144	know in advance. But L<AnyEvent::MP> supports the concept of 'registered
145	ports', which is basically a port on the server side registered under
146	a well known name.
147
148	For example, the server has a port for receiving chat messages with a
149	unique I<port id> and registers it under the name C<chatter>.
150
151	BTW, these "registered port names" should follow similar rules as Perl
152	identifiers, so you should prefix them with your package/module name to
153	make them unique, unless you use them in the main program.
154
155	As I<messages> can only be sent to a I<port id> and not just to some name
156	we have to ask the server node for the I<port id> of the port registered
157	as C<chatter>.
158
159	=head1 Finding The Chatter Port
160
161	Ok, lots of talk, now some code. Now we will actually get the
162	C<$server_port> from the backend:
163
164	...
165
166	use AnyEvent::MP;
167
168	my $server_node = "127.0.0.1:1299";
169
170	my $client_port = port;
171
172	snd $server_node, lookup => "chatter", $client_port, "resolved";
173
174	my $resolved_cv = AnyEvent->condvar;
175	my $server_port;
176
177	# setup a receiver callback for the 'resolved' message:
178	rcv $client_port, resolved => sub {
179	my ($tag, $chatter_port_id) = @_;
180
181	print "Resolved the server port 'chatter' to $chatter_port_id\n";
182	$server_port = $chatter_port_id;
183
184	$resolved_cv->send;
185	1
186	};
187
188	# lets block the client until we have resolved the server port.
189	$resolved_cv->recv;
190
191	# now setup another receiver callback for the chat messages:
192	rcv $client_port, message => sub {
193	my ($tag, $msg) = @_;
194
195	print "chat> $msg\n";
196	0
197	};
198
199	# send a 'join' message to the server:
200	snd $server_port, join => "$client_port";
201
202	sub send_message { ...
203
204	Now that was a lot of new stuff:
205
206	First we define the C<$server_node>: In order to refer to another node
207	we need some kind of string to reference it - the node reference. The
208	I<noderef> is basically a comma separated list of C<address:port>
209	pairs. We assume in this tutorial that the server runs on C<127.0.0.1>
210	(localhost) on port 1299, which results in the noderef C<127.0.0.1:1299>.
211
212	Next, in order to receive a reply from the other node or the server we
213	need to have a I<port> that messages can be sent to. This is what the
214	C<port> function will do for us, it just creates a new local port and
215	returns it's I<port ID> that can then be used to receive messages.
216
217	When you look carefully, you will see that the first C<snd> uses the
218	C<$server_node> (a noderef) as destination port. Well, what I didn't
219	tell you yet is that each I<node> has a default I<port> to receive
220	messages. The ID of this port is the same as the noderef.
221
222	This I<default port> provides some special services for us, for example
223	resolving a registered name to a I<port id> (a-ha! finally!).
224
225	This is exactly what this line does:
226
227	snd $server_node, lookup => "chatter", $client_port, "resolved";
228
229	This sends a message with first element being C<lookup>, followed by the
230	(hopefully) registered port name that we want to resolve to a I<port
231	id>: C<chatter>. And in order for the server node to be able to send us
232	back the resolved I<port ID> we have to tell it where to send it: The
233	result message will be sent to C<$client_port> (the I<port id> of the
234	port we just created), and will have the string C<resolved> as the first
235	element.
236
237	When the node receives this message, it will look up the name, gobble up
238	all the extra arguments we passed, append the resolved name, and send the
239	resulting list as a message.
240
241	Next we register a receiver for this C<lookup>-request.
242
243	rcv $client_port, resolved => sub {
244	my ($tag, $chatter_port_id) = @_;
245	...
246	1
247	};
248
249	This sets up a receiver on our own port for messages with the first
250	element being the string C<resolved>. Receivers can match the contents of
251	the messages before actually executing the specified callback.
252
253	B<Please note> that the every C<rcv> callback has to return either a true
254	or a false value, indicating whether it is B<successful>/B<done> (true) or
255	still wants to B<continue> (false) receiving messages.
256
257	In this case we tell the C<$client_port> to look into all the messages
258	it receives and look for the string C<resolved> in the first element of
259	the message. If it is found, the given callback will be called with the
260	message elements as arguments.
261
262	Using a string as the first element of the message is called I<tagging>
263	the message. It's common practise to code the 'type' of a message into
264	it's first element, as this allows for simple matching.
265
266	The result message will contain the I<port ID> of the well known port
267	C<chatter> as second element, which will be stored in C<$chatter_port_id>.
268
269	This port ID will then be stored in C<$server_port>, followed by calling
270	C<send> on $resolved_cv> so the program will continue.
271
272	The callback then returns a C<1> (a true value), to indicate that it has
273	done it's job and doesn't want to receive further C<resolved> messages.
274
275	After this the chat message receiver callback is registered with the port:
276
277	rcv $client_port, message => sub {
278	my ($tag, $msg) = @_;
279
280	print "chat> $msg\n";
281
282	0
283	};
284
285	We assume that all messages that are broadcast to the clients by the
286	server contain the string tag C<message> as first element, and the actual
287	message as second element. The callback returns a false value this time,
288	to indicate that it is not yet done and wants to receive further messages.
289
290	The last thing to do is to tell the server to send us new chat messages
291	from other clients. We do so by sending the message C<join> followed by
292	our own I<port ID>.
