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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. |
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|
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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 your |
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system. |
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|
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First we have to setup a I<shared secret>: for two L<AnyEvent::MP> I<nodes> to |
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be able to communicate with each other and authenticate each other it is |
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necessary to setup the same I<shared secret> for both of them (or use TLS |
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certificates). |
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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 random |
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shared secret. You can copy this file to any other system and then communicate |
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over the network (via TCP) with it. You can also select your own shared secret |
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(F<aemp setsecret>) and for increased security requirements you can even create |
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a TLS certificate (F<aemp gencert>), causing connections to not just be |
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authenticated, but also to be encrypted. |
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|
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Connections will only be successful when the I<nodes> that want to connect to |
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each other have the same I<shared secret> (or successfully verify the TLS |
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certificate of the other side). |
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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 all for now, there is more fiddling around with the C<aemp> utility |
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later. |
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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 small programs. First the |
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receiver application: |
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|
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#!/opt/perl/bin/perl |
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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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initialise_node "eg_simple_receiver"; |
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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, lets step through the new functions |
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and modules that have been used. For starters there is now an additional |
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module loaded: L<AnyEvent::MP::Global>. |
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|
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That module provides us with a I<global registry>, which lets us share data |
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among all I<nodes> in a network. Why do we need it you might ask? |
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|
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The thing is, that the I<port ids> are just random strings, assigned by |
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L<AnyEvent::MP>. We can't know those I<port ids> in advance, so we don't know |
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which I<port id> to send messages to if the message is to be passed between |
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I<nodes> (or UNIX processes). To find the right I<port> of another I<node> in |
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the network we will need to communicate that somehow to the sender. And |
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exactly that is what L<AnyEvent::MP::Global> provides. |
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|
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=head3 initialise_node And The Network |
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|
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Now, lets have a look at the next new thing, the C<initialise_node>: |
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|
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initialise_node "eg_simple_receiver"; |
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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. The first argument, the string C<"eg_simple_receiver">, is called the |
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I<profile> of this node. A profile holds some information about the application |
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that is going to be a node in an L<AnyEvent::MP> network. |
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|
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Most importantly the profile allows you to set the I<node id> that your |
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application will use. You can also set I<binds> in the profile, meaning that |
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you can define TCP ports that the application will listen on for incoming |
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connections from other nodes of the network. |
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|
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Next you can configure I<seeds> in profile. A I<seed> is just a TCP endpoint |
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which tells the application where to find other nodes of it's network. To |
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explain this a bit more detailed we have to look at the topology of an |
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L<AnyEvent::MP> network. The topology is called a I<fully connected mesh>, here |
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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 I<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 of those 4 already connected |
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nodes. And exactly this is what the I<seeds> are for. Now lets assume that |
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the new node C<N5> has as I<seed> the TCP endpoint of the node C<N2>. |
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It then connects to C<N2>: |
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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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C<N2> then tells C<N5> the I<binds> of the other nodes it is connected to, |
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and C<N5> builds up 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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Finished. C<N5> is now happily connected to the rest of the network. |
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|
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=head3 Setting Up The Profiles |
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|
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Ok, so much to the profile. Now lets setup the C<eg_simple_receiver> I<profile> |
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for later. For the receiver we just give the receiver a I<bind>: |
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|
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aemp profile eg_simple_receiver setbinds localhost:12266 |
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|
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And while we are at it, just setup the I<profile> for the sender in the second |
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part of this example too. We will call the sender I<profile> |
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C<eg_simple_sender>. For the sender we will just setup a I<seed> to the |
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receiver: |
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|
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aemp profile eg_simple_sender setseeds localhost:12266 |
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aemp profile eg_simple_sender setbinds |
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|
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You might wonder why we setup I<binds> to be empty here. Well, there can be |
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exceptions to the I<fully> in the I<fully connected mesh> in L<AnyEvent::MP>. |
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If you don't configure a I<bind> for a node's profile it won't bind itself |
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somewhere. These kinds of I<nodes> will not be able to send messages to other |
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I<nodes> that also didn't I<bind> them self to some TCP address. For this |
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example, as well as some cases in the real world, we can live with this |
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limitation. |
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|
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=head3 Registering The Receiver |
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|
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Ok, where were we. We now discussed the basic purpose of L<AnyEvent::MP::Global> |
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and initialise_node with it's relations to profiles. We also setup our profiles |
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for later use and now have to continue talking about the receiver example. |
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|
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Lets look at the next undiscussed line(s) of code: |
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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 already has been discussed. It just creates a new I<port> |
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and gives us the I<port id>. Now to the C<register> function of |
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L<AnyEvent::MP::Global>: The first argument is a I<port id> that we want to add |
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to a I<global group>, and it's second argument is the name of that I<global |
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group>. |
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|
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You can choose that name of such a I<global group> freely, and it's purpose is |
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to store a set of I<port ids>. That set is made available throughout the whole |
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L<AnyEvent::MP> network, so that each node can see which ports belong to that |
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group. |
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|
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The sender will later look for the ports in that I<global group> and send |
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messages to them. |
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|
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Last step in the example is to setup a receiver callback for those messages |
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like we have discussed in the first example. We again match for the I<tag> |
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C<test>. The difference is just that we don't end the application after |
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receiving the first message. We just infinitely continue to look out for new |
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messages. |
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|
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=head2 The Sender |
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|
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Ok, now lets take a look at the sender: |
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|
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#!/opt/perl/bin/perl |
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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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initialise_node "eg_simple_sender"; |
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|
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my $find_timer = |
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AnyEvent->timer (after => 0, interval => 1, cb => sub { |
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my $ports = AnyEvent::MP::Global::find "eg_receivers" |
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or return; |
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|
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snd $_, test => time |
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for @$ports; |
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}); |
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|
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AnyEvent->condvar->recv; |
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|
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It's even less code. The C<initialise_node> is known now from the receiver |
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above. As discussed in the section where we setup the profiles we configure |
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this application to use the I<profile> C<eg_simple_sender>. |
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|
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Next we setup a timer that repeatedly calls this chunk of code: |
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|
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my $ports = AnyEvent::MP::Global::find "eg_receivers" |
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or return; |
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|
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snd $_, test => time |
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for @$ports; |
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|
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The new function here is the C<find> function of L<AnyEvent::MP::Global>. It |
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searches in the I<global group> named C<eg_receivers> for ports. If none are |
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found C<undef> is returned and we wait for the next time the timer fires. |
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|
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In case the receiver application has been connected and the newly added port by |
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the receiver has propagated to the sender C<find> returns an array reference |
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that contains the I<port id> of the receiver I<port(s)>. |
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|
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We then just send to every I<port> in the I<global group> a message consisting |
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of the I<tag> C<test> and the current time in form of a UNIX timestamp. |
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|
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And thats all. |
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|
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=head1 SEE ALSO |
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|
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L<AnyEvent> |
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|
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L<AnyEvent::Handle> |
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|
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L<AnyEvent::MP> |
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|
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L<AnyEvent::MP::Global> |
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|
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=head1 AUTHOR |
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|
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Robin Redeker <elmex@ta-sa.org> |
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|
