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root |
1.4 |
=head1 Message Passing for the Non-Blocked Mind |
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elmex |
1.1 |
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root |
1.8 |
=head1 Introduction and Terminology |
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elmex |
1.1 |
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root |
1.4 |
This is a tutorial about how to get the swing of the new L<AnyEvent::MP> |
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root |
1.23 |
module, which allows programs to transparently pass messages within the |
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process and to other processes on the same or a different host. |
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elmex |
1.1 |
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root |
1.23 |
What kind of messages? Basically a message here means a list of Perl |
10 |
root |
1.15 |
strings, numbers, hashes and arrays, anything that can be expressed as a |
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root |
1.23 |
L<JSON> text (as JSON is used by default in the protocol). Here are two |
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examples: |
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elmex |
1.1 |
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root |
1.23 |
write_log => 1251555874, "action was successful.\n" |
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123, ["a", "b", "c"], { foo => "bar" } |
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elmex |
1.21 |
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root |
1.23 |
When using L<AnyEvent::MP> it is customary to use a descriptive string as |
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first element of a message, that indictes the type of the message. This |
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element is called a I<tag> in L<AnyEvent::MP>, as some API functions |
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(C<rcv>) support matching it directly. |
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Supposedly you want to send a ping message with your current time to |
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somewhere, this is how such a message might look like (in Perl syntax): |
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ping => 1251381636 |
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Now that we know what a message is, to which entities are those |
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messages being I<passed>? They are I<passed> to I<ports>. A I<port> is |
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a destination for messages but also a context to execute code: when |
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a runtime error occurs while executing code belonging to a port, the |
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exception will be raised on the port and can even travel to interested |
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parties on other nodes, which makes supervision of distributed processes |
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easy. |
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How do these ports relate to things you know? Each I<port> belongs |
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to a I<node>, and a I<node> is just the UNIX process that runs your |
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L<AnyEvent::MP> application. |
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Each I<node> is distinguished from other I<nodes> running on the same or |
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another host in a network by its I<node ID>. A I<node ID> is simply a |
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unique string chosen manually or assigned by L<AnyEvent::MP> in some way |
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(UNIX nodename, random string...). |
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Here is a diagram about how I<nodes>, I<ports> and UNIX processes relate |
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to each other. The setup consists of two nodes (more are of course |
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possible): Node C<A> (in UNIX process 7066) with the ports C<ABC> and |
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C<DEF>. And the node C<B> (in UNIX process 8321) with the ports C<FOO> and |
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C<BAR>. |
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elmex |
1.17 |
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|- PID: 7066 -| |- PID: 8321 -| |
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| | | | |
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| Node ID: A | | Node ID: B | |
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| | | | |
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| Port ABC =|= <----\ /-----> =|= Port FOO | |
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| | X | | |
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| Port DEF =|= <----/ \-----> =|= Port BAR | |
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| | | | |
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|-------------| |-------------| |
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root |
1.23 |
The strings for the I<port IDs> here are just for illustrative |
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purposes: Even though I<ports> in L<AnyEvent::MP> are also identified by |
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strings, they can't be choosen manually and are assigned by the system |
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dynamically. These I<port IDs> are unique within a network and can also be |
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used to identify senders or as message tags for instance. |
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The next sections will explain the API of L<AnyEvent::MP> by going through |
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a few simple examples. Later some more complex idioms are introduced, |
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which are hopefully useful to solve some real world problems. |
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root |
1.8 |
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elmex |
1.16 |
=head1 Passing Your First Message |
