=head1 Message Passing for the Non-Blocked Mind

=head1 Introduction and Terminology

This is a tutorial about how to get the swing of the new L<AnyEvent::MP>
module, which allows programs to transparently pass messages within the
process and to other processes on the same or a different host.

What kind of messages? Basically a message here means a list of Perl
strings, numbers, hashes and arrays, anything that can be expressed as a
L<JSON> text (as JSON is used by default in the protocol). Here are two
examples:

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

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

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

   ping => 1251381636

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

How do these ports relate to things you know? Each I<port> belongs
to a I<node>, and a I<node> is just the UNIX process that runs your
L<AnyEvent::MP> application.

Each I<node> is distinguished from other I<nodes> running on the same or
another host in a network by its I<node ID>. A I<node ID> is simply a
unique string chosen manually or assigned by L<AnyEvent::MP> in some way
(UNIX nodename, random string...).

Here is a diagram about how I<nodes>, I<ports> and UNIX processes relate
to each other. The setup consists of two nodes (more are of course
possible): Node C<A> (in UNIX process 7066) with the ports C<ABC> and
C<DEF>. And the node C<B> (in UNIX process 8321) with the ports C<FOO> and
C<BAR>.


  |- PID: 7066 -|                  |- PID: 8321 -|
  |             |                  |             |
  | Node ID: A  |                  | Node ID: B  |
  |             |                  |             |
  |   Port ABC =|= <----\ /-----> =|= Port FOO   |
  |             |        X         |             |
  |   Port DEF =|= <----/ \-----> =|= Port BAR   |
  |             |                  |             |
  |-------------|                  |-------------|

The strings for the I<port IDs> here are just for illustrative
purposes: Even though I<ports> in L<AnyEvent::MP> are also identified by
strings, they can't be choosen manually and are assigned by the system
dynamically. These I<port IDs> are unique within a network and can also be
used to identify senders or as message tags for instance.

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

=head1 Passing Your First Message

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

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

   use AnyEvent;
   use AnyEvent::MP;

   my $end_cv = AnyEvent->condvar;

   my $port = port;

   rcv $port, test => sub {
      my ($data) = @_;
      $end_cv->send ($data);
   };

   snd $port, test => 123;

   print "Ending with: " . $end_cv->recv . "\n";

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

This I<port ID> can be used to send messages to the port and install
handlers to receive messages on the port. Since it is a simple string
it can be safely passed to other I<nodes> in the network when you want
to refer to that specific port (usually used for RPC, where you need
to tell the other end which I<port> to send the reply to - messages in
L<AnyEvent::MP> have a destination, but no source).

The next function is C<rcv>:

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

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

Messages can be sent with the C<snd> function, which is used like this in
the example above:

   snd $port, test => 123;

This will send the message C<'test', 123> to the I<port> with the I<port
ID> stored in C<$port>. Since in this case the receiver has a I<tag> match
on C<test> it will call the callback with the first argument being the
number C<123>.

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

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

=head1 System Requirements and System Setup

Before we can start with real IPC we have to make sure some things work on your
system.

First we have to setup a I<shared secret>: for two L<AnyEvent::MP> I<nodes> to
be able to communicate with each other and authenticate each other it is
necessary to setup the same I<shared secret> for both of them (or use TLS
certificates).

The easiest way is to set this up is to use the F<aemp> utility:

   aemp gensecret

This creates a F<$HOME/.perl-anyevent-mp> config file and generates a random
shared secret. You can copy this file to any other system and then communicate
over the network (via TCP) with it. You can also select your own shared secret
(F<aemp setsecret>) and for increased security requirements you can even create
a TLS certificate (F<aemp gencert>), causing connections to not just be
authenticated, but also to be encrypted.

Connections will only be successful when the I<nodes> that want to connect to
each other have the same I<shared secret> (or successfully verify the TLS
certificate of the other side).

B<If something does not work as expected, and for example tcpdump shows
that the connections are closed almost immediately, you should make sure
that F<~/.perl-anyevent-mp> is the same on all hosts/user accounts that
you try to connect with each other!>

Thats all for now, there is more fiddling around with the C<aemp> utility
later.

=head1 Passing Messages Between Processes

=head2 The Receiver

Lets split the previous example up into two small programs. First the
receiver application:

   #!/opt/perl/bin/perl
   use AnyEvent;
   use AnyEvent::MP;
   use AnyEvent::MP::Global;

   initialise_node "eg_simple_receiver";

   my $port = port;

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

   rcv $port, test => sub {
      my ($data, $reply_port) = @_;

      print "Received data: " . $data . "\n";
   };

   AnyEvent->condvar->recv;

=head3 AnyEvent::MP::Global

Now, that wasn't too bad, was it? Ok, lets step through the new functions
and modules that have been used. For starters there is now an additional
module loaded: L<AnyEvent::MP::Global>.

That module provides us with a I<global registry>, which lets us share data
among all I<nodes> in a network. Why do we need it you might ask?

