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 programs to transparently pass messages within the
process and to other processes on the same or a different host.

What kind of messages? 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). Here are two
examples:

    write_log => 1251555874, "action was successful.\n"
    123, ["a", "b", "c"], { foo => "bar" }

When using L<AnyEvent::MP> it is customary to use a descriptive string as
first element of a message, that indictes the type of the message. This
element is called a I<tag> in L<AnyEvent::MP>, as some API functions
(C<rcv>) support matching it directly.

Supposedly you want to send a ping message with your current time to
somewhere, this is how such a message might look like (in Perl syntax):

   ping => 1251381636

Now that we know what a message is, to which entities are those
messages being I<passed>? They are I<passed> to I<ports>. A I<port> is
a destination for messages but also a context to execute code: when
a runtime error occurs while executing code belonging to a port, the
exception will be raised on the port and can even travel to interested
parties on other nodes, which makes supervision of distributed processes
easy.

How do these ports relate to things you know? 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 or
another host in a network by its I<node ID>. A I<node ID> is simply a
unique string chosen manually or assigned by L<AnyEvent::MP> in some way
(UNIX nodename, random string...).

Here is a diagram about how I<nodes>, I<ports> and UNIX processes relate
to each other. The setup consists of two nodes (more are of course
possible): Node C<A> (in UNIX process 7066) with the ports C<ABC> and
C<DEF>. And the node C<B> (in UNIX process 8321) with the 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 I<port IDs> here are just for illustrative
purposes: Even though I<ports> in L<AnyEvent::MP> are also identified by
strings, they can't be choosen manually and are assigned by the system
dynamically. These I<port IDs> are unique within a network and can also be
used to identify senders or as message tags for instance.

The next sections will explain the API of L<AnyEvent::MP> by going through
a few simple examples. Later some more complex idioms are introduced,
which are hopefully useful to solve some real world problems.

=head1 Passing Your First Message

As a start lets have a look at the messaging API. The following example
is just a demo to show the basic elements of message passing with
L<AnyEvent::MP>.

The example should print: C<Ending with: 123>, in a rather complicated
way, by passing some message to a port.

   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 uses most of the essential functions inside
L<AnyEvent::MP>: First there is the C<port> function which will create a
I<port> and will return it's I<port ID>, a simple string.

This I<port ID> can be used to send messages to the port and install
handlers to receive messages on the port. Since it is a simple string
it can be safely passed to other I<nodes> in the network when you want
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 - messages in
L<AnyEvent::MP> have a destination, but no source).

The next function is C<rcv>:

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

It installs a receiver callback on the I<port> that specified as the first
argument (it only works for "local" ports, i.e. ports created on the same
node). The next argument, in this example C<test>, specifies a I<tag> to
match. This means that whenever a message with the first element being
the string C<test> is received, the callback is called with the remaining
parts of that message.

Messages can be sent with the C<snd> function, which is used 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> stored in C<$port>. Since in this case the receiver has a I<tag> match
on C<test> it will call the callback with the first argument being the
number C<123>.

The callback is a typicall AnyEvent idiom: the callback just passes
that number on to the I<condition variable> C<$end_cv> which will then
pass the value to the print. Condition variables are out of the scope
of this tutorial and not often used with ports, so please consult the
L<AnyEvent::Intro> about them.

Passing messages inside just one process is boring. Before we can move on
and do interprocess message passing we first have to make sure some things
have been set up correctly for our nodes to talk to each other.

=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 over the network it is
necessary to setup the same I<shared secret> for both of them, so they can
prove their trustworthyness to each other.

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 (or configure) a TLS certificate (F<aemp
gencert>), causing connections to not just be securely authenticated, but
also to be encrypted and protected against tinkering.

Connections will only be successfully established 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, in which case
no shared secret is required).

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 is all for now, you will find some more advanced fiddling with the
C<aemp> utility later.


=head1 Passing Messages Between Processes

=head2 The Receiver

Lets split the previous example up into two programs: one that contains
the sender and one for the receiver. First the receiver application, in
full:

   use AnyEvent;
   use AnyEvent::MP;
   use AnyEvent::MP::Global;

   configure nodeid => "eg_receiver", binds => ["*:4040"];

   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, let's step through the new functions
and modules that have been used.

For starters, there is now an additional module being
used: L<AnyEvent::MP::Global>. This module provides us with a I<global
registry>, which lets us register ports in groups that are visible on all
I<nodes> in a network.

