AnyEvent-MP/MP/Intro.pod

=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 the default serialiser 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 indicates 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 chosen 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 even 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.

=head2 Passing Your First Message

For starters, let's 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 creates 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 typical 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.

=head2 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 over the network it is
necessary to setup the same I<shared secret> for both of them, so they can
prove their trustworthyness to each other.

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 (or configure) a TLS certificate (F<aemp
gencert>), causing connections to not just be securely authenticated, but
also to be encrypted and protected against tinkering.

Connections will only be successfully established 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, in which case
no shared secret is required).

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 is all for now, you will find some more advanced fiddling with the
C<aemp> utility later.

=head2 Shooting the Trouble

Sometimes things go wrong, and AnyEvent::MP, being a professional module,
does not gratuitously spill out messages to your screen.

To help troubleshooting any issues, there are two environment variables
that you can set. The first, C<PERL_ANYEVENT_MP_WARNLEVEL> sets the
logging level. The default is C<5>, which means nothing much is
printed. You can increase it to C<8> or C<9> to get more verbose
output. This is example output when starting a node:

   2012-03-04 19:41:10 <8> node cerebro starting up.
   2012-03-04 19:41:10 <8> node listens on [10.0.0.1:4040].
   2012-03-04 19:41:10 <9> trying connect to seed node 10.0.0.19:4040.
   2012-03-04 19:41:10 <9> 10.0.0.19:4040 connected as rain
   2012-03-04 19:41:10 <7> rain is up ()

A lot of info, but at least you can see that it does something.

The other environment variable that can be useful is
C<PERL_ANYEVENT_MP_TRACE>, which, when set to a true value, will cause
most messages that are sent or received to be printed. For example, F<aemp
restart rijk> might output these message exchanges:

   SND rijk <- [null,"eval","AnyEvent::Watchdog::Util::restart; ()","aemp/cerebro/z4kUPp2JT4#b"]
   SND rain <- [null,"g_slave",{"'l":{"aemp/cerebro/z4kUPp2JT4":["10.0.0.1:48168"]}}]
   SND rain <- [null,"g_find","rijk"]
   RCV rain -> ["","g_found","rijk",["10.0.0.23:4040"]]
   RCV rijk -> ["b",""]

=head1 PART 1: Passing Messages Between Processes

=head2 The Receiver

Lets split the previous example up into two programs: one that contains
the sender and one for the receiver. First the receiver application, in
full:

   use AnyEvent;
   use AnyEvent::MP;

   configure nodeid => "eg_receiver/%u", binds => ["*:4040"];

   my $port = port;
   my $db_guard = db_reg eg_receivers => $port;

   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, let's step through the new functions
and modules that have been used.

For starters, there is now an additional module being
used: L<AnyEvent::MP::Global>. This module provides us with a I<global
registry>, which lets us register ports in groups that are visible on all
I<nodes> in a network.

What is this useful for? Well, the I<port IDs> are random-looking strings,
assigned by L<AnyEvent::MP>. We cannot know those I<port IDs> in advance,
so we don't know which I<port ID> to send messages to, especially when the
message is to be passed between different I<nodes> (or UNIX processes). To
find the right I<port> of another I<node> in the network we will need
to communicate this somehow to the sender. And exactly that is what
L<AnyEvent::MP::Global> provides.

Especially in larger, more anonymous networks this is handy: imagine you
have a few database backends, a few web front-ends and some processing
distributed over a number of hosts: all of these would simply register
themselves in the appropriate group, and your web front-ends can start to
find some database backend.

=head3 C<configure> and Joining and Maintaining the Network

Now, let's have a look at the new function, C<configure>:

   configure nodeid => "eg_receiver", binds => ["*:4040"];

Before we are able to send messages to other nodes we have to initialise
ourself to become a "distributed node". Initialising a node means naming
the node, optionally binding some TCP listeners so that other nodes can
contact it and connecting to a predefined set of seed addresses so the
node can discover the existing network - and the existing network can
discover the node!

