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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 |
|
9 |
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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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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root |
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). |
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elmex |
1.17 |
|
107 |
root |
1.24 |
The next function is C<rcv>: |
108 |
elmex |
1.16 |
|
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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 |
112 |
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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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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<$end_cv> which will then |
130 |
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pass the value to the print. Condition variables are out of the scope |
131 |
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of this tutorial and not often used with ports, so please consult the |
132 |
elmex |
1.17 |
L<AnyEvent::Intro> about them. |
133 |
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134 |
root |
1.24 |
Passing messages inside just one process is boring. Before we can move on |
135 |
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and do interprocess message passing we first have to make sure some things |
136 |
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have been set up correctly for our nodes to talk to each other. |
137 |
elmex |
1.17 |
|
138 |
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=head1 System Requirements and System Setup |
139 |
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140 |
root |
1.25 |
Before we can start with real IPC we have to make sure some things work on |
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your system. |
142 |
elmex |
1.17 |
|
143 |
root |
1.25 |
First we have to setup a I<shared secret>: for two L<AnyEvent::MP> |
144 |
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I<nodes> to be able to communicate with each other over the network it is |
145 |
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necessary to setup the same I<shared secret> for both of them, so they can |
146 |
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prove their trustworthyness to each other. |
147 |
elmex |
1.17 |
|
148 |
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The easiest way is to set this up is to use the F<aemp> utility: |
149 |
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150 |
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aemp gensecret |
151 |
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152 |
root |
1.25 |
This creates a F<$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 |
root |
1.30 |
=head1 PART 1: Passing Messages Between Processes |
175 |
elmex |
1.18 |
|
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 |
root |
1.28 |
configure nodeid => "eg_receiver", binds => ["*:4040"]; |
187 |
elmex |
1.18 |
|
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 |
1.28 |
=head3 C<configure> and the Network |
225 |
elmex |
1.18 |
|
226 |
root |
1.28 |
Now, let's have a look at the new function, C<configure>: |
227 |
elmex |
1.18 |
|
228 |
root |
1.28 |
configure nodeid => "eg_receiver", binds => ["*:4040"]; |
229 |
elmex |
1.18 |
|
230 |
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Before we are able to send messages to other nodes we have to initialise |
231 |
root |
1.26 |
ourself to become a "distributed node". Initialising a node means naming |
232 |
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the node, optionally binding some TCP listeners so that other nodes can |
233 |
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contact it and connecting to a predefined set of seed addresses so the |
234 |
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node can discover the existing network - and the existing network can |