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root |
1.24 |
As a start lets have a look at the messaging API. The following example |
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is just a demo to show the basic elements of message passing with |
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L<AnyEvent::MP>. |
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The example should print: C<Ending with: 123>, in a rather complicated |
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way, by passing some message to a port. |
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elmex |
1.16 |
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use AnyEvent; |
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use AnyEvent::MP; |
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my $end_cv = AnyEvent->condvar; |
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my $port = port; |
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rcv $port, test => sub { |
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my ($data) = @_; |
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$end_cv->send ($data); |
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}; |
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snd $port, test => 123; |
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print "Ending with: " . $end_cv->recv . "\n"; |
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1.24 |
It already uses most of the essential functions inside |
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L<AnyEvent::MP>: First there is the C<port> function which will create a |
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I<port> and will return it's I<port ID>, a simple string. |
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This I<port ID> can be used to send messages to the port and install |
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handlers to receive messages on the port. Since it is a simple string |
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it can be safely passed to other I<nodes> in the network when you want |
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to refer to that specific port (usually used for RPC, where you need |
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to tell the other end which I<port> to send the reply to - messages in |
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L<AnyEvent::MP> have a destination, but no source). |
106 |
elmex |
1.17 |
|
107 |
root |
1.24 |
The next function is C<rcv>: |
108 |
elmex |
1.16 |
|
109 |
elmex |
1.17 |
rcv $port, test => sub { ... }; |
110 |
elmex |
1.16 |
|
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root |
1.24 |
It installs a receiver callback on the I<port> that specified as the first |
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argument (it only works for "local" ports, i.e. ports created on the same |
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node). The next argument, in this example C<test>, specifies a I<tag> to |
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match. This means that whenever a message with the first element being |
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the string C<test> is received, the callback is called with the remaining |
116 |
elmex |
1.17 |
parts of that message. |
117 |
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118 |
root |
1.24 |
Messages can be sent with the C<snd> function, which is used like this in |
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the example above: |
120 |
elmex |
1.17 |
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121 |
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snd $port, test => 123; |
122 |
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123 |
root |
1.24 |
This will send the message C<'test', 123> to the I<port> with the I<port |
124 |
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ID> stored in C<$port>. Since in this case the receiver has a I<tag> match |
125 |
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on C<test> it will call the callback with the first argument being the |
126 |
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number C<123>. |
127 |
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128 |
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The callback is a typicall AnyEvent idiom: the callback just passes |
129 |
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that number on to the I<condition variable> C<$end_cv> which will then |
130 |
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pass the value to the print. Condition variables are out of the scope |
131 |
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of this tutorial and not often used with ports, so please consult the |
132 |
elmex |
1.17 |
L<AnyEvent::Intro> about them. |
133 |
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134 |
root |
1.24 |
Passing messages inside just one process is boring. Before we can move on |
135 |
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and do interprocess message passing we first have to make sure some things |
136 |
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have been set up correctly for our nodes to talk to each other. |
137 |
elmex |
1.17 |
|
138 |
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=head1 System Requirements and System Setup |
139 |
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140 |
root |
1.25 |
Before we can start with real IPC we have to make sure some things work on |
141 |
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your system. |
142 |
elmex |
1.17 |
|
143 |
root |
1.25 |
First we have to setup a I<shared secret>: for two L<AnyEvent::MP> |
144 |
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I<nodes> to be able to communicate with each other over the network it is |
145 |
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necessary to setup the same I<shared secret> for both of them, so they can |
146 |
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prove their trustworthyness to each other. |
147 |
elmex |
1.17 |
|
148 |
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The easiest way is to set this up is to use the F<aemp> utility: |
149 |
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150 |
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aemp gensecret |
151 |
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152 |
root |
1.25 |
This creates a F<$HOME/.perl-anyevent-mp> config file and generates a |
153 |
