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 and System Setup

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 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 conenctions 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).

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:	1.13
Committed:	Thu Aug 13 20:47:54 2009 UTC (14 years, 9 months ago) by root
Branch:	MAIN
Changes since 1.12:	+6 -5 lines
Log Message:	* empty log message *
#	User	Rev	Content
1	root	1.4	=head1 Message Passing for the Non-Blocked Mind
2	elmex	1.1
3	root	1.8	=head1 Introduction and Terminology
4	elmex	1.1
5	root	1.4	This is a tutorial about how to get the swing of the new L<AnyEvent::MP>
6	root	1.8	module. Which allows us to transparently pass messages to our own process
7			and to other processes on another or the same host.
8	elmex	1.1
9	root	1.4	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	root	1.8	expressed as a L<JSON> text (as JSON is used by default in the protocol).
12	elmex	1.1
13	root	1.8	And next you might ask: between which entities are those messages
14	root	1.9	being "passed"? Effectively between I<nodes>: a nodes is basically a
15	root	1.8	process/program that use L<AnyEvent::MP> and can run either on the same or
16			different hosts.
17	elmex	1.1
18	root	1.9	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	root	1.4	In this tutorial I'll show you how to write a simple chat server based on
22	elmex	1.1	L<AnyEvent::MP>.
23
24	root	1.13	=head1 System Requirements and System Setup
25	elmex	1.7
26			Before we can start we have to make sure some things work on your
27	root	1.8	system.
28	elmex	1.7
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	root	1.13	Then we have to setup a I<shared secret>: for two L<AnyEvent::MP> nodes to
33	root	1.8	be able to communicate with each other and authenticate each other it is
34	root	1.12	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	root	1.13	communicate with it. You can also select your own shared secret (F<aemp
43			setsecret>) and for increased security requirements you can even create
44			a TLS certificate (F<aemp gencert>), causing conenctions to not just be
45			authenticated, but also to be encrypted.
46	root	1.12
47			Connections will only be successful when the nodes that want to connect to
48			each other have the same I<shared secret> (or successfully verify the TLS
49			certificate).
50	elmex	1.7
51			B<If something does not work as expected, and for example tcpdump shows
52	root	1.8	that the connections are closed almost immediatly, you should make sure
53	root	1.12	that F<~/.perl-anyevent-mp> is the same on all hosts/user accounts that
54			you try to connect with each other!>
55	root	1.8
56			=head1 The Chat Client
57
58			OK, lets start by implementing the "frontend" of the client. We will
59			develop the client first and postpone the server for later, as the most
60			complex things actually happen in the client.
61
62			We will use L<AnyEvent::Handle> to do non-blocking IO read on standard
63			input (all of this code deals with actually handling user input, no
64			message passing yet):
65	elmex	1.7
66	root	1.8	#!perl
67	elmex	1.1
68			use AnyEvent;
69			use AnyEvent::Handle;
70
71			sub send_message {
72			die "This is where we will send the messages to the server"
73			. "in the next step of this tutorial.\n"
74			}
75
76			# make an AnyEvent condition variable for the 'quit' condition
77			# (when we want to exit the client).
78			my $quit_cv = AnyEvent->condvar;
79
80			my $stdin_hdl = AnyEvent::Handle->new (
81	root	1.8	fh => *STDIN,
82			on_error => sub { $quit_cv->send },
83			on_read => sub {
84	elmex	1.1	my ($hdl) = @_;
85
86			$hdl->push_read (line => sub {
87			my ($hdl, $line) = @_;
88
89			if ($line =~ /^\/quit/) { # /quit will end the client
90			$quit_cv->send;
91			} else {
92			send_message ($line);
93			}
94			});
95			}
96			);
97
98			$quit_cv->recv;
99
100	root	1.4	This is now a very basic client. Explaining explicitly what
101			L<AnyEvent::Handle> does or what a I<condvar> is all about is out of scope
102			of this document, please consult L<AnyEvent::Intro> or the manual pages
103			for L<AnyEvent> and L<AnyEvent::Handle>.
104	elmex	1.1
105	root	1.9	=head1 First Steps Into Messaging
106	elmex	1.1
107	root	1.8	To supply the C<send_message> function we now take a look at
108			L<AnyEvent::MP>. This is an example of how it might look like:
109	elmex	1.1
110			... # the use lines from the above snippet
111
112			use AnyEvent::MP;
113
114			sub send_message {
115			my ($msg) = @_;
116
117			snd $server_port, message => $msg;
118			}
119
120			... # the rest of the above script
121
122	root	1.8	The C<snd> function is exported by L<AnyEvent::MP>, it stands for 'send
123			a message'. The first argument is the I<port> (a I<port> is something
124			that can receive messages, represented by a printable string) of the
125			server which will receive the message. How we get this port will be
126			explained in the next step.