235 |
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discover the node! |
236 |
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|
237 |
root |
1.28 |
All of this (and more) can be passed to the C<configure> function - later |
238 |
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we will see how we can do all this without even passing anything to |
239 |
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C<configure>! |
240 |
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241 |
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The first parameter, C<nodeid>, specified the node ID (in this case |
242 |
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C<eg_receiver> - the default is to use the node name of the current host, |
243 |
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but for this example we want to be able to run many nodes on the same |
244 |
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machine). Node IDs need to be unique within the network and can be almost |
245 |
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any string - if you don't care, you can specify a node ID of C<anon/> |
246 |
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which will then be replaced by a random node name. |
247 |
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|
248 |
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The second parameter, C<binds>, specifies a list of C<address:port> pairs |
249 |
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to bind TCP listeners on. The special "address" of C<*> means to bind on |
250 |
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every local IP address. |
251 |
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|
252 |
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The reason to bind on a TCP port is not just that other nodes can connect |
253 |
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to us: if no binds are specified, the node will still bind on a dynamic |
254 |
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port on all local addresses - but in this case we won't know the port, and |
255 |
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cannot tell other nodes to connect to it as seed node. |
256 |
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|
257 |
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A I<seed> is a (fixed) TCP address of some other node in the network. To |
258 |
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explain the need for seeds we have to look at the topology of a typical |
259 |
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L<AnyEvent::MP> network. The topology is called a I<fully connected mesh>, |
260 |
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here an example with 4 nodes: |
261 |
elmex |
1.18 |
|
262 |
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N1--N2 |
263 |
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| \/ | |
264 |
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| /\ | |
265 |
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N3--N4 |
266 |
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|
267 |
root |
1.28 |
Now imagine another node - C<N5> - wants to connect itself to that network: |
268 |
elmex |
1.18 |
|
269 |
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N1--N2 |
270 |
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| \/ | N5 |
271 |
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| /\ | |
272 |
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N3--N4 |
273 |
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|
274 |
root |
1.26 |
The new node needs to know the I<binds> of all nodes already |
275 |
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connected. Exactly this is what the I<seeds> are for: Let's assume that |
276 |
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the new node (C<N5>) uses the TCP address of the node C<N2> as seed. This |
277 |
