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, anything that can be expressed as a
L<JSON> text (as JSON is used by default in the protocol).

It's custom in L<AnyEvent::MP> to have a string which describes the type of the
message as first element (this is called a I<tag> in L<AnyEvent::MP>), as some
API functions (C<rcv>) support matching it directly. So supposedly you want to
send a ping message with your current time to something, this is how such a
message might look like (in Perl syntax):

   ['ping', 1251381636]

And next you might ask: between which entities are those messages being
I<passed>? They are I<passed> between I<ports>. I<ports> are just sources and
destinations for messages. How do these ports relate to things you know?  Well,
each I<port> belongs to a I<node>, and a I<node> is just the UNIX process that
runs your L<AnyEvent::MP> application.

Each I<node> is distinguished from other I<nodes> running on the same host or
multiple hosts in a network by it's I<node ID>. A I<node ID> can be manually
assigned or L<AnyEvent::MP> will assign one it self for you.

So, you might want to visualize it like this (setup is two nodes (more are of
course possible): Node C<A> (in UNIX process 7066) with ports C<ABC> and C<DEF>
and C<B> (in UNIX process 8321) with ports C<FOO> and C<BAR>).


  |- PID: 7066 -|                  |- PID: 8321 -|
  |             |                  |             |
  | Node ID: A  |                  | Node ID: B  |
  |             |                  |             |
  |   Port ABC =|= <----\ /-----> =|= Port FOO   |
  |             |        X         |             |
  |   Port DEF =|= <----/ \-----> =|= Port BAR   |
  |             |                  |             |
  |-------------|                  |-------------|

The strings for the ports here are just for illustrative purposes.  Even if in
reality I<ports> in L<AnyEvent::MP> are also identified by strings they can't
be choosen manually and are assigned randomly. These I<port ids> should also
not be used directly for other purposes than referring to an endpoint for
messages.

The next sections will explain the API of L<AnyEvent::MP>. First the API is
laid out by simple examples. Later some more complex idioms are introduced,
which are maybe useful to solve some real world purposes.

# 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 public chat server and client system,
# but it can be used to implement a distributed chat server for example.

=head1 Passing Your First Message

As start lets have a look at the messaging API. The next example is just a
demo to show the basic elements of message passing with L<AnyEvent::MP>.
It shout just print: "Ending with: 123". So here the code:

   use AnyEvent;
   use AnyEvent::MP;

   my $end_cv = AnyEvent->condvar;

   my $port = port;

   rcv $port, test => sub {
      my ($data) = @_;
      $end_cv->send ($data);
   };

   snd $port, test => 123;

   print "Ending with: " . $end_cv->recv . "\n";

It already contains most functions of the essential L<AnyEvent::MP> API.

First there is the C<port> function which will create a I<port> and will return
it's I<port id>.

That I<port id> can be used to send and receive messages. That I<port id> is a
simple string and can be safely passed to other I<nodes> in the network to
refer to that specific port (usually used for RPC, where you need to
tell the other end which I<port> to send the reply to).

Next function is C<rcv>:

   rcv $port, test => sub { ... };

It sets up a receiver callback on a specific I<port> which needs to be
specified as the first argument. The next argument, in this example C<test>, is
a I<tag> match. This means that whenever a message, with the first element
being the string C<tag>, is received the callback is called with the remaining
parts of that message.

Messages can be send with the C<snd> function, which looks like this in the
example above:

   snd $port, test => 123;

This will send the message C<['test', 123]> to the I<port> with the I<port id>
in C<$port>. The receiver got a I<tag> match on C<test> and will call the
callback with the first argument being the number C<123>.

That callback then just passes that number on to the I<condition variable>
C<$end_cv> which will then pass the value to the print. But I<condition
variables> are out of the scope of this tutorial. So please consult the
L<AnyEvent::Intro> about them.

But passing messages inside one process is boring, but before we can continue
and take the next step to interprocess message passing we first have to make
sure some things have been setup.

=head1 System Requirements and System Setup

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

First we have to setup a I<shared secret>: for two L<AnyEvent::MP> I<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 (or 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
over the network (via TCP) 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 I<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 immediately, 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!>

Thats all for now, there is more fiddling around with the C<aemp> utility
later.