293
294	# send the server a 'join' message:
295	snd $server_port, join => $client_port;
296
297	This way the server knows where to send all the new messages to.
298
299	=head1 The Completed Client
300
301	This is the complete client script:
302
303	#!perl
304
305	use AnyEvent;
306	use AnyEvent::Handle;
307	use AnyEvent::MP;
308
309	my $server_node = "127.0.0.1:1299";
310
311	my $client_port = port;
312
313	snd $server_node, lookup => "chatter", $client_port, "resolved";
314
315	my $resolved_cv = AnyEvent->condvar;
316	my $server_port;
317
318	# setup a receiver callback for the 'resolved' message:
319	rcv $client_port, resolved => sub {
320	my ($tag, $chatter_port_id) = @_;
321
322	print "Resolved the server port 'chatter' to $chatter_port_id\n";
323	$server_port = $chatter_port_id;
324
325	$resolved_cv->send;
326	1
327	};
328
329	# lets block the client until we have resolved the server port.
330	$resolved_cv->recv;
331
332	# now setup another receiver callback for the chat messages:
333	rcv $client_port, message => sub {
334	my ($tag, $msg) = @_;
335
336	print "chat> $msg\n";
337	0
338	};
339
340	# send a 'join' message to the server:
341	snd $server_port, join => "$client_port";
342
343	sub send_message {
344	my ($msg) = @_;
345
346	snd $server_port, message => $msg;
347	}
348
349	# make an AnyEvent condition variable for the 'quit' condition
350	# (when we want to exit the client).
351	my $quit_cv = AnyEvent->condvar;
352
353	my $stdin_hdl = AnyEvent::Handle->new (
354	fh => *STDIN,
355	on_error => sub { $quit_cv->send },
356	on_read => sub {
357	my ($hdl) = @_;
358
359	$hdl->push_read (line => sub {
360	my ($hdl, $line) = @_;
361
362	if ($line =~ /^\/quit/) { # /quit will end the client
363	$quit_cv->send;
364	} else {
365	send_message ($line);
366	}
367	});
368	}
369	);
370
371	$quit_cv->recv;
372
373	=head1 The Server
374
375	Ok, we finally come to the server.
376
377	The server of course also needs to set up a port, and in addition needs to
378	I<register> it, so the clients can find it.
379
380	Again, let's jump directly into the code:
381
382	#!perl
383
384	use AnyEvent;
385	use AnyEvent::MP;
386
387	become_public "127.0.0.1:1299";
388
389	my $chatter_port = port;
390
391	reg $chatter_port, "chatter";
392
393	my %client_ports;
394
395	rcv $chatter_port,
396	join => sub {
397	my ($tag, $client_port) = @_;
398
399	print "got new client port: $client_port\n";
400	$client_ports{$client_port} = 1;
401
402	0
403	},
404	message => sub {
405	my ($tag, $msg) = @_;
406
407	print "message> $msg\n";
408
409	snd $_, message => $msg
410	for keys %client_ports;
411
412	0
413	};
414
415	AnyEvent->condvar->recv;
416
417	That is all. Looks much simpler than the client, doesn't it?
418
419	Let's quickly look over it, as C<rcv> has already been discussed in the
420	client part of this tutorial above.
421
422	First this:
423
424	become_public "127.0.0.1:1299";
425
426	This will tell our I<node> to become a I<public> node, which means that it
427	can be contacted via TCP. The first argument should be the I<noderef> the
428	server wants to be reachable at. In this case it's the TCP port 1299 on
429	C<127.0.0.1>.
430
431	Next we set up two receivers, one for the C<join> messages and another one
432	for the actual messages of type C<messsage>. This is done with a single
433	call to C<rcv>, which allows multiple C<< match => $callback >> pairs.
434
435	In the C<join> callback we receive the client port, which is simply
436	remembered in the C<%client_ports> hash. In the C<message> callback we
437	just iterate through all known C<%client_ports> and relay the message to
438	them.
439
440	That concludes the server.
441
442	=head1 The Remaining Problems
443
444	The implementation as shown still has some bugs. For instance: How does
445	the server know that the client isn't there anymore, so it can clean up
446	the C<%client_ports> hash? Also, the chat messages have no originator, so
447	we don't know who actually sent the message (which would be quite useful
448	for human-to-human interaction: to know who the other one is :).
449
450	But aside from these issues I hope this tutorial showed you the basics of
451	L<AnyEvent::MP> and explained some common idioms.
452
453	How to solve the reliability and C<%client_ports> cleanup problem will
454	be explained later in this tutorial (TODO).
455
456	=head1 Inside The Protocol
457
458	Now, for the interested parties, let me explain some details about the protocol
459	that L<AnyEvent::MP> nodes use to communicate to each other. If you are not
460	interested you can skip this section.
461
462	Usually TCP is used for communication. Each I<node>, if configured to be
463	a I<public> node with the C<initialise_node> function will listen on the
464	configured TCP port (default is 4040).
465
466	If then one I<node> wants to send a message to another I<node> it will
467	connect to the host and port given in the I<port ID>.
468
469	Then some handshaking occurs to check whether both I<nodes> know the
470	I<shared secret>. Optionally, TLS can be enabled (about how to do this
471	exactly please consult the L<AnyEvent::MP> man page, just a hint: It
472	should be enough to put the private key and (self signed) certificate in
473	the C<~/.aemp-secret> file of all nodes).
474
475	After the handshake, messages will be exchanged using a serialiser
476	(usually L<JSON> is used for this, but it is also possible to use other
477	serialization formats such as L<Storable>).
478
479	=head1 SEE ALSO
480
481	L<AnyEvent>
482
483	L<AnyEvent::Handle>
484
485	L<AnyEvent::MP>
486
487	=head1 AUTHOR
488
489	Robin Redeker <elmex@ta-sa.org>
490