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cuases it to connect to C<N2>: |
278 |
elmex |
1.18 |
|
279 |
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N1--N2____ |
280 |
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| \/ | N5 |
281 |
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| /\ | |
282 |
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N3--N4 |
283 |
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284 |
root |
1.26 |
C<N2> then tells C<N5> about the I<binds> of the other nodes it is |
285 |
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connected to, and C<N5> creates the rest of the connections: |
286 |
elmex |
1.18 |
|
287 |
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/--------\ |
288 |
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N1--N2____| |
289 |
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| \/ | N5 |
290 |
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| /\ | /| |
291 |
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N3--N4--- | |
292 |
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\________/ |
293 |
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|
294 |
root |
1.26 |
All done: C<N5> is now happily connected to the rest of the network. |
295 |
elmex |
1.18 |
|
296 |
root |
1.28 |
Of course, this process takes time, during which the node is already |
297 |
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running. This also means it takes time until the node is fully connected, |
298 |
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and global groups and other information is available. The best way to deal |
299 |
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with this is to either retry regularly until you found the resource you |
300 |
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were looking for, or to only start services on demand after a node has |
301 |
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become available. |
302 |
elmex |
1.19 |
|
303 |
root |
1.28 |
=head3 Registering the Receiver |
304 |
elmex |
1.19 |
|
305 |
root |
1.27 |
Coming back to our example, we have now introduced the basic purpose of |
306 |
root |
1.28 |
L<AnyEvent::MP::Global> and C<configure> and its use of profiles. We |
307 |
root |
1.27 |
also set up our profiles for later use and now we will finally continue |
308 |
|
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talking about the receiver. |
309 |
elmex |
1.19 |
|
310 |
root |
1.27 |
Let's look at the next line(s): |
311 |
elmex |
1.19 |
|
312 |
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my $port = port; |
313 |
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AnyEvent::MP::Global::register $port, "eg_receivers"; |
314 |
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|
315 |
root |
1.27 |
The C<port> function has already been discussed. It simply creates a new |
316 |
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I<port> and returns the I<port ID>. The C<register> function, however, |
317 |
root |
1.28 |
is new: The first argument is the I<port ID> that we want to add to a |
318 |
root |
1.27 |
I<global group>, and its second argument is the name of that I<global |
319 |
elmex |
1.19 |
group>. |
320 |
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|
321 |
root |
1.27 |
You can choose the name of such a I<global group> freely (prefixing your |
322 |
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package name is highly recommended!). The purpose of such a group is to |
323 |
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store a set of I<port IDs>. This set is made available throughout the |
324 |
root |
1.28 |
L<AnyEvent::MP> network, so that each node can see which ports belong to |
325 |
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that group. |
326 |
root |
1.27 |
|
327 |
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Later we will see how the sender looks for the ports in this I<global |
328 |
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group> to send messages to them. |