What is this useful for? Well, the I<port IDs> are random-looking strings,
assigned by L<AnyEvent::MP>. We cannot know those I<port IDs> in advance,
so we don't know which I<port ID> to send messages to, especially when the
message is to be passed between different I<nodes> (or UNIX processes). To
find the right I<port> of another I<node> in the network we will need
to communicate this somehow to the sender. And exactly that is what
L<AnyEvent::MP::Global> provides.

Especially in larger, more anonymous networks this is handy: imagine you
have a few database backends, a few web frontends and some processing
distributed over a number of hosts: all of these would simply register
themselves in the appropriate group, and your web frontends can start to
find some database backend.

=head3 C<configure> and the Network

Now, let's have a look at the new function, C<configure>:

   configure nodeid => "eg_receiver", binds => ["*:4040"];

Before we are able to send messages to other nodes we have to initialise
ourself to become a "distributed node". Initialising a node means naming
the node, optionally binding some TCP listeners so that other nodes can
contact it and connecting to a predefined set of seed addresses so the
node can discover the existing network - and the existing network can
discover the node!

All of this (and more) can be passed to the C<configure> function - later
we will see how we can do all this without even passing anything to
C<configure>!

The first parameter, C<nodeid>, specified the node ID (in this case
C<eg_receiver> - the default is to use the node name of the current host,
but for this example we want to be able to run many nodes on the same
machine). Node IDs need to be unique within the network and can be almost
any string - if you don't care, you can specify a node ID of C<anon/>
which will then be replaced by a random node name.

The second parameter, C<binds>, specifies a list of C<address:port> pairs
to bind TCP listeners on. The special "address" of C<*> means to bind on
every local IP address.

The reason to bind on a TCP port is not just that other nodes can connect
to us: if no binds are specified, the node will still bind on a dynamic
port on all local addresses - but in this case we won't know the port, and
cannot tell other nodes to connect to it as seed node.

A I<seed> is a (fixed) TCP address of some other node in the network. To
explain the need for seeds we have to look at the topology of a typical
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 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 nodes already
connected. Exactly this is what the I<seeds> are for: Let's assume that
the new node (C<N5>) uses the TCP address of the node C<N2> as seed.  This
cuases it to connect to C<N2>:

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

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

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

All done: C<N5> is now happily connected to the rest of the network.

Of course, this process takes time, during which the node is already
running. This also means it takes time until the node is fully connected,
and global groups and other information is available. The best way to deal
with this is to either retry regularly until you found the resource you
were looking for, or to only start services on demand after a node has
become available.

=head3 Registering the Receiver

Coming back to our example, we have now introduced the basic purpose of
L<AnyEvent::MP::Global> and C<configure> and its use of profiles. We
also set up our profiles for later use and now we will finally continue
talking about the receiver.

Let's look at the next line(s):

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

The C<port> function has already been discussed. It simply creates a new
I<port> and returns the I<port ID>. The C<register> function, however,
is new: The first argument is the I<port ID> that we want to add to a
I<global group>, and its second argument is the name of that I<global
group>.

You can choose the name of such a I<global group> freely (prefixing your
package name is highly recommended!). The purpose of such a group is to
store a set of I<port IDs>. This set is made available throughout the
L<AnyEvent::MP> network, so that each node can see which ports belong to
that group.

Later we will see how the sender looks for the ports in this I<global
group> to send messages to them.

The last step in the example is to set up a receiver callback for those
messages, just as was discussed in the first example. We again match
for the tag C<test>. The difference is that this time we don't exit the
application after receiving the first message. Instead we continue to wait
for new messages indefinitely.

=head2 The Sender

Ok, now let's take a look at the sender code:

   use AnyEvent;
   use AnyEvent::MP;
   use AnyEvent::MP::Global;

   configure nodeid => "eg_sender", seeds => ["*:4040"];

   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<configure> serves the same purpose as in the
receiver, but instead of specifying binds we specify a list of seeds -
which happens to be the same as the binds used by the receiver, which
becomes our seed node.

Next we set up a timer that repeatedly (every second) calls this chunk of
code:

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

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

The only new function here is the C<find> function of
L<AnyEvent::MP::Global>. It searches in the global group named
C<eg_receivers> for ports. If none are found, it returns C<undef>, which
makes our code return instantly and wait for the next round, as nobody is
interested in our message.

As soon as the receiver application has connected and the information
about the newly added port in the receiver has propagated to the sender
node, C<find> returns an array reference that contains the I<port ID> of
the receiver I<port(s)>.

We then just send a message with a tag and the current time to every
I<port> in the global group.