All of this (and more) can be passed to the C<configure> function - later
we will see how we can do all this without even passing anything to
C<configure>!

The first parameter, C<nodeid>, specified the node ID (in this case
C<eg_receiver> - the default is to use the node name of the current host,
but for this example we want to be able to run many nodes on the same
machine). Node IDs need to be unique within the network and can be almost
any string - if you don't care, you can specify a node ID of C<anon/>
which will then be replaced by a random node name.

The second parameter, C<binds>, specifies a list of C<address:port> pairs
to bind TCP listeners on. The special "address" of C<*> means to bind on
every local IP address (this might not work on every OS, so it should not
be used unless you know it works).

The reason to bind on a TCP port is not just that other nodes can connect
to us: if no binds are specified, the node will still bind on a dynamic
port on all local addresses - but in this case we won't know the port, and
cannot tell other nodes to connect to it as seed node.

A I<seed> is a (fixed) TCP address of some other node in the network. To
explain the need for seeds we have to look at the topology of a typical
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 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 nodes already
connected. Exactly this is what the I<seeds> are for: Let's assume that
the new node (C<N5>) uses the TCP address of the node C<N2> as seed. This
causes it to connect to C<N2>:

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

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

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

All done: C<N5> is now happily connected to the rest of the network.

Apart form the obvious function - joining the network - seed nodes fulfill
another very important function: the connections created by connecting
to seed nodes are used to keep the network together - by trying to keep
connections to all seed nodes active, the network ensures that it will not
split into multiple networks without connection to each other.

This means that the graph created by all seed node connections must span
the whole network, in some way.

There are many ways of doing this - the most simple is probably to use
a single set of one or more seednodes as seednodes for all nodes in the
network - this creates a "hub" of seednodes that connect to each other,
and "leaf" nodes that connect to the nodes in the hub, keeping everything
together.

The process of joining a network takes time, during which the node is
already running. This also means it takes time until the node is fully
connected, and global groups and other information is available. The best
way to deal with this is to either retry regularly until you found the
resource you were looking for, or to only start services on demand after a
node has become available.

=head3 Registering the Receiver

Coming back to our example, we have now introduced the basic purpose of
L<AnyEvent::MP::Global> and C<configure> and its use of profiles. We
also set up our profiles for later use and now we will finally continue
talking about the receiver.

Let's look at the next line(s):

   my $port = port;
   my $db_guard = db_reg eg_receivers => $port;

The C<port> function has already been discussed. It simply creates a new
I<port> and returns the I<port ID>. The C<grp_reg> function, however, is
new: The first argument is the name of a I<global group>, and the second
argument is the I<port ID> to register in that group.

You can choose the name of such a I<global group> freely (prefixing your
package name is I<highly recommended> however and might be enforce din
future versions!). The purpose of such a group is to store a set of port
IDs. This set is made available throughout the L<AnyEvent::MP> network,
so that each node can see which ports belong to that group.

Later we will see how the sender looks for the ports in this global
group to send messages to them.

The last step in the example is to set up a receiver callback for those
messages, just as was discussed in the first example. We again match
for the tag C<test>. The difference is that this time we don't exit the
application after receiving the first message. Instead we continue to wait
for new messages indefinitely.

=head2 The Sender

Ok, now let's take a look at the sender code:

   use AnyEvent;
   use AnyEvent::MP;

   configure nodeid => "eg_sender/%u", seeds => ["*:4040"];

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

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

   AnyEvent->condvar->recv;

It's even less code. The C<configure> serves the same purpose as in the
receiver, but instead of specifying binds we specify a list of seeds -
which happens to be the same as the binds used by the receiver, which
becomes our seed node.