=head1 Passing Messages Between Processes

=head2 The Receiver

Lets split the previous example up into two small programs. First the
receiver application:

   #!/opt/perl/bin/perl
   use AnyEvent;
   use AnyEvent::MP;
   use AnyEvent::MP::Global;

   initialise_node "eg_simple_receiver";

   my $port = port;

   AnyEvent::MP::Global::register $port, "eg_receivers";

   rcv $port, test => sub {
      my ($data, $reply_port) = @_;

      print "Received data: " . $data . "\n";
   };

   AnyEvent->condvar->recv;

=head3 AnyEvent::MP::Global

Now, that wasn't too bad, was it? Ok, lets step through the new functions
and modules that have been used. For starters there is now an additional
module loaded: L<AnyEvent::MP::Global>.

That module provides us with a I<global registry>, which lets us share data
among all I<nodes> in a network. Why do we need it you might ask?

The thing is, that the I<port ids> are just random strings, assigned by
L<AnyEvent::MP>.  We can't know those I<port ids> in advance, so we don't know
which I<port id> to send messages to if the message is to be passed between
I<nodes> (or UNIX processes). To find the right I<port> of another I<node> in
the network we will need to communicate that somehow to the sender.  And
exactly that is what L<AnyEvent::MP::Global> provides.

=head3 initialise_node And The Network

Now, lets have a look at the next new thing, the C<initialise_node>:

   initialise_node "eg_simple_receiver";

Before we are able to send messages to other nodes we have to initialise
ourself. The first argument, the string C<"eg_simple_receiver">, is called the
I<profile> of this node. A profile holds some information about the application
that is going to be a node in an L<AnyEvent::MP> network.

Most importantly the profile allows you to set the I<node id> that your
application will use. You can also set I<binds> in the profile, meaning that
you can define TCP ports that the application will listen on for incoming
connections from other nodes of the network.

Next you can configure I<seeds> in profile. A I<seed> is just a TCP endpoint
which tells the application where to find other nodes of it's network. To
explain this a bit more detailed we have to look at the topology of an
L<AnyEvent::MP> network. The topology is called a I<fully connected mesh>, here
an example with 4 nodes:

   N1--N2
   | \/ |
   | /\ |
   N3--N4

Now imagine another I<node> C<N5>. wants to connect itself to that network:

   N1--N2
   | \/ |    N5
   | /\ |
   N3--N4

The new node needs to know the I<binds> of all of those 4 already connected
nodes. And exactly this is what the I<seeds> are for. Now lets assume that
the new node C<N5> has as I<seed> the TCP endpoint of the node C<N2>.
It then connects to C<N2>:

   N1--N2____
   | \/ |    N5
   | /\ |
   N3--N4

C<N2> then tells C<N5> the I<binds> of the other nodes it is connected to,
and C<N5> builds up the rest of the connections:

    /--------\
   N1--N2____|
   | \/ |    N5
   | /\ |   /|
   N3--N4--- |
    \________/

Finished. C<N5> is now happily connected to the rest of the network.

=head3 Setting Up The Profiles

Ok, so much to the profile. Now lets setup the C<eg_simple_receiver> I<profile>
for later. For the receiver we just give the receiver a I<bind>:

   aemp profile eg_simple_receiver setbinds localhost:12266

And while we are at it, just setup the I<profile> for the sender in the second
part of this example too. We will call the sender I<profile>
C<eg_simple_sender>. For the sender we will just setup a I<seed> to the
receiver:

   aemp profile eg_simple_sender setseeds localhost:12266

You might wonder why we don't setup a I<bind> here. Well, there can be
exceptions to the I<fully> in the I<fully connected mesh> in L<AnyEvent::MP>.
If you don't configure a I<bind> for a node's profile it won't bind itself
somewhere. These kinds of I<nodes> will not be able to send messages to other
I<nodes> that also didn't I<bind> them self to some TCP address. For this
example, as well as some cases in the real world, we can live with this
limitation.

=head3 Registering The Receiver

Ok, where were we. We now discussed the basic purpose of L<AnyEvent::MP::Global>
and initialise_node with it's relations to profiles. We also setup our profiles
for later use and now have to continue talking about the receiver example.