=head3 Splitting Network Configuration and Application Code

Ok, so far, this works. In the real world, however, the person configuring
your application to run on a specific network (the end user or network
administrator) is often different to the person coding the application.

Or to put it differently: the arguments passed to configure are usually
provided not by the programmer, but by whoeever is deplying the program.

To make this easy, AnyEvent::MP supports a simple configuration database,
using profiles, which can be managed using the F<aemp> command-line
utility.

When you change both programs above to simply call

   configure;

then AnyEvent::MP tries to look up a profile using the current node name
in its configuration database, falling back to some global default.

You can run "generic" nodes using the F<aemp> utility as well, and we will
exploit this in the following way: we configure a profile "seed" and run
a node using it, whose sole purpose is to be a seed node for our example
programs.

We bind the seed node to port 4040 on all interfaces:

   aemp profile seed binds "*:4040"

And we configure all nodes to use this as seed node (this only works when
running on the same host, for multiple machines you would provide the IP
address or hostname of the node running the seed):

   aemp seeds "*:4040"

Then we run the seed node:

   aemp run profile seed

After that, we can start as many other nodes as we want, and they will all
use our generic seed node to discover each other.

In fact, starting many receivers nicely illustrates that the time sender
can have multiple receivers.

That's all for now - next time we will teach you about monitoring by
writing a simple chat client and server :)

