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=head1 Message Passing for the Non-Blocked Mind |
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|
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=head1 Introduction and Terminology |
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|
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This is a tutorial about how to get the swing of the new L<AnyEvent::MP> |
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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. |
8 |
|
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What kind of messages? Basically a message here means a list of Perl |
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strings, numbers, hashes and arrays, anything that can be expressed as a |
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L<JSON> text (as JSON is used by default in the protocol). Here are two |
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examples: |
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|
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write_log => 1251555874, "action was successful.\n" |
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123, ["a", "b", "c"], { foo => "bar" } |
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|
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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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|
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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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|
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ping => 1251381636 |
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|
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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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|
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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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|
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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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|
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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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|
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|
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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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|
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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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|
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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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|
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=head1 Passing Your First Message |
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|
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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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|
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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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|
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use AnyEvent; |
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use AnyEvent::MP; |
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|
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my $end_cv = AnyEvent->condvar; |
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|
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my $port = port; |
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|
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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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|
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snd $port, test => 123; |
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|
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print "Ending with: " . $end_cv->recv . "\n"; |
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|
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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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|
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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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|
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The next function is C<rcv>: |
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|
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rcv $port, test => sub { ... }; |
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|
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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 |
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parts of that message. |
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|
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Messages can be sent with the C<snd> function, which is used like this in |
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the example above: |
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|
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snd $port, test => 123; |
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|
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This will send the message C<'test', 123> to the I<port> with the I<port |
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ID> stored in C<$port>. Since in this case the receiver has a I<tag> match |
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on C<test> it will call the callback with the first argument being the |
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number C<123>. |
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|
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The callback is a typicall AnyEvent idiom: the callback just passes |
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that number on to the I<condition variable> C<$end_cv> which will then |
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pass the value to the print. Condition variables are out of the scope |
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of this tutorial and not often used with ports, so please consult the |
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L<AnyEvent::Intro> about them. |
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|
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Passing messages inside just one process is boring. Before we can move on |
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and do interprocess message passing we first have to make sure some things |
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have been set up correctly for our nodes to talk to each other. |
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|
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=head1 System Requirements and System Setup |
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|
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Before we can start with real IPC we have to make sure some things work on |
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your system. |
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|
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First we have to setup a I<shared secret>: for two L<AnyEvent::MP> |
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I<nodes> to be able to communicate with each other over the network it is |
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necessary to setup the same I<shared secret> for both of them, so they can |
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prove their trustworthyness to each other. |
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|
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The easiest way is to set this up is to use the F<aemp> utility: |
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|
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aemp gensecret |
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|
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This creates a F<$HOME/.perl-anyevent-mp> config file and generates a |
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random shared secret. You can copy this file to any other system and |
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then communicate over the network (via TCP) with it. You can also select |
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your own shared secret (F<aemp setsecret>) and for increased security |
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requirements you can even create (or configure) a TLS certificate (F<aemp |
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gencert>), causing connections to not just be securely authenticated, but |
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also to be encrypted and protected against tinkering. |
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|
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Connections will only be successfully established when the I<nodes> |
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that want to connect to each other have the same I<shared secret> (or |
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successfully verify the TLS certificate of the other side, in which case |
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no shared secret is required). |
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|
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B<If something does not work as expected, and for example tcpdump shows |
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that the connections are closed almost immediately, you should make sure |
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that F<~/.perl-anyevent-mp> is the same on all hosts/user accounts that |
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you try to connect with each other!> |
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|
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Thats is all for now, you will find some more advanced fiddling with the |
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C<aemp> utility later. |
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|
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|
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=head1 Passing Messages Between Processes |
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|
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=head2 The Receiver |
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|
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Lets split the previous example up into two programs: one that contains |
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the sender and one for the receiver. First the receiver application, in |
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full: |
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|
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use AnyEvent; |
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use AnyEvent::MP; |
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use AnyEvent::MP::Global; |
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|
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configure nodeid => "eg_receiver", binds => ["*:4040"]; |