329 |
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|
330 |
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The last step in the example is to set up a receiver callback for those |
331 |
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messages, just as was discussed in the first example. We again match |
332 |
|
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for the tag C<test>. The difference is that this time we don't exit the |
333 |
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application after receiving the first message. Instead we continue to wait |
334 |
|
|
for new messages indefinitely. |
335 |
elmex |
1.19 |
|
336 |
elmex |
1.20 |
=head2 The Sender |
337 |
root |
1.8 |
|
338 |
root |
1.27 |
Ok, now let's take a look at the sender code: |
339 |
root |
1.4 |
|
340 |
elmex |
1.1 |
use AnyEvent; |
341 |
|
|
use AnyEvent::MP; |
342 |
elmex |
1.20 |
use AnyEvent::MP::Global; |
343 |
elmex |
1.1 |
|
344 |
root |
1.28 |
configure nodeid => "eg_sender", seeds => ["*:4040"]; |
345 |
elmex |
1.1 |
|
346 |
elmex |
1.20 |
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 |
elmex |
1.1 |
|
355 |
|
|
AnyEvent->condvar->recv; |
356 |
|
|
|
357 |
root |
1.28 |
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 |
root |
1.10 |
|
362 |
root |
1.27 |
Next we set up a timer that repeatedly (every second) calls this chunk of |
363 |
|
|
code: |
364 |
elmex |
1.1 |
|
365 |
elmex |
1.20 |
my $ports = AnyEvent::MP::Global::find "eg_receivers" |
366 |
|
|
or return; |
367 |
elmex |
1.2 |
|
368 |
elmex |
1.20 |
snd $_, test => time |
369 |
|
|
for @$ports; |
370 |
elmex |
1.1 |
|
371 |
root |
1.27 |
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 |
root |
1.28 |
=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 |
elmex |
1.31 |
provided not by the programmer, but by whoever is deploying the program. |
393 |
root |
1.28 |
|
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 |
root |
1.30 |
utility (yes, this section is about the advanced tinkering we mentioned |
397 |
|
|
before). |
398 |
root |
1.28 |
|
399 |
|
|
When you change both programs above to simply call |
400 |
|
|
|
401 |
|
|
configure; |
402 |
|
|
|
403 |
|
|
then AnyEvent::MP tries to look up a profile using the current node name |
404 |
|
|
in its configuration database, falling back to some global default. |
405 |
|
|
|
406 |
|
|
You can run "generic" nodes using the F<aemp> utility as well, and we will |
407 |
|
|
exploit this in the following way: we configure a profile "seed" and run |
408 |
|
|
a node using it, whose sole purpose is to be a seed node for our example |
409 |
|
|
programs. |
410 |
|
|
|
411 |
|
|
We bind the seed node to port 4040 on all interfaces: |
412 |
|
|
|
413 |
root |
1.29 |
aemp profile seed binds "*:4040" |
414 |
root |
1.28 |
|
415 |
|
|
And we configure all nodes to use this as seed node (this only works when |
416 |
|
|
running on the same host, for multiple machines you would provide the IP |
417 |
root |
1.30 |
address or hostname of the node running the seed), and use a random name |
418 |
|
|
(because we want to start multiple nodes on the same host): |
419 |
root |
1.28 |
|
420 |
root |
1.30 |
aemp seeds "*:4040" nodeid anon/ |
421 |
root |
1.28 |
|
422 |
|
|
Then we run the seed node: |
423 |
|
|
|
424 |
|
|
aemp run profile seed |
425 |
|
|
|
426 |
|
|
After that, we can start as many other nodes as we want, and they will all |
427 |
|
|
use our generic seed node to discover each other. |
428 |
root |
1.27 |
|
429 |
root |
1.28 |
In fact, starting many receivers nicely illustrates that the time sender |
430 |
|
|
can have multiple receivers. |
431 |
elmex |
1.7 |
|
432 |
root |
1.30 |
That's all for now - next we will teach you about monitoring by writing a |
433 |
|
|
simple chat client and server :) |
434 |
|
|
|
435 |
|
|
=head1 PART 2: Monitoring, Supervising, Exception Handling and Recovery |
436 |
|
|
|
437 |
|
|
That's a mouthful, so what does it mean? Our previous example is what one |
438 |
|
|