Next we set up a timer that repeatedly (every second) calls this chunk of
code:

   my $ports = grp_get "eg_receivers"
      or return;

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

The only new function here is the C<grp_get> function of
L<AnyEvent::MP::Global>. It searches in the global group named
C<eg_receivers> for ports. If none are found, it returns C<undef>, which
makes our code return instantly and wait for the next round, as nobody is
interested in our message.

As soon as the receiver application has connected and the information
about the newly added port in the receiver has propagated to the sender
node, C<grp_get> returns an array reference that contains the I<port ID> of
the receiver I<port(s)>.

We then just send a message with a tag and the current time to every
I<port> in the global group.

=head3 Splitting Network Configuration and Application Code

OK, so far, this works. In the real world, however, the person configuring
your application to run on a specific network (the end user or network
administrator) is often different to the person coding the application.

Or to put it differently: the arguments passed to configure are usually
provided not by the programmer, but by whoever is deploying the program.

To make this easy, AnyEvent::MP supports a simple configuration database,
using profiles, which can be managed using the F<aemp> command-line
utility (yes, this section is about the advanced tinkering we mentioned
before).

When you change both programs above to simply call

   configure;

then AnyEvent::MP tries to look up a profile using the current node name
in its configuration database, falling back to some global default.

You can run "generic" nodes using the F<aemp> utility as well, and we will
exploit this in the following way: we configure a profile "seed" and run
a node using it, whose sole purpose is to be a seed node for our example
programs.

We bind the seed node to port 4040 on all interfaces:

   aemp profile seed binds "*:4040"

And we configure all nodes to use this as seed node (this only works when
running on the same host, for multiple machines you would provide the IP
address or hostname of the node running the seed), and use a random name
(because we want to start multiple nodes on the same host):

   aemp seeds "*:4040" nodeid anon/

Then we run the seed node:

   aemp run profile seed

After that, we can start as many other nodes as we want, and they will all
use our generic seed node to discover each other.

In fact, starting many receivers nicely illustrates that the time sender
can have multiple receivers.

That's all for now - next we will teach you about monitoring by writing a
simple chat client and server :)

=head1 PART 2: Monitoring, Supervising, Exception Handling and Recovery

That's a mouthful, so what does it mean? Our previous example is what one
could call "very loosely coupled" - the sender doesn't care about whether
there are any receivers, and the receivers do not care if there is any
sender.

This can work fine for simple services, but most real-world applications
want to ensure that the side they are expecting to be there is actually
there. Going one step further: most bigger real-world applications even
want to ensure that if some component is missing, or has crashed, it will
still be there, by recovering and restarting the service.

AnyEvent::MP supports this by catching exceptions and network problems,
and notifying interested parties of this.

=head2 Exceptions, Port Context, Network Errors and Monitors

=head3 Exceptions

Exceptions are handled on a per-port basis: receive callbacks are executed
in a special context, the so-called I<port-context>: code that throws an
otherwise uncaught exception will cause the port to be C<kil>led. Killed
ports are destroyed automatically (killing ports is the only way to free
ports, incidentally).

Ports can be monitored, even from a different host, and when a port is
killed any entity monitoring it will be notified.

Here is a simple example:

  use AnyEvent::MP;

  # create a port, it always dies
  my $port = port { die "oops" };

  # monitor it
  mon $port, sub {
     warn "$port was killed (with reason @_)";
  };

  # now send it some message, causing it to die:
  snd $port;

It first creates a port whose only action is to throw an exception,
and the monitors it with the C<mon> function. Afterwards it sends it a
message, causing it to die and call the monitoring callback:

   anon/6WmIpj.a was killed (with reason die oops at xxx line 5.) at xxx line 9.

The callback was actually passed two arguments: C<die> (to indicate it did
throw an exception as opposed to, say, a network error) and the exception
message itself.

What happens when a port is killed before we have a chance to monitor
it? Granted, this is highly unlikely in our example, but when you program
in a network this can easily happen due to races between nodes.

  use AnyEvent::MP;

  my $port = port { die "oops" };

  snd $port;

  mon $port, sub {
     warn "$port was killed (with reason @_)";
  };

This time we will get something like:

   anon/zpX.a was killed (with reason no_such_port cannot monitor nonexistent port)

Since the port was already gone, the kill reason is now C<no_such_port>
with some descriptive (we hope) error message.