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random shared secret. You can copy this file to any other system and |
154 |
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then communicate over the network (via TCP) with it. You can also select |
155 |
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your own shared secret (F<aemp setsecret>) and for increased security |
156 |
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requirements you can even create (or configure) a TLS certificate (F<aemp |
157 |
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gencert>), causing connections to not just be securely authenticated, but |
158 |
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also to be encrypted and protected against tinkering. |
159 |
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160 |
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Connections will only be successfully established when the I<nodes> |
161 |
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that want to connect to each other have the same I<shared secret> (or |
162 |
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successfully verify the TLS certificate of the other side, in which case |
163 |
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no shared secret is required). |
164 |
elmex |
1.17 |
|
165 |
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B<If something does not work as expected, and for example tcpdump shows |
166 |
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that the connections are closed almost immediately, you should make sure |
167 |
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that F<~/.perl-anyevent-mp> is the same on all hosts/user accounts that |
168 |
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you try to connect with each other!> |
169 |
elmex |
1.16 |
|
170 |
root |
1.25 |
Thats is all for now, you will find some more advanced fiddling with the |
171 |
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C<aemp> utility later. |
172 |
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173 |
elmex |
1.18 |
|
174 |
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=head1 Passing Messages Between Processes |
175 |
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176 |
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=head2 The Receiver |
177 |
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178 |
root |
1.25 |
Lets split the previous example up into two programs: one that contains |
179 |
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the sender and one for the receiver. First the receiver application, in |
180 |
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full: |
181 |
elmex |
1.18 |
|
182 |
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use AnyEvent; |
183 |
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use AnyEvent::MP; |
184 |
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use AnyEvent::MP::Global; |
185 |
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186 |
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initialise_node "eg_simple_receiver"; |
187 |
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188 |
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my $port = port; |
189 |
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190 |
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AnyEvent::MP::Global::register $port, "eg_receivers"; |
191 |
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192 |
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rcv $port, test => sub { |
193 |
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my ($data, $reply_port) = @_; |
194 |
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195 |
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print "Received data: " . $data . "\n"; |
196 |
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}; |
197 |
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198 |
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AnyEvent->condvar->recv; |
199 |
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200 |
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=head3 AnyEvent::MP::Global |
201 |
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202 |
root |
1.25 |
Now, that wasn't too bad, was it? Ok, let's step through the new functions |
203 |
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and modules that have been used. |
204 |
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205 |
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For starters, there is now an additional module being |
206 |
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used: L<AnyEvent::MP::Global>. This module provides us with a I<global |
207 |
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registry>, which lets us register ports in groups that are visible on all |
208 |
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I<nodes> in a network. |
209 |
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210 |
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What is this useful for? Well, the I<port IDs> are random-looking strings, |
211 |
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assigned by L<AnyEvent::MP>. We cannot know those I<port IDs> in advance, |
212 |
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so we don't know which I<port ID> to send messages to, especially when the |
213 |
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message is to be passed between different I<nodes> (or UNIX processes). To |
214 |
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find the right I<port> of another I<node> in the network we will need |
215 |
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to communicate this somehow to the sender. And exactly that is what |
216 |
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L<AnyEvent::MP::Global> provides. |
217 |
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218 |
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Especially in larger, more anonymous networks this is handy: imagine you |
219 |
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have a few database backends, a few web frontends and some processing |
220 |
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distributed over a number of hosts: all of these would simply register |
221 |
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themselves in the appropriate group, and your web frontends can start to |
222 |