could call "very loosely coupled" - the sender doesn't care about whether |
439 |
|
|
there are any receivers, and the receivers do not care if there is any |
440 |
|
|
sender. |
441 |
|
|
|
442 |
|
|
This can work fine for simple services, but most real-world applications |
443 |
|
|
want to ensure that the side they are expecting to be there is actually |
444 |
|
|
there. Going one step further: most bigger real-world applications even |
445 |
|
|
want to ensure that if some component is missing, or has crashed, it will |
446 |
|
|
still be there, by recovering and restarting the service. |
447 |
|
|
|
448 |
|
|
AnyEvent::MP supports this by catching exceptions and network problems, |
449 |
|
|
and notifying interested parties of this. |
450 |
|
|
|
451 |
|
|
=head2 Exceptions, Network Errors and Monitors |
452 |
|
|
|
453 |
|
|
=head3 Exceptions |
454 |
|
|
|
455 |
|
|
Exceptions are handled on a per-port basis: receive callbacks are executed |
456 |
|
|
in a special context, the port-context, and code that throws an uncaught |
457 |
|
|
exception will cause the port to be C<kil>led. Killed ports are destroyed |
458 |
|
|
automatically (killing ports is the only way to free ports, incidentally). |
459 |
|
|
|
460 |
|
|
Ports can be monitored, even from a different host, and when a port is |
461 |
|
|
killed any entity monitoring it will be notified. |
462 |
|
|
|
463 |
|
|
Here is a simple example: |
464 |
|
|
|
465 |
|
|
use AnyEvent::MP; |
466 |
|
|
|
467 |
|
|
# create a port, it always dies |
468 |
|
|
my $port = port { die "oops" }; |
469 |
|
|
|
470 |
|
|
# monitor it |
471 |
|
|
mon $port, sub { |
472 |
|
|
warn "$port was killed (with reason @_)"; |
473 |
|
|
}; |
474 |
|
|
|
475 |
|
|
# now send it some message, causing it to die: |
476 |
|
|
snd $port; |
477 |
|
|
|
478 |
|
|
It first creates a port whose only action is to throw an exception, |
479 |
|
|
and the monitors it with the C<mon> function. Afterwards it sends it a |
480 |
|
|
message, causing it to die and call the monitoring callback: |
481 |
|
|
|
482 |
|
|
anon/6WmIpj.a was killed (with reason die oops at xxx line 5.) at xxx line 9. |
483 |
|
|
|
484 |
|
|
The callback was actually passed two arguments: C<die> (to indicate it did |
485 |
|
|
throw an exception as opposed to, say, a network error) and the exception |
486 |
|
|
message itself. |
487 |
|
|
|
488 |
|
|
What happens when a port is killed before we have a chance to monitor |
489 |
|
|
it? Granted, this is highly unlikely in our example, but when you program |
490 |
|
|
in a network this can easily happen due to races between nodes. |
491 |
|
|
|
492 |
|
|
use AnyEvent::MP; |
493 |
|
|
|
494 |
|
|
my $port = port { die "oops" }; |
495 |
|
|
|
496 |
|
|
snd $port; |
497 |
|
|
|
498 |
|
|
mon $port, sub { |
499 |
|
|
warn "$port was killed (with reason @_)"; |
500 |
|
|
}; |
501 |
|
|
|
502 |
|
|
This time we will get something like: |
503 |
|
|
|
504 |
|
|
anon/zpX.a was killed (with reason no_such_port cannot monitor nonexistent port) |
505 |
|
|
|
506 |
|
|
Since the port was already gone, the kill reason is now C<no_such_port> |
507 |
|
|
with some descriptive (we hope) error message. |
508 |
|
|
|
509 |
|
|
In fact, the kill reason is usually some identifier as first argument |
510 |
|
|
and a human-readable error message as second argument, but can be about |
511 |
|
|
anything (it's a list) or even nothing - which is called a "normal" kill. |
512 |
|
|
|
513 |
|
|
You can kill ports manually using the C<kil> function, which will be |
514 |
|
|
treated like an error when any reason is specified: |
515 |
|
|
|
516 |
|
|
kil $port, custom_error => "don't like your steenking face"; |
517 |
|
|
|
518 |
|
|
And a clean kill without any reason arguments: |
519 |
|
|
|
520 |
|
|
kil $port; |
521 |
|
|
|
522 |
|
|
By now you probably wonder what this "normal" kill business is: A common |
523 |
|
|
idiom is to not specify a callback to C<mon>, but another port, such as |
524 |
|
|
C<$SELF>: |
525 |
|
|
|
526 |
|
|
mon $port, $SELF; |
527 |
|
|
|
528 |
|
|