In fact, the kill reason is usually some identifier as first argument
and a human-readable error message as second argument, but can be about
anything (it's a list) or even nothing - which is called a "normal" kill.

You can kill ports manually using the C<kil> function, which will be
treated like an error when any reason is specified:

   kil $port, custom_error => "don't like your steenking face";

And a clean kill without any reason arguments:
  
   kil $port;

By now you probably wonder what this "normal" kill business is: A common
idiom is to not specify a callback to C<mon>, but another port, such as
C<$SELF>:

   mon $port, $SELF;

This basically means "monitor $port and kill me when it crashes". And a
"normal" kill does not count as a crash. This way you can easily link
ports together and make them crash together on errors (but allow you to
remove a port silently).

=head3 Port Context

When code runs in an environment where C<$SELF> contains its own port ID
and exceptions will be caught, it is said to run in a port context.

Since AnyEvent::MP is event-based, it is not uncommon to register
callbacks from C<rcv> handlers. As example, assume that the port receive
handler wants to C<die> a second later, using C<after>:

  my $port = port {
     after 1, sub { die "oops" };
  };

Then you will find it does not work - when the after callback is executed,
it does not run in port context anymore, so exceptions will not be caught.

For these cases, AnyEvent::MP exports a special "closure constructor"
called C<psub>, which works just like perls built-in C<sub>:

  my $port = port {
     after 1, psub { die "oops" };
  };

C<psub> stores C<$SELF> and returns a code reference. When the code
reference is invoked, it will run the code block within the context of
that port, so exception handling once more works as expected.

There is also a way to temporarily execute code in the context of some
port, namely C<peval>:

  peval $port, sub {
     # die'ing here will kil $port
  };

The C<peval> function temporarily replaces C<$SELF> by the given C<$port>
and then executes the given sub in a port context.

=head3 Network Errors and the AEMP Guarantee

I mentioned another important source of monitoring failures: network
problems. When a node loses connection to another node, it will invoke all
monitoring actions as if the port was killed, even if it is possible that
the port still lives happily on another node (not being able to talk to a
node means we have no clue what's going on with it, it could be crashed,
but also still running without knowing we lost the connection).

So another way to view monitors is "notify me when some of my messages
couldn't be delivered". AEMP has a guarantee about message delivery to a
port:  After starting a monitor, any message sent to a port will either
be delivered, or, when it is lost, any further messages will also be lost
until the monitoring action is invoked. After that, further messages
I<might> get delivered again.

This doesn't sound like a very big guarantee, but it is kind of the best
you can get while staying sane: Specifically, it means that there will
be no "holes" in the message sequence: all messages sent are delivered
in order, without any missing in between, and when some were lost, you
I<will> be notified of that, so you can take recovery action.

=head3 Supervising

Ok, so what is this crashing-everything-stuff going to make applications
I<more> stable? Well in fact, the goal is not really to make them more
stable, but to make them more resilient against actual errors and
crashes. And this is not done by crashing I<everything>, but by crashing
everything except a supervisor.

A supervisor is simply some code that ensures that an application (or a
part of it) is running, and if it crashes, is restarted properly.

To show how to do all this we will create a simple chat server that can
handle many chat clients. Both server and clients can be killed and
restarted, and even crash, to some extent.