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find some database backend. |
223 |
elmex |
1.18 |
|
224 |
root |
1.25 |
=head3 C<initialise_node> And The Network |
225 |
elmex |
1.18 |
|
226 |
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Now, lets have a look at the next new thing, the C<initialise_node>: |
227 |
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228 |
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initialise_node "eg_simple_receiver"; |
229 |
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230 |
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Before we are able to send messages to other nodes we have to initialise |
231 |
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ourself. The first argument, the string C<"eg_simple_receiver">, is called the |
232 |
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I<profile> of this node. A profile holds some information about the application |
233 |
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that is going to be a node in an L<AnyEvent::MP> network. |
234 |
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235 |
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Most importantly the profile allows you to set the I<node id> that your |
236 |
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application will use. You can also set I<binds> in the profile, meaning that |
237 |
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you can define TCP ports that the application will listen on for incoming |
238 |
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connections from other nodes of the network. |
239 |
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240 |
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Next you can configure I<seeds> in profile. A I<seed> is just a TCP endpoint |
241 |
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which tells the application where to find other nodes of it's network. To |
242 |
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explain this a bit more detailed we have to look at the topology of an |
243 |
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L<AnyEvent::MP> network. The topology is called a I<fully connected mesh>, here |
244 |
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an example with 4 nodes: |
245 |
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246 |
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N1--N2 |
247 |
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| \/ | |
248 |
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| /\ | |
249 |
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N3--N4 |
250 |
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251 |
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Now imagine another I<node> C<N5>. wants to connect itself to that network: |
252 |
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253 |
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N1--N2 |
254 |
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| \/ | N5 |
255 |
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| /\ | |
256 |
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N3--N4 |
257 |
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258 |
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The new node needs to know the I<binds> of all of those 4 already connected |
259 |
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nodes. And exactly this is what the I<seeds> are for. Now lets assume that |
260 |
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the new node C<N5> has as I<seed> the TCP endpoint of the node C<N2>. |
261 |
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It then connects to C<N2>: |
262 |
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263 |
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N1--N2____ |
264 |
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| \/ | N5 |
265 |
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| /\ | |
266 |
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N3--N4 |
267 |
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268 |
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C<N2> then tells C<N5> the I<binds> of the other nodes it is connected to, |
269 |
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and C<N5> builds up the rest of the connections: |
270 |
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271 |
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/--------\ |
272 |
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N1--N2____| |
273 |
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| \/ | N5 |
274 |
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| /\ | /| |
275 |
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N3--N4--- | |
276 |
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\________/ |
277 |
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278 |
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Finished. C<N5> is now happily connected to the rest of the network. |
279 |
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280 |
elmex |
1.19 |
=head3 Setting Up The Profiles |
281 |
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282 |
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Ok, so much to the profile. Now lets setup the C<eg_simple_receiver> I<profile> |
283 |
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for later. For the receiver we just give the receiver a I<bind>: |
284 |
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285 |
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aemp profile eg_simple_receiver setbinds localhost:12266 |
286 |
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|
287 |
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And while we are at it, just setup the I<profile> for the sender in the second |
288 |
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part of this example too. We will call the sender I<profile> |
289 |
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C<eg_simple_sender>. For the sender we will just setup a I<seed> to the |
290 |
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receiver: |
291 |
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292 |
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aemp profile eg_simple_sender setseeds localhost:12266 |
293 |
elmex |
1.22 |
aemp profile eg_simple_sender setbinds |
294 |
elmex |
1.19 |
|
295 |
elmex |
1.22 |
You might wonder why we setup I<binds> to be empty here. Well, there can be |
296 |
elmex |
1.19 |