=head1 SEE ALSO

L<AnyEvent>

L<AnyEvent::Handle>

L<AnyEvent::MP>

L<AnyEvent::MP::Global>

=head1 AUTHOR

  Robin Redeker <elmex@ta-sa.org>

Revision:	1.29
Committed:	Mon Aug 31 11:11:27 2009 UTC (14 years, 9 months ago) by root
Branch:	MAIN
Changes since 1.28:	+2 -2 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 programs to transparently pass messages within the
7	process and to other processes on the same or a different host.
8
9	What kind of messages? 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). Here are two
12	examples:
13
14	write_log => 1251555874, "action was successful.\n"
15	123, ["a", "b", "c"], { foo => "bar" }
16
17	When using L<AnyEvent::MP> it is customary to use a descriptive string as
18	first element of a message, that indictes the type of the message. This
19	element is called a I<tag> in L<AnyEvent::MP>, as some API functions
20	(C<rcv>) support matching it directly.
21
22	Supposedly you want to send a ping message with your current time to
23	somewhere, this is how such a message might look like (in Perl syntax):
24
25	ping => 1251381636
26
27	Now that we know what a message is, to which entities are those
28	messages being I<passed>? They are I<passed> to I<ports>. A I<port> is
29	a destination for messages but also a context to execute code: when
30	a runtime error occurs while executing code belonging to a port, the
31	exception will be raised on the port and can even travel to interested
32	parties on other nodes, which makes supervision of distributed processes
33	easy.
34
35	How do these ports relate to things you know? Each I<port> belongs
36	to a I<node>, and a I<node> is just the UNIX process that runs your
37	L<AnyEvent::MP> application.
38
39	Each I<node> is distinguished from other I<nodes> running on the same or
40	another host in a network by its I<node ID>. A I<node ID> is simply a
41	unique string chosen manually or assigned by L<AnyEvent::MP> in some way
42	(UNIX nodename, random string...).
43
44	Here is a diagram about how I<nodes>, I<ports> and UNIX processes relate
45	to each other. The setup consists of two nodes (more are of course
46	possible): Node C<A> (in UNIX process 7066) with the ports C<ABC> and
47	C<DEF>. And the node C<B> (in UNIX process 8321) with the ports C<FOO> and
48	C<BAR>.
49
50
51	\|- PID: 7066 -\| \|- PID: 8321 -\|
52	\| \| \| \|
53	\| Node ID: A \| \| Node ID: B \|
54	\| \| \| \|
55	\| Port ABC =\|= <----\ /-----> =\|= Port FOO \|
56	\| \| X \| \|
57	\| Port DEF =\|= <----/ \-----> =\|= Port BAR \|
58	\| \| \| \|
59	\|-------------\| \|-------------\|
60
61	The strings for the I<port IDs> here are just for illustrative
62	purposes: Even though I<ports> in L<AnyEvent::MP> are also identified by
63	strings, they can't be choosen manually and are assigned by the system
64	dynamically. These I<port IDs> are unique within a network and can also be
65	used to identify senders or as message tags for instance.
66
67	The next sections will explain the API of L<AnyEvent::MP> by going through
68	a few simple examples. Later some more complex idioms are introduced,
69	which are hopefully useful to solve some real world problems.
70
71	=head1 Passing Your First Message
72
73	As a start lets have a look at the messaging API. The following example
74	is just a demo to show the basic elements of message passing with
75	L<AnyEvent::MP>.
76
77	The example should print: C<Ending with: 123>, in a rather complicated
78	way, by passing some message to a port.
79
80	use AnyEvent;
81	use AnyEvent::MP;
82
83	my $end_cv = AnyEvent->condvar;
84
85	my $port = port;
86
87	rcv $port, test => sub {
88	my ($data) = @_;
89	$end_cv->send ($data);
90	};
91
92	snd $port, test => 123;
93
94	print "Ending with: " . $end_cv->recv . "\n";
95
96	It already uses most of the essential functions inside
97	L<AnyEvent::MP>: First there is the C<port> function which will create a
98	I<port> and will return it's I<port ID>, a simple string.
99
100	This I<port ID> can be used to send messages to the port and install
101	handlers to receive messages on the port. Since it is a simple string
102	it can be safely passed to other I<nodes> in the network when you want
103	to refer to that specific port (usually used for RPC, where you need
104	to tell the other end which I<port> to send the reply to - messages in
105	L<AnyEvent::MP> have a destination, but no source).
106
107	The next function is C<rcv>:
108
109	rcv $port, test => sub { ... };
110
111	It installs a receiver callback on the I<port> that specified as the first
112	argument (it only works for "local" ports, i.e. ports created on the same
113	node). The next argument, in this example C<test>, specifies a I<tag> to
114	match. This means that whenever a message with the first element being
115	the string C<test> is received, the callback is called with the remaining
116	parts of that message.
117
118	Messages can be sent with the C<snd> function, which is used like this in
119	the example above:
120
121	snd $port, test => 123;
122
123	This will send the message C<'test', 123> to the I<port> with the I<port
124	ID> stored in C<$port>. Since in this case the receiver has a I<tag> match
125	on C<test> it will call the callback with the first argument being the
126	number C<123>.
127
128	The callback is a typicall AnyEvent idiom: the callback just passes
129	that number on to the I<condition variable> C<$end_cv> which will then
130	pass the value to the print. Condition variables are out of the scope
131	of this tutorial and not often used with ports, so please consult the
132	L<AnyEvent::Intro> about them.
133
134	Passing messages inside just one process is boring. Before we can move on
135	and do interprocess message passing we first have to make sure some things
136	have been set up correctly for our nodes to talk to each other.
137
138	=head1 System Requirements and System Setup
139
140	Before we can start with real IPC we have to make sure some things work on