This basically means "monitor $port and kill me when it crashes". And a |
529 |
|
|
"normal" kill does not count as a crash. This way you can easily link |
530 |
|
|
ports together and make them crash together on errors (but allow you to |
531 |
|
|
remove a port silently). |
532 |
|
|
|
533 |
|
|
=head3 Network Errors and the AEMP Guarantee |
534 |
|
|
|
535 |
|
|
I mentioned another important source of monitoring failures: network |
536 |
|
|
problems. When a node loses connection to another node, it will invoke all |
537 |
|
|
monitoring actions as if the port was killed, even if it is possible that |
538 |
elmex |
1.31 |
the port still lives happily on another node (not being able to talk to a |
539 |
root |
1.30 |
node means we have no clue what's going on with it, it could be crashed, |
540 |
|
|
but also still running without knowing we lost the connection). |
541 |
|
|
|
542 |
|
|
So another way to view monitors is "notify me when some of my messages |
543 |
|
|
couldn't be delivered". AEMP has a guarantee about message delivery to a |
544 |
|
|
port: After starting a monitor, any message sent to a port will either |
545 |
|
|
be delivered, or, when it is lost, any further messages will also be lost |
546 |
elmex |
1.31 |
until the monitoring action is invoked. After that, further messages |
547 |
root |
1.30 |
I<might> get delivered again. |
548 |
|
|
|
549 |
|
|
This doesn't sound like a very big guarantee, but it is kind of the best |
550 |
elmex |
1.31 |
you can get while staying sane: Specifically, it means that there will |
551 |
|
|
be no "holes" in the message sequence: all messages sent are delivered |
552 |
root |
1.30 |
in order, without any missing in between, and when some were lost, you |
553 |
|
|
I<will> be notified of that, so you can take recovery action. |
554 |
|
|
|
555 |
|
|
=head3 Supervising |
556 |
|
|
|
557 |
|
|
Ok, so what is this crashing-everything-stuff going to make applications |
558 |
|
|
I<more> stable? Well in fact, the goal is not really to make them more |
559 |
|
|
stable, but to make them more resilient against actual errors and |
560 |
|
|
crashes. And this is not done by crashing I<everything>, but by crashing |
561 |
|
|
everything except a supervisor. |
562 |
|
|
|
563 |
elmex |
1.31 |
A supervisor is simply some code that ensures that an application (or a |
564 |
root |
1.30 |
part of it) is running, and if it crashes, is restarted properly. |
565 |
|
|
|
566 |
|
|
To show how to do all this we will create a simple chat server that can |
567 |
|
|
handle many chat clients. Both server and clients can be killed and |
568 |
|
|
restarted, and even crash, to some extent. |
569 |
|
|
|
570 |
|
|
=head2 Chatting, the Resilient Way |
571 |
|
|
|
572 |
|
|
Without further ado, here is the chat server (to run it, we assume the |
573 |
|
|
set-up explained earlier, with a separate F<aemp run> seed node): |
574 |
|
|
|
575 |
|
|
use common::sense; |
576 |
|
|
use AnyEvent::MP; |
577 |
|
|
use AnyEvent::MP::Global; |
578 |
|
|
|
579 |
|
|
configure; |
580 |
|
|
|
581 |
|
|
my %clients; |
582 |
|
|
|
583 |
|
|
sub msg { |
584 |
|
|
print "relaying: $_[0]\n"; |
585 |
|
|
snd $_, $_[0] |
586 |
|
|
for values %clients; |
587 |
|
|
} |
588 |
|
|
|
589 |
|
|
our $server = port; |
590 |
|
|
|
591 |
|
|
rcv $server, join => sub { |
592 |
|
|
my ($client, $nick) = @_; |
593 |
|
|
|
594 |
|
|
$clients{$client} = $client; |
595 |
|
|
|
596 |
|
|
mon $client, sub { |
597 |
|
|
delete $clients{$client}; |
598 |
|
|
msg "$nick (quits, @_)"; |
599 |
|
|
}; |
600 |
|
|
msg "$nick (joins)"; |
601 |
|
|
}; |
602 |
|
|
|
603 |
|
|
rcv $server, privmsg => sub { |
604 |
|
|
my ($nick, $msg) = @_; |
605 |
|
|
msg "$nick: $msg"; |
606 |
|
|
}; |
607 |
|
|
|
608 |
|
|
AnyEvent::MP::Global::register $server, "eg_chat_server"; |
609 |
|
|
|
610 |
|
|
warn "server ready.\n"; |
611 |
|
|
|
612 |
|
|
AnyEvent->condvar->recv; |
613 |
|
|
|
614 |
elmex |
1.31 |
Looks like a lot, but it is actually quite simple: after your usual |
615 |
root |
1.30 |
preamble (this time we use common sense), we define a helper function that |