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|
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my $port = port; |
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|
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AnyEvent::MP::Global::register $port, "eg_receivers"; |
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|
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rcv $port, test => sub { |
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my ($data, $reply_port) = @_; |
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|
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print "Received data: " . $data . "\n"; |
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}; |
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|
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AnyEvent->condvar->recv; |
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|
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=head3 AnyEvent::MP::Global |
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|
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Now, that wasn't too bad, was it? Ok, let's step through the new functions |
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and modules that have been used. |
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|
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For starters, there is now an additional module being |
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used: L<AnyEvent::MP::Global>. This module provides us with a I<global |
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registry>, which lets us register ports in groups that are visible on all |
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I<nodes> in a network. |
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|
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What is this useful for? Well, the I<port IDs> are random-looking strings, |
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assigned by L<AnyEvent::MP>. We cannot know those I<port IDs> in advance, |
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so we don't know which I<port ID> to send messages to, especially when the |
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message is to be passed between different I<nodes> (or UNIX processes). To |
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find the right I<port> of another I<node> in the network we will need |
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to communicate this somehow to the sender. And exactly that is what |
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L<AnyEvent::MP::Global> provides. |
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|
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Especially in larger, more anonymous networks this is handy: imagine you |
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have a few database backends, a few web frontends and some processing |
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distributed over a number of hosts: all of these would simply register |
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themselves in the appropriate group, and your web frontends can start to |
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find some database backend. |
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|
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=head3 C<configure> and the Network |
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|
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Now, let's have a look at the new function, C<configure>: |
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|
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configure nodeid => "eg_receiver", binds => ["*:4040"]; |
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|
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Before we are able to send messages to other nodes we have to initialise |
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ourself to become a "distributed node". Initialising a node means naming |
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the node, optionally binding some TCP listeners so that other nodes can |
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contact it and connecting to a predefined set of seed addresses so the |
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node can discover the existing network - and the existing network can |
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discover the node! |
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|
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All of this (and more) can be passed to the C<configure> function - later |
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we will see how we can do all this without even passing anything to |
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C<configure>! |
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|
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The first parameter, C<nodeid>, specified the node ID (in this case |
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C<eg_receiver> - the default is to use the node name of the current host, |
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but for this example we want to be able to run many nodes on the same |
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machine). Node IDs need to be unique within the network and can be almost |
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any string - if you don't care, you can specify a node ID of C<anon/> |
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which will then be replaced by a random node name. |
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|
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The second parameter, C<binds>, specifies a list of C<address:port> pairs |
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to bind TCP listeners on. The special "address" of C<*> means to bind on |
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every local IP address. |
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|
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The reason to bind on a TCP port is not just that other nodes can connect |
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to us: if no binds are specified, the node will still bind on a dynamic |
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port on all local addresses - but in this case we won't know the port, and |
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cannot tell other nodes to connect to it as seed node. |
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|
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A I<seed> is a (fixed) TCP address of some other node in the network. To |
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explain the need for seeds we have to look at the topology of a typical |
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L<AnyEvent::MP> network. The topology is called a I<fully connected mesh>, |
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here an example with 4 nodes: |
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|
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N1--N2 |
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| \/ | |
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| /\ | |
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N3--N4 |
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|
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Now imagine another node - C<N5> - wants to connect itself to that network: |
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|
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N1--N2 |
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| \/ | N5 |
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| /\ | |
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N3--N4 |
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|
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The new node needs to know the I<binds> of all nodes already |
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connected. Exactly this is what the I<seeds> are for: Let's assume that |
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the new node (C<N5>) uses the TCP address of the node C<N2> as seed. This |
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cuases it to connect to C<N2>: |
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|
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N1--N2____ |
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| \/ | N5 |
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| /\ | |
282 |
N3--N4 |
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|
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C<N2> then tells C<N5> about the I<binds> of the other nodes it is |
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connected to, and C<N5> creates the rest of the connections: |
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|
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/--------\ |
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N1--N2____| |
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| \/ | N5 |
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| /\ | /| |
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N3--N4--- | |
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\________/ |
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|
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All done: C<N5> is now happily connected to the rest of the network. |
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|
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Of course, this process takes time, during which the node is already |
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running. This also means it takes time until the node is fully connected, |
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and global groups and other information is available. The best way to deal |
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with this is to either retry regularly until you found the resource you |
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were looking for, or to only start services on demand after a node has |
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become available. |
302 |
|
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=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 |
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talking about the receiver. |
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|
310 |
Let's look at the next line(s): |
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|
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my $port = port; |
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AnyEvent::MP::Global::register $port, "eg_receivers"; |
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|
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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 |
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L<AnyEvent::MP> network, so that each node can see which ports belong to |
325 |
that group. |
326 |
|
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Later we will see how the sender looks for the ports in this I<global |
328 |
group> to send messages to them. |
329 |
|
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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 |
|
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=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 |
|