141	your system.
142
143	First we have to setup a I<shared secret>: for two L<AnyEvent::MP>
144	I<nodes> to be able to communicate with each other over the network it is
145	necessary to setup the same I<shared secret> for both of them, so they can
146	prove their trustworthyness to each other.
147
148	The easiest way is to set this up is to use the F<aemp> utility:
149
150	aemp gensecret
151
152	This creates a F<$HOME/.perl-anyevent-mp> config file and generates a
153	random shared secret. You can copy this file to any other system and
154	then communicate over the network (via TCP) with it. You can also select
155	your own shared secret (F<aemp setsecret>) and for increased security
156	requirements you can even create (or configure) a TLS certificate (F<aemp
157	gencert>), causing connections to not just be securely authenticated, but
158	also to be encrypted and protected against tinkering.
159
160	Connections will only be successfully established when the I<nodes>
161	that want to connect to each other have the same I<shared secret> (or
162	successfully verify the TLS certificate of the other side, in which case
163	no shared secret is required).
164
165	B<If something does not work as expected, and for example tcpdump shows
166	that the connections are closed almost immediately, you should make sure
167	that F<~/.perl-anyevent-mp> is the same on all hosts/user accounts that
168	you try to connect with each other!>
169
170	Thats is all for now, you will find some more advanced fiddling with the
171	C<aemp> utility later.
172
173
174	=head1 Passing Messages Between Processes
175
176	=head2 The Receiver
177
178	Lets split the previous example up into two programs: one that contains
179	the sender and one for the receiver. First the receiver application, in
180	full:
181
182	use AnyEvent;
183	use AnyEvent::MP;
184	use AnyEvent::MP::Global;
185
186	configure nodeid => "eg_receiver", binds => ["*:4040"];
187
188	my $port = port;
189
190	AnyEvent::MP::Global::register $port, "eg_receivers";
191
192	rcv $port, test => sub {
193	my ($data, $reply_port) = @_;
194
195	print "Received data: " . $data . "\n";
196	};
197
198	AnyEvent->condvar->recv;
199
200	=head3 AnyEvent::MP::Global
201
202	Now, that wasn't too bad, was it? Ok, let's step through the new functions
203	and modules that have been used.
204
205	For starters, there is now an additional module being
206	used: L<AnyEvent::MP::Global>. This module provides us with a I<global
207	registry>, which lets us register ports in groups that are visible on all
208	I<nodes> in a network.
209
210	What is this useful for? Well, the I<port IDs> are random-looking strings,
211	assigned by L<AnyEvent::MP>. We cannot know those I<port IDs> in advance,
212	so we don't know which I<port ID> to send messages to, especially when the
213	message is to be passed between different I<nodes> (or UNIX processes). To
214	find the right I<port> of another I<node> in the network we will need
215	to communicate this somehow to the sender. And exactly that is what
216	L<AnyEvent::MP::Global> provides.
217
218	Especially in larger, more anonymous networks this is handy: imagine you
219	have a few database backends, a few web frontends and some processing
220	distributed over a number of hosts: all of these would simply register
221	themselves in the appropriate group, and your web frontends can start to
222	find some database backend.
223
224	=head3 C<configure> and the Network
225
226	Now, let's have a look at the new function, C<configure>:
227
228	configure nodeid => "eg_receiver", binds => ["*:4040"];
229
230	Before we are able to send messages to other nodes we have to initialise
231	ourself to become a "distributed node". Initialising a node means naming
232	the node, optionally binding some TCP listeners so that other nodes can
233	contact it and connecting to a predefined set of seed addresses so the
234	node can discover the existing network - and the existing network can
235	discover the node!
236
237	All of this (and more) can be passed to the C<configure> function - later
238	we will see how we can do all this without even passing anything to
239	C<configure>!
240
241	The first parameter, C<nodeid>, specified the node ID (in this case
242	C<eg_receiver> - the default is to use the node name of the current host,
243	but for this example we want to be able to run many nodes on the same
244	machine). Node IDs need to be unique within the network and can be almost
245	any string - if you don't care, you can specify a node ID of C<anon/>
246	which will then be replaced by a random node name.
247
248	The second parameter, C<binds>, specifies a list of C<address:port> pairs
249	to bind TCP listeners on. The special "address" of C<*> means to bind on
250	every local IP address.
251
252	The reason to bind on a TCP port is not just that other nodes can connect
253	to us: if no binds are specified, the node will still bind on a dynamic
254	port on all local addresses - but in this case we won't know the port, and
255	cannot tell other nodes to connect to it as seed node.
256
257	A I<seed> is a (fixed) TCP address of some other node in the network. To
258	explain the need for seeds we have to look at the topology of a typical
259	L<AnyEvent::MP> network. The topology is called a I<fully connected mesh>,
260	here an example with 4 nodes:
261
262	N1--N2
263	\| \/ \|
264	\| /\ \|
265	N3--N4
266
267	Now imagine another node - C<N5> - wants to connect itself to that network:
268
269	N1--N2
270	\| \/ \| N5
271	\| /\ \|
272	N3--N4
273
274	The new node needs to know the I<binds> of all nodes already
275	connected. Exactly this is what the I<seeds> are for: Let's assume that
276	the new node (C<N5>) uses the TCP address of the node C<N2> as seed. This
277	cuases it to connect to C<N2>:
278
279	N1--N2____
280	\| \/ \| N5
281	\| /\ \|
282	N3--N4
283