=head2 Chatting, the Resilient Way

Without further ado, here is the chat server (to run it, we assume the
set-up explained earlier, with a separate F<aemp run> seed node):

   use common::sense;
   use AnyEvent::MP;
   use AnyEvent::MP::Global;

   configure;

   my %clients;

   sub msg {
      print "relaying: $_[0]\n";
      snd $_, $_[0]
         for values %clients;
   }

   our $server = port;

   rcv $server, join => sub {
      my ($client, $nick) = @_;

      $clients{$client} = $client;

      mon $client, sub {
         delete $clients{$client};
         msg "$nick (quits, @_)";
      };
      msg "$nick (joins)";
   };

   rcv $server, privmsg => sub {
      my ($nick, $msg) = @_;
      msg "$nick: $msg";
   };

   grp_reg eg_chat_server => $server;

   warn "server ready.\n";

   AnyEvent->condvar->recv;

Looks like a lot, but it is actually quite simple: after your usual
preamble (this time we use common sense), we define a helper function that
sends some message to every registered chat client:

   sub msg {
      print "relaying: $_[0]\n";
      snd $_, $_[0]
         for values %clients;
   }

The clients are stored in the hash C<%client>. Then we define a server
port and install two receivers on it, C<join>, which is sent by clients
to join the chat, and C<privmsg>, that clients use to send actual chat
messages.

C<join> is most complicated. It expects the client port and the nickname
to be passed in the message, and registers the client in C<%clients>.

   rcv $server, join => sub {
      my ($client, $nick) = @_;

      $clients{$client} = $client;

The next step is to monitor the client. The monitoring action removes the
client and sends a quit message with the error to all remaining clients.

      mon $client, sub {
         delete $clients{$client};
         msg "$nick (quits, @_)";
      };

And finally, it creates a join message and sends it to all clients.

      msg "$nick (joins)";
   };

The C<privmsg> callback simply broadcasts the message to all clients:

   rcv $server, privmsg => sub {
      my ($nick, $msg) = @_;
      msg "$nick: $msg";
   };

And finally, the server registers itself in the server group, so that
clients can find it:

   grp_reg eg_chat_server => $server;

Well, well... and where is this supervisor stuff? Well... we cheated,
it's not there. To not overcomplicate the example, we only put it into
the..... CLIENT!

=head3 The Client, and a Supervisor!

Again, here is the client, including supervisor, which makes it a bit
longer:

   use common::sense;
   use AnyEvent::MP;
   use AnyEvent::MP::Global;

   my $nick = shift;

   configure;

   my ($client, $server);

   sub server_connect {
      my $servernodes = grp_get "eg_chat_server"
         or return after 1, \&server_connect;

      print "\rconnecting...\n";

      $client = port { print "\r  \r@_\n> " };
      mon $client, sub {
         print "\rdisconnected @_\n";
         &server_connect;
      };

      $server = $servernodes->[0];
      snd $server, join => $client, $nick;
      mon $server, $client;
   }

   server_connect;

   my $w = AnyEvent->io (fh => 0, poll => 'r', cb => sub {
      chomp (my $line = <STDIN>);
      print "> ";
      snd $server, privmsg => $nick, $line
        if $server;
   });

   $| = 1;
   print "> ";
   AnyEvent->condvar->recv;

The first thing the client does is to store the nick name (which is
expected as the only command line argument) in C<$nick>, for further
usage.

The next relevant thing is... finally... the supervisor:

   sub server_connect {
      my $servernodes = grp_get "eg_chat_server"
         or return after 1, \&server_connect;

This looks up the server in the C<eg_chat_server> global group. If it
cannot find it (which is likely when the node is just starting up),
it will wait a second and then retry. This "wait a bit and retry"
is an important pattern, as distributed programming means lots of
things are going on asynchronously. In practise, one should use a more
intelligent algorithm, to possibly warn after an excessive number of
retries. Hopefully future versions of AnyEvent::MP will offer some
predefined supervisors, for now you will have to code it on your own.

Next it creates a local port for the server to send messages to, and
monitors it. When the port is killed, it will print "disconnected" and
tell the supervisor function to retry again.