Lets look at the next undiscussed line(s) of code:

   my $port = port;
   AnyEvent::MP::Global::register $port, "eg_receivers";

The C<port> function already has been discussed. It just creates a new I<port>
and gives us the I<port id>. Now to the C<register> function of
L<AnyEvent::MP::Global>: The first argument is a I<port id> that we want to add
to a I<global group>, and it's second argument is the name of that I<global
group>.

You can choose that name of such a I<global group> freely, and it's purpose is
to store a set of I<port ids>. That set is made available throughout the whole
L<AnyEvent::MP> network, so that each node can see which ports belong to that
group.

The sender will later look for the ports in that I<global group> and send
messages to them.

Last step in the example is to setup a receiver callback for those messages
like we have discussed in the first example. We again match for the I<tag>
C<test>. The difference is just that we don't end the application after
receiving the first message. We just infinitely continue to look out for new
messages.

=head2 The Sender

Ok, now lets take a look at the sender:

   #!/opt/perl/bin/perl
   use AnyEvent;
   use AnyEvent::MP;
   use AnyEvent::MP::Global;

   initialise_node "eg_simple_sender";

   my $find_timer =
      AnyEvent->timer (after => 0, interval => 1, cb => sub {
         my $ports = AnyEvent::MP::Global::find "eg_receivers"
            or return;

         snd $_, test => time
            for @$ports;
      });

   AnyEvent->condvar->recv;

It's even less code. The C<initialise_node> is known now from the receiver
above. As discussed in the section where we setup the profiles we configure
this application to use the I<profile> C<eg_simple_sender>.

Next we setup a timer that repeatedly calls this chunk of code:

   my $ports = AnyEvent::MP::Global::find "eg_receivers"
      or return;

   snd $_, test => time
      for @$ports;

The new function here is the C<find> function of L<AnyEvent::MP::Global>.  It
searches in the I<global group> named C<eg_receivers> for ports. If none are
found C<undef> is returned and we wait for the next time the timer fires.

In case the receiver application has been connected and the newly added port by
the receiver has propagated to the sender C<find> returns an array reference
that contains the I<port id> of the receiver I<port(s)>.

We then just send to every I<port> in the I<global group> a message consisting
of the I<tag> C<test> and the current time in form of a UNIX timestamp.

And thats all.