616 |
|
|
sends some message to every registered chat client: |
617 |
|
|
|
618 |
|
|
sub msg { |
619 |
|
|
print "relaying: $_[0]\n"; |
620 |
|
|
snd $_, $_[0] |
621 |
|
|
for values %clients; |
622 |
|
|
} |
623 |
|
|
|
624 |
|
|
The clients are stored in the hash C<%client>. Then we define a server |
625 |
|
|
port and install two receivers on it, C<join>, which is sent by clients |
626 |
|
|
to join the chat, and C<privmsg>, that clients use to send actual chat |
627 |
|
|
messages. |
628 |
|
|
|
629 |
|
|
C<join> is most complicated. It expects the client port and the nickname |
630 |
|
|
to be passed in the message, and registers the client in C<%clients>. |
631 |
|
|
|
632 |
|
|
rcv $server, join => sub { |
633 |
|
|
my ($client, $nick) = @_; |
634 |
|
|
|
635 |
|
|
$clients{$client} = $client; |
636 |
|
|
|
637 |
|
|
The next step is to monitor the client. The monitoring action removes the |
638 |
|
|
client and sends a quit message with the error to all remaining clients. |
639 |
|
|
|
640 |
|
|
mon $client, sub { |
641 |
|
|
delete $clients{$client}; |
642 |
|
|
msg "$nick (quits, @_)"; |
643 |
|
|
}; |
644 |
|
|
|
645 |
|
|
And finally, it creates a join message and sends it to all clients. |
646 |
|
|
|
647 |
|
|
msg "$nick (joins)"; |
648 |
|
|
}; |
649 |
|
|
|
650 |
|
|
The C<privmsg> callback simply broadcasts the message to all clients: |
651 |
|
|
|
652 |
|
|
rcv $server, privmsg => sub { |
653 |
|
|
my ($nick, $msg) = @_; |
654 |
|
|
msg "$nick: $msg"; |
655 |
|
|
}; |
656 |
|
|
|
657 |
elmex |
1.31 |
And finally, the server registers itself in the server group, so that |
658 |
root |
1.30 |
clients can find it: |
659 |
|
|
|
660 |
|
|
AnyEvent::MP::Global::register $server, "eg_chat_server"; |
661 |
|
|
|
662 |
|
|
Well, well... and where is this supervisor stuff? Well... we cheated, |
663 |
|
|
it's not there. To not overcomplicate the example, we only put it into |
664 |
|
|
the..... CLIENT! |
665 |
|
|
|
666 |
|
|
=head3 The Client, and a Supervisor! |
667 |
|
|
|
668 |
|
|
Again, here is the client, including supervisor, which makes it a bit |
669 |
|
|
longer: |
670 |
|
|
|
671 |
|
|
use common::sense; |
672 |
|
|
use AnyEvent::MP; |
673 |
|
|
use AnyEvent::MP::Global; |
674 |
|
|
|
675 |
|
|
my $nick = shift; |
676 |
|
|
|
677 |
|
|
configure; |
678 |
|
|
|
679 |
|
|
my ($client, $server); |
680 |
|
|
|
681 |
|
|
sub server_connect { |
682 |
|
|
my $servernodes = AnyEvent::MP::Global::find "eg_chat_server" |
683 |
|
|
or return after 1, \&server_connect; |
684 |
|
|
|
685 |
|
|
print "\rconnecting...\n"; |
686 |
|
|
|
687 |
|
|
$client = port { print "\r \r@_\n> " }; |
688 |
|
|
mon $client, sub { |
689 |
|
|
print "\rdisconnected @_\n"; |
690 |
|
|
&server_connect; |
691 |
|
|
}; |
692 |
|
|
|
693 |
|
|
$server = $servernodes->[0]; |
694 |
|
|
snd $server, join => $client, $nick; |
695 |
|
|
mon $server, $client; |
696 |
|
|
} |
697 |
|
|
|
698 |
|
|
server_connect; |
699 |
|
|
|
700 |
|
|
my $w = AnyEvent->io (fh => *STDIN, poll => 'r', cb => sub { |
701 |
|
|
chomp (my $line = <STDIN>); |
702 |
|
|
print "> "; |
703 |
|
|
snd $server, privmsg => $nick, $line |
704 |
|
|
if $server; |
705 |
|
|
}); |
706 |
|
|
|
707 |
|
|
$| = 1; |
708 |
|
|
print "> "; |
709 |
|
|
AnyEvent->condvar->recv; |
710 |
|
|
|
711 |
|
|
The first thing the client does is to store the nick name (which is |
712 |
|
|
expected as the only command line argument) in C<$nick>, for further |
713 |
|
|
usage. |
714 |
|
|
|
715 |
|
|
The next relevant thing is... finally... the supervisor: |
716 |
|
|
|
717 |
|
|
sub server_connect { |
718 |
|
|
my $servernodes = AnyEvent::MP::Global::find "eg_chat_server" |
719 |
|
|
or return after 1, \&server_connect; |
720 |
|
|
|
721 |
|
|
This looks up the server in the C<eg_chat_server> global group. If it |
722 |
|
|
cannot find it (which is likely when the node is just starting up), |
723 |
|
|
it will wait a second and then retry. This "wait a bit and retry" |
724 |
|
|
is an important pattern, as distributed programming means lots of |