The thing is, that the I<port ids> are just random strings, assigned by
L<AnyEvent::MP>.  We can't know those I<port ids> in advance, so we don't know
which I<port id> to send messages to if the message is to be passed between
I<nodes> (or UNIX processes). To find the right I<port> of another I<node> in
the network we will need to communicate that somehow to the sender.  And
exactly that is what L<AnyEvent::MP::Global> provides.

=head3 initialise_node And The Network

Now, lets have a look at the next new thing, the C<initialise_node>:

   initialise_node "eg_simple_receiver";

Before we are able to send messages to other nodes we have to initialise
ourself. The first argument, the string C<"eg_simple_receiver">, is called the
I<profile> of this node. A profile holds some information about the application
that is going to be a node in an L<AnyEvent::MP> network.

Most importantly the profile allows you to set the I<node id> that your
application will use. You can also set I<binds> in the profile, meaning that
you can define TCP ports that the application will listen on for incoming
connections from other nodes of the network.

Next you can configure I<seeds> in profile. A I<seed> is just a TCP endpoint
which tells the application where to find other nodes of it's network. To
explain this a bit more detailed we have to look at the topology of an
L<AnyEvent::MP> network. The topology is called a I<fully connected mesh>, here
an example with 4 nodes:

   N1--N2
   | \/ |
   | /\ |
   N3--N4

Now imagine another I<node> C<N5>. wants to connect itself to that network:

   N1--N2
   | \/ |    N5
   | /\ |
   N3--N4

The new node needs to know the I<binds> of all of those 4 already connected
nodes. And exactly this is what the I<seeds> are for. Now lets assume that
the new node C<N5> has as I<seed> the TCP endpoint of the node C<N2>.
It then connects to C<N2>:

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

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

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

Finished. C<N5> is now happily connected to the rest of the network.

=head3 Setting Up The Profiles

Ok, so much to the profile. Now lets setup the C<eg_simple_receiver> I<profile>
for later. For the receiver we just give the receiver a I<bind>:

   aemp profile eg_simple_receiver setbinds localhost:12266

And while we are at it, just setup the I<profile> for the sender in the second
part of this example too. We will call the sender I<profile>
C<eg_simple_sender>. For the sender we will just setup a I<seed> to the
receiver:

   aemp profile eg_simple_sender setseeds localhost:12266
   aemp profile eg_simple_sender setbinds

You might wonder why we setup I<binds> to be empty here. Well, there can be
exceptions to the I<fully> in the I<fully connected mesh> in L<AnyEvent::MP>.
If you don't configure a I<bind> for a node's profile it won't bind itself
somewhere. These kinds of I<nodes> will not be able to send messages to other
I<nodes> that also didn't I<bind> them self to some TCP address. For this
example, as well as some cases in the real world, we can live with this
limitation.

=head3 Registering The Receiver

Ok, where were we. We now discussed the basic purpose of L<AnyEvent::MP::Global>
and initialise_node with it's relations to profiles. We also setup our profiles
for later use and now have to continue talking about the receiver example.

Lets look at the next undiscussed line(s) of code:

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

The C<port> function already has been discussed. It just creates a new I<port>
and gives us the I<port id>. Now to the C<register> function of
L<AnyEvent::MP::Global>: The first argument is a I<port id> that we want to add
to a I<global group>, and it's second argument is the name of that I<global
group>.

You can choose that name of such a I<global group> freely, and it's purpose is
to store a set of I<port ids>. That set is made available throughout the whole
L<AnyEvent::MP> network, so that each node can see which ports belong to that
group.

The sender will later look for the ports in that I<global group> and send
messages to them.

Last step in the example is to setup a receiver callback for those messages
like we have discussed in the first example. We again match for the I<tag>
C<test>. The difference is just that we don't end the application after
receiving the first message. We just infinitely continue to look out for new
messages.

=head2 The Sender

Ok, now lets take a look at the sender:

   #!/opt/perl/bin/perl
   use AnyEvent;
   use AnyEvent::MP;
   use AnyEvent::MP::Global;

   initialise_node "eg_simple_sender";

   my $find_timer =
      AnyEvent->timer (after => 0, interval => 1, cb => sub {
         my $ports = AnyEvent::MP::Global::find "eg_receivers"
            or return;

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

   AnyEvent->condvar->recv;

It's even less code. The C<initialise_node> is known now from the receiver
above. As discussed in the section where we setup the profiles we configure
this application to use the I<profile> C<eg_simple_sender>.

Next we setup a timer that repeatedly calls this chunk of code:

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

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

The new function here is the C<find> function of L<AnyEvent::MP::Global>.  It
searches in the I<global group> named C<eg_receivers> for ports. If none are
found C<undef> is returned and we wait for the next time the timer fires.

In case the receiver application has been connected and the newly added port by
the receiver has propagated to the sender C<find> returns an array reference
that contains the I<port id> of the receiver I<port(s)>.

We then just send to every I<port> in the I<global group> a message consisting
of the I<tag> C<test> and the current time in form of a UNIX timestamp.

And thats all.

=head1 SEE ALSO

L<AnyEvent>

L<AnyEvent::Handle>

L<AnyEvent::MP>

L<AnyEvent::MP::Global>

=head1 AUTHOR

  Robin Redeker <elmex@ta-sa.org>