=head1 SEE ALSO

L<AnyEvent>

L<AnyEvent::Handle>

L<AnyEvent::MP>

L<AnyEvent::MP::Global>

=head1 AUTHOR

  Robin Redeker <elmex@ta-sa.org>

Revision:	1.20
Committed:	Fri Aug 28 15:44:39 2009 UTC (14 years, 9 months ago) by elmex
Branch:	MAIN
Changes since 1.19:	+38 -407 lines
Log Message:	finished intro of second example.
#	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 Perl
10	strings, numbers, hashes and arrays, anything that can be expressed as a
11	L<JSON> text (as JSON is used by default in the protocol).
12
13	It's custom in L<AnyEvent::MP> to have a string which describes the type of the
14	message as first element (this is called a I<tag> in L<AnyEvent::MP>), as some
15	API functions (C<rcv>) support matching it directly. So supposedly you want to
16	send a ping message with your current time to something, this is how such a
17	message might look like (in Perl syntax):
18
19	['ping', 1251381636]
20
21	And next you might ask: between which entities are those messages being
22	I<passed>? They are I<passed> between I<ports>. I<ports> are just sources and
23	destinations for messages. How do these ports relate to things you know? Well,
24	each I<port> belongs to a I<node>, and a I<node> is just the UNIX process that
25	runs your L<AnyEvent::MP> application.
26
27	Each I<node> is distinguished from other I<nodes> running on the same host or
28	multiple hosts in a network by it's I<node ID>. A I<node ID> can be manually
29	assigned or L<AnyEvent::MP> will assign one it self for you.
30
31	So, you might want to visualize it like this (setup is two nodes (more are of
32	course possible): Node C<A> (in UNIX process 7066) with ports C<ABC> and C<DEF>
33	and C<B> (in UNIX process 8321) with ports C<FOO> and C<BAR>).
34
35
36	\|- PID: 7066 -\| \|- PID: 8321 -\|
37	\| \| \| \|
38	\| Node ID: A \| \| Node ID: B \|
39	\| \| \| \|
40	\| Port ABC =\|= <----\ /-----> =\|= Port FOO \|
41	\| \| X \| \|
42	\| Port DEF =\|= <----/ \-----> =\|= Port BAR \|
43	\| \| \| \|
44	\|-------------\| \|-------------\|
45
46	The strings for the ports here are just for illustrative purposes. Even if in
47	reality I<ports> in L<AnyEvent::MP> are also identified by strings they can't
48	be choosen manually and are assigned randomly. These I<port ids> should also
49	not be used directly for other purposes than referring to an endpoint for
50	messages.
51
52	The next sections will explain the API of L<AnyEvent::MP>. First the API is
53	laid out by simple examples. Later some more complex idioms are introduced,
54	which are maybe useful to solve some real world purposes.
55
56	# In this tutorial I'll show you how to write a simple chat server based on
57	# L<AnyEvent::MP>. This example is used because it nicely shows how to organise a
58	# simple application, but keep in mind that every node trusts any other, so this
59	# chat cannot be used to implement a real public chat server and client system,
60	# but it can be used to implement a distributed chat server for example.
61
62	=head1 Passing Your First Message
63
64	As start lets have a look at the messaging API. The next example is just a
65	demo to show the basic elements of message passing with L<AnyEvent::MP>.
66	It shout just print: "Ending with: 123". So here the code:
67
68	use AnyEvent;
69	use AnyEvent::MP;
70
71	my $end_cv = AnyEvent->condvar;
72
73	my $port = port;
74
75	rcv $port, test => sub {
76	my ($data) = @_;
77	$end_cv->send ($data);
78	};
79
80	snd $port, test => 123;
81
82	print "Ending with: " . $end_cv->recv . "\n";
83
84	It already contains most functions of the essential L<AnyEvent::MP> API.
85
86	First there is the C<port> function which will create a I<port> and will return
87	it's I<port id>.
88
89	That I<port id> can be used to send and receive messages. That I<port id> is a
90	simple string and can be safely passed to other I<nodes> in the network to
91	refer to that specific port (usually used for RPC, where you need to
92	tell the other end which I<port> to send the reply to).
93
94	Next function is C<rcv>:
95
96	rcv $port, test => sub { ... };
97
98	It sets up a receiver callback on a specific I<port> which needs to be
99	specified as the first argument. The next argument, in this example C<test>, is
100	a I<tag> match. This means that whenever a message, with the first element
101	being the string C<tag>, is received the callback is called with the remaining
102	parts of that message.
103
104	Messages can be send with the C<snd> function, which looks like this in the
105	example above:
106
107	snd $port, test => 123;
108
109	This will send the message C<['test', 123]> to the I<port> with the I<port id>
110	in C<$port>. The receiver got a I<tag> match on C<test> and will call the
111	callback with the first argument being the number C<123>.
112