725 |
|
|
things are going on asynchronously. In practise, one should use a more |
726 |
|
|
intelligent algorithm, to possibly warn after an excessive number of |
727 |
|
|
retries. Hopefully future versions of AnyEvent::MP will offer some |
728 |
|
|
predefined supervisors, for now you will have to code it on your own. |
729 |
|
|
|
730 |
|
|
Next it creates a local port for the server to send messages to, and |
731 |
|
|
monitors it. When the port is killed, it will print "disconnected" and |
732 |
|
|
tell the supervisor function to retry again. |
733 |
|
|
|
734 |
|
|
$client = port { print "\r \r@_\n> " }; |
735 |
|
|
mon $client, sub { |
736 |
|
|
print "\rdisconnected @_\n"; |
737 |
|
|
&server_connect; |
738 |
|
|
}; |
739 |
|
|
|
740 |
|
|
Then everything is ready: the client will send a C<join> message with it's |
741 |
|
|
local port to the server, and start monitoring it: |
742 |
|
|
|
743 |
|
|
$server = $servernodes->[0]; |
744 |
|
|
snd $server, join => $client, $nick; |
745 |
|
|
mon $server, $client; |
746 |
|
|
} |
747 |
|
|
|
748 |
|
|
The monitor will ensure that if the server crashes or goes away, the |
749 |
|
|
client will be killed as well. This tells the user that the client was |
750 |
|
|
disconnected, and will then start to connect the server again. |
751 |
|
|
|
752 |
|
|
The rest of the program deals with the boring details of actually invoking |
753 |
|
|
the supervisor function to start the whole client process and handle the |
754 |
|
|
actual terminal input, sending it to the server. |
755 |
|
|
|
756 |
elmex |
1.31 |
You should now try to start the server and one or more clients in different |
757 |
root |
1.30 |
terminal windows (and the seed node): |
758 |
|
|
|
759 |
|
|
perl eg/chat_client nick1 |
760 |
|
|
perl eg/chat_client nick2 |
761 |
|
|
perl eg/chat_server |
762 |
|
|
aemp run profile seed |
763 |
|
|
|
764 |
|
|
And then you can experiment with chatting, killing one or more clients, or |
765 |
|
|
stopping and restarting the server, to see the monitoring in action. |
766 |
|
|
|
767 |
root |
1.33 |
The crucial point you should understand from this example is that |
768 |
|
|
monitoring is usually symmetric: when you monitor some other port, |
769 |
|
|
potentially on another node, that other port usually should monitor you, |
770 |
|
|
too, so when the connection dies, both ports get killed, or at least both |
771 |
|
|
sides can take corrective action. Exceptions are "servers" that serve |
772 |
|
|
multiple clients at once and might only wish to clean up, and supervisors, |
773 |
|
|
who of course should not normally get killed (unless they, too, have a |
774 |
|
|
supervisor). |
775 |
|
|
|
776 |
|
|
If you often think in object-oriented terms, then treat a port as an |
777 |
|
|
object, C<port> is the constructor, the receive callbacks set by C<rcv> |
778 |
|
|
act as methods, the C<kil> function becomes the explicit destructor and |
779 |
|
|
C<mon> installs a destructor hook. Unlike conventional object oriented |
780 |
|
|
programming, it can make sense to exchange ports more freely (for example, |
781 |
|
|
to monitor one port from another). |
782 |
|
|
|
783 |
root |
1.30 |
There is ample room for improvement: the server should probably remember |
784 |
|
|
the nickname in the C<join> handler instead of expecting it in every chat |
785 |
|
|
message, it should probably monitor itself, and the client should not try |
786 |
|
|
to send any messages unless a server is actually connected. |
787 |
|
|
|
788 |
|
|
=head1 PART 3: TIMTOWTDI: Virtual Connections |
789 |
|
|
|
790 |
|
|
#TODO |
791 |
elmex |
1.7 |
|
792 |
elmex |
1.1 |
=head1 SEE ALSO |
793 |
|
|
|
794 |
|
|
L<AnyEvent> |
795 |
|
|
|
796 |
|
|
L<AnyEvent::Handle> |
797 |
|
|
|
798 |
|
|
L<AnyEvent::MP> |
799 |
|
|
|
800 |
elmex |
1.20 |
L<AnyEvent::MP::Global> |
801 |
|
|
|
802 |
elmex |
1.1 |
=head1 AUTHOR |
803 |
|
|
|
804 |
|
|
Robin Redeker <elmex@ta-sa.org> |
805 |
root |
1.32 |
Marc Lehmann <schmorp@schmorp.de> |
806 |
root |
1.4 |
|