exceptions to the I<fully> in the I<fully connected mesh> in L<AnyEvent::MP>. |
297 |
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If you don't configure a I<bind> for a node's profile it won't bind itself |
298 |
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somewhere. These kinds of I<nodes> will not be able to send messages to other |
299 |
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I<nodes> that also didn't I<bind> them self to some TCP address. For this |
300 |
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example, as well as some cases in the real world, we can live with this |
301 |
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limitation. |
302 |
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303 |
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=head3 Registering The Receiver |
304 |
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305 |
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Ok, where were we. We now discussed the basic purpose of L<AnyEvent::MP::Global> |
306 |
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and initialise_node with it's relations to profiles. We also setup our profiles |
307 |
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for later use and now have to continue talking about the receiver example. |
308 |
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|
309 |
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Lets look at the next undiscussed line(s) of code: |
310 |
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311 |
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my $port = port; |
312 |
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AnyEvent::MP::Global::register $port, "eg_receivers"; |
313 |
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314 |
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The C<port> function already has been discussed. It just creates a new I<port> |
315 |
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and gives us the I<port id>. Now to the C<register> function of |
316 |
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L<AnyEvent::MP::Global>: The first argument is a I<port id> that we want to add |
317 |
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to a I<global group>, and it's second argument is the name of that I<global |
318 |
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group>. |
319 |
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320 |
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You can choose that name of such a I<global group> freely, and it's purpose is |
321 |
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to store a set of I<port ids>. That set is made available throughout the whole |
322 |
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L<AnyEvent::MP> network, so that each node can see which ports belong to that |
323 |
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group. |
324 |
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325 |
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The sender will later look for the ports in that I<global group> and send |
326 |
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messages to them. |
327 |
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328 |
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Last step in the example is to setup a receiver callback for those messages |
329 |
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like we have discussed in the first example. We again match for the I<tag> |
330 |
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C<test>. The difference is just that we don't end the application after |
331 |
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receiving the first message. We just infinitely continue to look out for new |
332 |
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messages. |
333 |
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|
334 |
elmex |
1.20 |
=head2 The Sender |
335 |
root |
1.8 |
|
336 |
elmex |
1.20 |
Ok, now lets take a look at the sender: |
337 |
root |
1.4 |
|
338 |
elmex |
1.20 |
#!/opt/perl/bin/perl |
339 |
elmex |
1.1 |
use AnyEvent; |
340 |
|
|
use AnyEvent::MP; |
341 |
elmex |
1.20 |
use AnyEvent::MP::Global; |
342 |
elmex |
1.1 |
|
343 |
elmex |
1.20 |
initialise_node "eg_simple_sender"; |
344 |
elmex |
1.1 |
|
345 |
elmex |
1.20 |
my $find_timer = |
346 |
|
|
AnyEvent->timer (after => 0, interval => 1, cb => sub { |
347 |
|
|
my $ports = AnyEvent::MP::Global::find "eg_receivers" |
348 |
|
|
or return; |
349 |
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|
350 |
|
|
snd $_, test => time |
351 |
|
|
for @$ports; |
352 |
|
|
}); |
353 |
elmex |
1.1 |
|
354 |
|
|
AnyEvent->condvar->recv; |
355 |
|
|
|
356 |
elmex |
1.20 |
It's even less code. The C<initialise_node> is known now from the receiver |
357 |
|
|
above. As discussed in the section where we setup the profiles we configure |
358 |
|
|
this application to use the I<profile> C<eg_simple_sender>. |
359 |
root |
1.10 |
|
360 |
elmex |
1.20 |
Next we setup a timer that repeatedly calls this chunk of code: |
361 |
elmex |
1.1 |
|
362 |
elmex |
1.20 |
my $ports = AnyEvent::MP::Global::find "eg_receivers" |
363 |
|
|
or return; |
364 |
elmex |
1.2 |
|
365 |
elmex |
1.20 |
snd $_, test => time |
366 |
|
|
for @$ports; |
367 |
elmex |
1.1 |
|
368 |
elmex |
1.20 |
The new function here is the C<find> function of L<AnyEvent::MP::Global>. It |
369 |
|
|
searches in the I<global group> named C<eg_receivers> for ports. If none are |
370 |
|
|
found C<undef> is returned and we wait for the next time the timer fires. |
371 |
elmex |
1.1 |
|
372 |
elmex |
1.20 |
In case the receiver application has been connected and the newly added port by |
373 |
|
|
the receiver has propagated to the sender C<find> returns an array reference |
374 |
|
|
that contains the I<port id> of the receiver I<port(s)>. |
375 |
elmex |
1.1 |
|
376 |
elmex |
1.20 |
We then just send to every I<port> in the I<global group> a message consisting |
377 |
|
|
of the I<tag> C<test> and the current time in form of a UNIX timestamp. |
378 |
elmex |
1.7 |
|
379 |
elmex |
1.20 |
And thats all. |
380 |
elmex |
1.7 |
|
381 |
elmex |
1.1 |
=head1 SEE ALSO |
382 |
|
|
|
383 |
|
|
L<AnyEvent> |
384 |
|
|
|
385 |
|
|
L<AnyEvent::Handle> |
386 |
|
|
|
387 |
|
|
L<AnyEvent::MP> |
388 |
|
|
|
389 |
elmex |
1.20 |
L<AnyEvent::MP::Global> |
390 |
|
|
|
391 |
elmex |
1.1 |
=head1 AUTHOR |
392 |
|
|
|
393 |
|
|
Robin Redeker <elmex@ta-sa.org> |
394 |
root |
1.4 |
|