127
128			The remaining arguments of C<snd> are C<message> and C<$msg>, the first
129			two elements of the I<message> (a I<message> in L<AnyEvent::MP> is a
130			simple list of values, which can be sent to a I<port>).
131
132			So all the function does is send the two values C<message> (a constant
133			string to tell the server what to expect) and the actual message string.
134
135			Thats all fine and simple so far, but where do we get the
136			C<$server_port>? Well, we need to get the unique I<port id> of the
137			server's port where it wants to receive all the incoming chat messages. A
138	root	1.9	I<port id> is unfortunately a very unique string, which we are unable to
139			know in advance. But L<AnyEvent::MP> supports the concept of 'registered
140			ports', which is basically a port on the server side registered under
141			a well known name.
142
143			For example, the server has a port for receiving chat messages with a
144			unique I<port id> and registers it under the name C<chatter>.
145	root	1.4
146	root	1.9	BTW, these "registered port names" should follow similar rules as Perl
147	root	1.4	identifiers, so you should prefix them with your package/module name to
148			make them unique, unless you use them in the main program.
149	elmex	1.1
150	root	1.9	As I<messages> can only be sent to a I<port id> and not just to some name
151			we have to ask the server node for the I<port id> of the port registered
152			as C<chatter>.
153	elmex	1.1
154	root	1.9	=head1 Finding The Chatter Port
155	elmex	1.1
156	root	1.9	Ok, lots of talk, now some code. Now we will actually get the
157			C<$server_port> from the backend:
158	elmex	1.1
159			...
160
161			use AnyEvent::MP;
162
163	root	1.9	my $server_node = "127.0.0.1:1299";
164	elmex	1.1
165	root	1.9	my $client_port = port;
166	elmex	1.1
167	root	1.9	snd $server_node, lookup => "chatter", $client_port, "resolved";
168	elmex	1.1
169	root	1.9	my $resolved_cv = AnyEvent->condvar;
170	elmex	1.1	my $server_port;
171
172			# setup a receiver callback for the 'resolved' message:
173	root	1.9	rcv $client_port, resolved => sub {
174			my ($tag, $chatter_port_id) = @_;
175	elmex	1.1
176			print "Resolved the server port 'chatter' to $chatter_port_id\n";
177			$server_port = $chatter_port_id;
178
179			$resolved_cv->send;
180			1
181	root	1.9	};
182	elmex	1.1
183	root	1.9	# lets block the client until we have resolved the server port.
184	elmex	1.1	$resolved_cv->recv;
185
186			# now setup another receiver callback for the chat messages:
187	root	1.9	rcv $client_port, message => sub {
188			my ($tag, $msg) = @_;
189	elmex	1.1
190			print "chat> $msg\n";
191			0
192	root	1.9	};
193	elmex	1.1
194	root	1.9	# send a 'join' message to the server:
195	elmex	1.1	snd $server_port, join => "$client_port";
196
197			sub send_message { ...
198
199	root	1.9	Now that was a lot of new stuff:
200	elmex	1.1
201	root	1.9	First we define the C<$server_node>: In order to refer to another node
202			we need some kind of string to reference it - the node reference. The
203			I<noderef> is basically a comma separated list of C<address:port>
204			pairs. We assume in this tutorial that the server runs on C<127.0.0.1>
205			(localhost) on port 1299, which results in the noderef C<127.0.0.1:1299>.
206
207			Next, in order to receive a reply from the other node or the server we
208			need to have a I<port> that messages can be sent to. This is what the
209			C<port> function will do for us, it just creates a new local port and
210			returns it's I<port ID> that can then be used to receive messages.
211
212			When you look carefully, you will see that the first C<snd> uses the
213			C<$server_node> (a noderef) as destination port. Well, what I didn't
214			tell you yet is that each I<node> has a default I<port> to receive
215			messages. The ID of this port is the same as the noderef.
216
217			This I<default port> provides some special services for us, for example
218			resolving a registered name to a I<port id> (a-ha! finally!).
219
220			This is exactly what this line does:
221
222			snd $server_node, lookup => "chatter", $client_port, "resolved";
223
224			This sends a message with first element being C<lookup>, followed by the
225			(hopefully) registered port name that we want to resolve to a I<port
226			id>: C<chatter>. And in order for the server node to be able to send us
227			back the resolved I<port ID> we have to tell it where to send it: The
228			result message will be sent to C<$client_port> (the I<port id> of the
229			port we just created), and will have the string C<resolved> as the first
230			element.
231
232			When the node receives this message, it will look up the name, gobble up
233			all the extra arguments we passed, append the resolved name, and send the
234			resulting list as a message.
235	elmex	1.1
236	root	1.9	Next we register a receiver for this C<lookup>-request.