113	That callback then just passes that number on to the I<condition variable>
114	C<$end_cv> which will then pass the value to the print. But I<condition
115	variables> are out of the scope of this tutorial. So please consult the
116	L<AnyEvent::Intro> about them.
117
118	But passing messages inside one process is boring, but before we can continue
119	and take the next step to interprocess message passing we first have to make
120	sure some things have been setup.
121
122	=head1 System Requirements and System Setup
123
124	Before we can start with real IPC we have to make sure some things work on your
125	system.
126
127	First we have to setup a I<shared secret>: for two L<AnyEvent::MP> I<nodes> to
128	be able to communicate with each other and authenticate each other it is
129	necessary to setup the same I<shared secret> for both of them (or use TLS
130	certificates).
131
132	The easiest way is to set this up is to use the F<aemp> utility:
133
134	aemp gensecret
135
136	This creates a F<$HOME/.perl-anyevent-mp> config file and generates a random
137	shared secret. You can copy this file to any other system and then communicate
138	over the network (via TCP) with it. You can also select your own shared secret
139	(F<aemp setsecret>) and for increased security requirements you can even create
140	a TLS certificate (F<aemp gencert>), causing connections to not just be
141	authenticated, but also to be encrypted.
142
143	Connections will only be successful when the I<nodes> that want to connect to
144	each other have the same I<shared secret> (or successfully verify the TLS
145	certificate of the other side).
146
147	B<If something does not work as expected, and for example tcpdump shows
148	that the connections are closed almost immediately, you should make sure
149	that F<~/.perl-anyevent-mp> is the same on all hosts/user accounts that
150	you try to connect with each other!>
151
152	Thats all for now, there is more fiddling around with the C<aemp> utility
153	later.
154
155	=head1 Passing Messages Between Processes
156
157	=head2 The Receiver
158
159	Lets split the previous example up into two small programs. First the
160	receiver application:
161
162	#!/opt/perl/bin/perl
163	use AnyEvent;
164	use AnyEvent::MP;
165	use AnyEvent::MP::Global;
166
167	initialise_node "eg_simple_receiver";
168
169	my $port = port;
170
171	AnyEvent::MP::Global::register $port, "eg_receivers";
172
173	rcv $port, test => sub {
174	my ($data, $reply_port) = @_;
175
176	print "Received data: " . $data . "\n";
177	};
178
179	AnyEvent->condvar->recv;
180
181	=head3 AnyEvent::MP::Global
182
183	Now, that wasn't too bad, was it? Ok, lets step through the new functions
184	and modules that have been used. For starters there is now an additional
185	module loaded: L<AnyEvent::MP::Global>.
186
187	That module provides us with a I<global registry>, which lets us share data
188	among all I<nodes> in a network. Why do we need it you might ask?
189
190	The thing is, that the I<port ids> are just random strings, assigned by
191	L<AnyEvent::MP>. We can't know those I<port ids> in advance, so we don't know
192	which I<port id> to send messages to if the message is to be passed between
193	I<nodes> (or UNIX processes). To find the right I<port> of another I<node> in
194	the network we will need to communicate that somehow to the sender. And
195	exactly that is what L<AnyEvent::MP::Global> provides.
196
197	=head3 initialise_node And The Network
198
199	Now, lets have a look at the next new thing, the C<initialise_node>:
200
201	initialise_node "eg_simple_receiver";
202
203	Before we are able to send messages to other nodes we have to initialise
204	ourself. The first argument, the string C<"eg_simple_receiver">, is called the
205	I<profile> of this node. A profile holds some information about the application
206	that is going to be a node in an L<AnyEvent::MP> network.
207
208	Most importantly the profile allows you to set the I<node id> that your
209	application will use. You can also set I<binds> in the profile, meaning that
210	you can define TCP ports that the application will listen on for incoming
211	connections from other nodes of the network.
212
213	Next you can configure I<seeds> in profile. A I<seed> is just a TCP endpoint
214	which tells the application where to find other nodes of it's network. To
215	explain this a bit more detailed we have to look at the topology of an
216	L<AnyEvent::MP> network. The topology is called a I<fully connected mesh>, here
217	an example with 4 nodes:
218
219	N1--N2
220	\| \/ \|
221	\| /\ \|
222	N3--N4
223
224	Now imagine another I<node> C<N5>. wants to connect itself to that network:
225
226	N1--N2
227	\| \/ \| N5
228	\| /\ \|
229	N3--N4
230
231	The new node needs to know the I<binds> of all of those 4 already connected