284	C<N2> then tells C<N5> about the I<binds> of the other nodes it is
285	connected to, and C<N5> creates the rest of the connections:
286
287	/--------\
288	N1--N2____\|
289	\| \/ \| N5
290	\| /\ \| /\|
291	N3--N4--- \|
292	\________/
293
294	All done: C<N5> is now happily connected to the rest of the network.
295
296	Of course, this process takes time, during which the node is already
297	running. This also means it takes time until the node is fully connected,
298	and global groups and other information is available. The best way to deal
299	with this is to either retry regularly until you found the resource you
300	were looking for, or to only start services on demand after a node has
301	become available.
302
303	=head3 Registering the Receiver
304
305	Coming back to our example, we have now introduced the basic purpose of
306	L<AnyEvent::MP::Global> and C<configure> and its use of profiles. We
307	also set up our profiles for later use and now we will finally continue
308	talking about the receiver.
309
310	Let's look at the next line(s):
311
312	my $port = port;
313	AnyEvent::MP::Global::register $port, "eg_receivers";
314
315	The C<port> function has already been discussed. It simply creates a new
316	I<port> and returns the I<port ID>. The C<register> function, however,
317	is new: The first argument is the I<port ID> that we want to add to a
318	I<global group>, and its second argument is the name of that I<global
319	group>.
320
321	You can choose the name of such a I<global group> freely (prefixing your
322	package name is highly recommended!). The purpose of such a group is to
323	store a set of I<port IDs>. This set is made available throughout the
324	L<AnyEvent::MP> network, so that each node can see which ports belong to
325	that group.
326
327	Later we will see how the sender looks for the ports in this I<global
328	group> to send messages to them.
329
330	The last step in the example is to set up a receiver callback for those
331	messages, just as was discussed in the first example. We again match
332	for the tag C<test>. The difference is that this time we don't exit the
333	application after receiving the first message. Instead we continue to wait
334	for new messages indefinitely.
335
336	=head2 The Sender
337
338	Ok, now let's take a look at the sender code:
339
340	use AnyEvent;
341	use AnyEvent::MP;
342	use AnyEvent::MP::Global;
343
344	configure nodeid => "eg_sender", seeds => ["*:4040"];
345
346	my $find_timer =
347	AnyEvent->timer (after => 0, interval => 1, cb => sub {
348	my $ports = AnyEvent::MP::Global::find "eg_receivers"
349	or return;
350
351	snd $_, test => time
352	for @$ports;
353	});
354
355	AnyEvent->condvar->recv;
356
357	It's even less code. The C<configure> serves the same purpose as in the
358	receiver, but instead of specifying binds we specify a list of seeds -
359	which happens to be the same as the binds used by the receiver, which
360	becomes our seed node.
361
362	Next we set up a timer that repeatedly (every second) calls this chunk of
363	code:
364
365	my $ports = AnyEvent::MP::Global::find "eg_receivers"
366	or return;
367
368	snd $_, test => time
369	for @$ports;
370
371	The only new function here is the C<find> function of
372	L<AnyEvent::MP::Global>. It searches in the global group named
373	C<eg_receivers> for ports. If none are found, it returns C<undef>, which
374	makes our code return instantly and wait for the next round, as nobody is
375	interested in our message.
376
377	As soon as the receiver application has connected and the information
378	about the newly added port in the receiver has propagated to the sender
379	node, C<find> returns an array reference that contains the I<port ID> of
380	the receiver I<port(s)>.
381
382	We then just send a message with a tag and the current time to every
383	I<port> in the global group.
384
385	=head3 Splitting Network Configuration and Application Code
386
387	Ok, so far, this works. In the real world, however, the person configuring
388	your application to run on a specific network (the end user or network
389	administrator) is often different to the person coding the application.
390
391	Or to put it differently: the arguments passed to configure are usually
392	provided not by the programmer, but by whoeever is deplying the program.
393
394	To make this easy, AnyEvent::MP supports a simple configuration database,
395	using profiles, which can be managed using the F<aemp> command-line
396	utility.
397
398	When you change both programs above to simply call
399
400	configure;
401
402	then AnyEvent::MP tries to look up a profile using the current node name
403	in its configuration database, falling back to some global default.
404
405	You can run "generic" nodes using the F<aemp> utility as well, and we will
406	exploit this in the following way: we configure a profile "seed" and run
407	a node using it, whose sole purpose is to be a seed node for our example
408	programs.
409
410	We bind the seed node to port 4040 on all interfaces:
411
412	aemp profile seed binds "*:4040"
413
414	And we configure all nodes to use this as seed node (this only works when
415	running on the same host, for multiple machines you would provide the IP
416	address or hostname of the node running the seed):
417
418	aemp seeds "*:4040"
419
420	Then we run the seed node:
421
422	aemp run profile seed
423
424	After that, we can start as many other nodes as we want, and they will all
425	use our generic seed node to discover each other.
426
427	In fact, starting many receivers nicely illustrates that the time sender
428	can have multiple receivers.
429
430	That's all for now - next time we will teach you about monitoring by
431	writing a simple chat client and server :)
432
433	=head1 SEE ALSO
434
435	L<AnyEvent>
436
437	L<AnyEvent::Handle>
438
439	L<AnyEvent::MP>
440
441	L<AnyEvent::MP::Global>
442
443	=head1 AUTHOR
444
445	Robin Redeker <elmex@ta-sa.org>
446