237
238			rcv $client_port, resolved => sub {
239			my ($tag, $chatter_port_id) = @_;
240			...
241			1
242			};
243
244			This sets up a receiver on our own port for messages with the first
245			element being the string C<resolved>. Receivers can match the contents of
246			the messages before actually executing the specified callback.
247
248			B<Please note> that the every C<rcv> callback has to return either a true
249			or a false value, indicating whether it is B<successful>/B<done> (true) or
250			still wants to B<continue> (false) receiving messages.
251	elmex	1.1
252	root	1.9	In this case we tell the C<$client_port> to look into all the messages
253			it receives and look for the string C<resolved> in the first element of
254			the message. If it is found, the given callback will be called with the
255			message elements as arguments.
256	elmex	1.1
257	root	1.9	Using a string as the first element of the message is called I<tagging>
258			the message. It's common practise to code the 'type' of a message into
259			it's first element, as this allows for simple matching.
260	elmex	1.1
261	root	1.9	The result message will contain the I<port ID> of the well known port
262			C<chatter> as second element, which will be stored in C<$chatter_port_id>.
263	elmex	1.1
264	root	1.9	This port ID will then be stored in C<$server_port>, followed by calling
265			C<send> on $resolved_cv> so the program will continue.
266	elmex	1.1
267	root	1.9	The callback then returns a C<1> (a true value), to indicate that it has
268			done it's job and doesn't want to receive further C<resolved> messages.
269	elmex	1.1
270	root	1.9	After this the chat message receiver callback is registered with the port:
271	elmex	1.1
272	root	1.9	rcv $client_port, message => sub {
273			my ($tag, $msg) = @_;
274	elmex	1.1
275			print "chat> $msg\n";
276	root	1.9
277	elmex	1.1	0
278	root	1.9	};
279	elmex	1.1
280	root	1.9	We assume that all messages that are broadcast to the clients by the
281			server contain the string tag C<message> as first element, and the actual
282			message as second element. The callback returns a false value this time,
283			to indicate that it is not yet done and wants to receive further messages.
284
285			The last thing to do is to tell the server to send us new chat messages
286			from other clients. We do so by sending the message C<join> followed by
287			our own I<port ID>.
288	elmex	1.1
289			# send the server a 'join' message:
290	root	1.9	snd $server_port, join => $client_port;
291	elmex	1.1
292	root	1.9	This way the server knows where to send all the new messages to.
293	elmex	1.1
294	root	1.8	=head1 The Completed Client
295	elmex	1.1
296			This is the complete client script:
297
298			#!perl
299	root	1.4
300	elmex	1.1	use AnyEvent;
301			use AnyEvent::Handle;
302			use AnyEvent::MP;
303
304	root	1.10	my $server_node = "127.0.0.1:1299";
305	elmex	1.1
306	root	1.10	my $client_port = port;
307	elmex	1.1
308	root	1.10	snd $server_node, lookup => "chatter", $client_port, "resolved";
309	elmex	1.1
310	root	1.10	my $resolved_cv = AnyEvent->condvar;
311	elmex	1.1	my $server_port;
312
313			# setup a receiver callback for the 'resolved' message:
314	root	1.10	rcv $client_port, resolved => sub {
315			my ($tag, $chatter_port_id) = @_;
316	elmex	1.1
317			print "Resolved the server port 'chatter' to $chatter_port_id\n";
318			$server_port = $chatter_port_id;
319
320			$resolved_cv->send;
321			1
322	root	1.10	};
323	elmex	1.1
324	root	1.10	# lets block the client until we have resolved the server port.
325	elmex	1.1	$resolved_cv->recv;
326
327			# now setup another receiver callback for the chat messages:
328	root	1.10	rcv $client_port, message => sub {
329			my ($tag, $msg) = @_;
330	elmex	1.1
331			print "chat> $msg\n";
332			0
333	root	1.10	};
334	elmex	1.1
335	root	1.10	# send a 'join' message to the server:
336	elmex	1.1	snd $server_port, join => "$client_port";
337
338			sub send_message {
339			my ($msg) = @_;
340
341			snd $server_port, message => $msg;
342			}
343
344			# make an AnyEvent condition variable for the 'quit' condition
345			# (when we want to exit the client).
346			my $quit_cv = AnyEvent->condvar;
347
348			my $stdin_hdl = AnyEvent::Handle->new (
349	root	1.10	fh => *STDIN,
350			on_error => sub { $quit_cv->send },
351			on_read => sub {
352	elmex	1.1	my ($hdl) = @_;
353
354			$hdl->push_read (line => sub {
355			my ($hdl, $line) = @_;
356
357			if ($line =~ /^\/quit/) { # /quit will end the client
358			$quit_cv->send;
359			} else {
360			send_message ($line);
361			}
362			});
363			}
364			);
365
366			$quit_cv->recv;
367
368	root	1.8	=head1 The Server
369	elmex	1.1
370	root	1.10	Ok, we finally come to the server.