232	nodes. And exactly this is what the I<seeds> are for. Now lets assume that
233	the new node C<N5> has as I<seed> the TCP endpoint of the node C<N2>.
234	It then connects to C<N2>:
235
236	N1--N2____
237	\| \/ \| N5
238	\| /\ \|
239	N3--N4
240
241	C<N2> then tells C<N5> the I<binds> of the other nodes it is connected to,
242	and C<N5> builds up the rest of the connections:
243
244	/--------\
245	N1--N2____\|
246	\| \/ \| N5
247	\| /\ \| /\|
248	N3--N4--- \|
249	\________/
250
251	Finished. C<N5> is now happily connected to the rest of the network.
252
253	=head3 Setting Up The Profiles
254
255	Ok, so much to the profile. Now lets setup the C<eg_simple_receiver> I<profile>
256	for later. For the receiver we just give the receiver a I<bind>:
257
258	aemp profile eg_simple_receiver setbinds localhost:12266
259
260	And while we are at it, just setup the I<profile> for the sender in the second
261	part of this example too. We will call the sender I<profile>
262	C<eg_simple_sender>. For the sender we will just setup a I<seed> to the
263	receiver:
264
265	aemp profile eg_simple_sender setseeds localhost:12266
266
267	You might wonder why we don't setup a I<bind> here. Well, there can be
268	exceptions to the I<fully> in the I<fully connected mesh> in L<AnyEvent::MP>.
269	If you don't configure a I<bind> for a node's profile it won't bind itself
270	somewhere. These kinds of I<nodes> will not be able to send messages to other
271	I<nodes> that also didn't I<bind> them self to some TCP address. For this
272	example, as well as some cases in the real world, we can live with this
273	limitation.
274
275	=head3 Registering The Receiver
276
277	Ok, where were we. We now discussed the basic purpose of L<AnyEvent::MP::Global>
278	and initialise_node with it's relations to profiles. We also setup our profiles
279	for later use and now have to continue talking about the receiver example.
280
281	Lets look at the next undiscussed line(s) of code:
282
283	my $port = port;
284	AnyEvent::MP::Global::register $port, "eg_receivers";
285
286	The C<port> function already has been discussed. It just creates a new I<port>
287	and gives us the I<port id>. Now to the C<register> function of
288	L<AnyEvent::MP::Global>: The first argument is a I<port id> that we want to add
289	to a I<global group>, and it's second argument is the name of that I<global
290	group>.
291
292	You can choose that name of such a I<global group> freely, and it's purpose is
293	to store a set of I<port ids>. That set is made available throughout the whole
294	L<AnyEvent::MP> network, so that each node can see which ports belong to that
295	group.
296
297	The sender will later look for the ports in that I<global group> and send
298	messages to them.
299
300	Last step in the example is to setup a receiver callback for those messages
301	like we have discussed in the first example. We again match for the I<tag>
302	C<test>. The difference is just that we don't end the application after
303	receiving the first message. We just infinitely continue to look out for new
304	messages.
305
306	=head2 The Sender
307
308	Ok, now lets take a look at the sender:
309
310	#!/opt/perl/bin/perl
311	use AnyEvent;
312	use AnyEvent::MP;
313	use AnyEvent::MP::Global;
314
315	initialise_node "eg_simple_sender";
316
317	my $find_timer =
318	AnyEvent->timer (after => 0, interval => 1, cb => sub {
319	my $ports = AnyEvent::MP::Global::find "eg_receivers"
320	or return;
321
322	snd $_, test => time
323	for @$ports;
324	});
325
326	AnyEvent->condvar->recv;
327
328	It's even less code. The C<initialise_node> is known now from the receiver
329	above. As discussed in the section where we setup the profiles we configure
330	this application to use the I<profile> C<eg_simple_sender>.
331
332	Next we setup a timer that repeatedly calls this chunk of code:
333
334	my $ports = AnyEvent::MP::Global::find "eg_receivers"
335	or return;
336
337	snd $_, test => time
338	for @$ports;
339
340	The new function here is the C<find> function of L<AnyEvent::MP::Global>. It
341	searches in the I<global group> named C<eg_receivers> for ports. If none are
342	found C<undef> is returned and we wait for the next time the timer fires.
343
344	In case the receiver application has been connected and the newly added port by
345	the receiver has propagated to the sender C<find> returns an array reference
346	that contains the I<port id> of the receiver I<port(s)>.
347
348	We then just send to every I<port> in the I<global group> a message consisting
349	of the I<tag> C<test> and the current time in form of a UNIX timestamp.
350
351	And thats all.
352
353	=head1 SEE ALSO
354
355	L<AnyEvent>
356
357	L<AnyEvent::Handle>
358
359	L<AnyEvent::MP>
360
361	L<AnyEvent::MP::Global>
362
363	=head1 AUTHOR
364
365	Robin Redeker <elmex@ta-sa.org>
366