371
372			The server of course also needs to set up a port, and in addition needs to
373			I<register> it, so the clients can find it.
374	elmex	1.1
375	root	1.10	Again, let's jump directly into the code:
376	elmex	1.1
377			#!perl
378	root	1.4
379	elmex	1.1	use AnyEvent;
380			use AnyEvent::MP;
381
382	root	1.10	become_public "127.0.0.1:1299";
383	elmex	1.1
384	root	1.10	my $chatter_port = port;
385
386			reg $chatter_port, "chatter";
387	elmex	1.1
388			my %client_ports;
389
390	root	1.10	rcv $chatter_port,
391			join => sub {
392			my ($tag, $client_port) = @_;
393	elmex	1.1
394	root	1.10	print "got new client port: $client_port\n";
395			$client_ports{$client_port} = 1;
396	elmex	1.5
397	root	1.10	0
398			},
399			message => sub {
400			my ($tag, $msg) = @_;
401	elmex	1.1
402	root	1.10	print "message> $msg\n";
403	elmex	1.1
404	root	1.10	snd $_, message => $msg
405			for keys %client_ports;
406	elmex	1.5
407	root	1.10	0
408	root	1.11	};
409	elmex	1.1
410			AnyEvent->condvar->recv;
411
412	root	1.10	That is all. Looks much simpler than the client, doesn't it?
413
414			Let's quickly look over it, as C<rcv> has already been discussed in the
415			client part of this tutorial above.
416	elmex	1.2
417			First this:
418
419	root	1.10	become_public "127.0.0.1:1299";
420	elmex	1.1
421	root	1.10	This will tell our I<node> to become a I<public> node, which means that it
422			can be contacted via TCP. The first argument should be the I<noderef> the
423			server wants to be reachable at. In this case it's the TCP port 1299 on
424			C<127.0.0.1>.
425
426			Next we set up two receivers, one for the C<join> messages and another one
427			for the actual messages of type C<messsage>. This is done with a single
428			call to C<rcv>, which allows multiple C<< match => $callback >> pairs.
429
430			In the C<join> callback we receive the client port, which is simply
431			remembered in the C<%client_ports> hash. In the C<message> callback we
432			just iterate through all known C<%client_ports> and relay the message to
433			them.
434	elmex	1.1
435	root	1.10	That concludes the server.
436	elmex	1.2
437	root	1.8	=head1 The Remaining Problems
438	elmex	1.1
439	root	1.10	The implementation as shown still has some bugs. For instance: How does
440			the server know that the client isn't there anymore, so it can clean up
441			the C<%client_ports> hash? Also, the chat messages have no originator, so
442			we don't know who actually sent the message (which would be quite useful
443	elmex	1.1	for human-to-human interaction: to know who the other one is :).
444
445	root	1.10	But aside from these issues I hope this tutorial showed you the basics of
446	elmex	1.1	L<AnyEvent::MP> and explained some common idioms.
447
448	elmex	1.7	How to solve the reliability and C<%client_ports> cleanup problem will
449			be explained later in this tutorial (TODO).
450
451	root	1.8	=head1 Inside The Protocol
452	elmex	1.7
453			Now, for the interested parties, let me explain some details about the protocol
454			that L<AnyEvent::MP> nodes use to communicate to each other. If you are not
455			interested you can skip this section.
456
457			Usually TCP is used for communication. Each I<node>, if configured to be a
458			I<public> node with the C<become_public> function will listen on the configured
459	root	1.10	TCP port (default is 4040).
460	elmex	1.7
461	root	1.10	If then one I<node> wants to send a message to another I<node> it will
462			connect to the host and port given in the I<port ID>.
463	elmex	1.7
464	root	1.10	Then some handshaking occurs to check whether both I<nodes> know the
465			I<shared secret>. Optionally, TLS can be enabled (about how to do this
466			exactly please consult the L<AnyEvent::MP> man page, just a hint: It
467			should be enough to put the private key and (self signed) certificate in
468			the C<~/.aemp-secret> file of all nodes).
469
470			After the handshake, messages will be exchanged using a serialiser
471			(usually L<JSON> is used for this, but it is also possible to use other
472			serialization formats such as L<Storable>).
473	elmex	1.7
474	elmex	1.1	=head1 SEE ALSO
475
476			L<AnyEvent>
477
478			L<AnyEvent::Handle>
479
480			L<AnyEvent::MP>
481
482			=head1 AUTHOR
483
484			Robin Redeker <elmex@ta-sa.org>
485	root	1.4