      $client = port { print "\r  \r@_\n> " };
      mon $client, sub {
         print "\rdisconnected @_\n";
         &server_connect;
      };

Then everything is ready: the client will send a C<join> message with it's
local port to the server, and start monitoring it:

      $server = $servernodes->[0];
      snd $server, join => $client, $nick;
      mon $server, $client;
   }

The monitor will ensure that if the server crashes or goes away, the
client will be killed as well. This tells the user that the client was
disconnected, and will then start to connect the server again.

The rest of the program deals with the boring details of actually invoking
the supervisor function to start the whole client process and handle the
actual terminal input, sending it to the server.

You should now try to start the server and one or more clients in different
terminal windows (and the seed node):

   perl eg/chat_client nick1
   perl eg/chat_client nick2
   perl eg/chat_server
   aemp run profile seed

And then you can experiment with chatting, killing one or more clients, or
stopping and restarting the server, to see the monitoring in action.

The crucial point you should understand from this example is that
monitoring is usually symmetric: when you monitor some other port,
potentially on another node, that other port usually should monitor you,
too, so when the connection dies, both ports get killed, or at least both
sides can take corrective action. Exceptions are "servers" that serve
multiple clients at once and might only wish to clean up, and supervisors,
who of course should not normally get killed (unless they, too, have a
supervisor).

If you often think in object-oriented terms, then treat a port as an
object, C<port> is the constructor, the receive callbacks set by C<rcv>
act as methods, the C<kil> function becomes the explicit destructor and
C<mon> installs a destructor hook. Unlike conventional object oriented
programming, it can make sense to exchange ports more freely (for example,
to monitor one port from another).

There is ample room for improvement: the server should probably remember
the nickname in the C<join> handler instead of expecting it in every chat
message, it should probably monitor itself, and the client should not try
to send any messages unless a server is actually connected.

=head1 PART 3: TIMTOWTDI: Virtual Connections

The chat system developed in the previous sections is very "traditional"
in a way: you start some server(s) and some clients statically and they
start talking to each other.

Sometimes applications work more like "services": They can run on almost
any node and talks to itself on other nodes. The L<AnyEvent::MP::Global>
service for example monitors nodes joining the network and starts itself
automatically on other nodes (if it isn't running already).

A good way to design such applications is to put them into a module and
create "virtual connections" to other nodes - we call this the "bridge
head" method, because you start by creating a remote port (the bridge
head) and from that you start to bootstrap your application.

Since that sounds rather theoretical, let's redesign the chat server and
client using this design method.

Here is the server:

   use common::sense;
   use AnyEvent::MP;
   use AnyEvent::MP::Global;

   configure;

   grp_reg eg_chat_server2 => $NODE;

   my %clients;

   sub msg {
      print "relaying: $_[0]\n";
      snd $_, $_[0]
         for values %clients;
   }

   sub client_connect {
      my ($client, $nick) = @_;

      mon $client;
      mon $client, sub {
         delete $clients{$client};
         msg "$nick (quits, @_)";
      };

      $clients{$client} = $client;

      msg "$nick (joins)";

      rcv $SELF, sub { msg "$nick: $_[0]" };
   }

   warn "server ready.\n";

   AnyEvent->condvar->recv;

It starts out not much different then the previous example, except that
this time, we register the node port in the global group and not any port
we created - the clients only want to know which node the server should be
running on. In fact, they could also use some kind of election mechanism,
to find the node with lowest load or something like that.

The more interesting change is that indeed no server port is created -
the server consists only of code, and "does" nothing by itself. All it
does is define a function C<client_connect>, which expects a client port
and a nick name as arguments. It then monitors the client port and binds
a receive callback on C<$SELF>, which expects messages that in turn are
broadcast to all clients.

The two C<mon> calls are a bit tricky - the first C<mon> is a shorthand
for C<mon $client, $SELF>. The second does the normal "client has gone
away" clean-up action. Both could actually be rolled into one C<mon>
action.

C<$SELF> is a good hint that something interesting is going on. And
indeed, when looking at the client code, there is a new function,
C<spawn>:

   use common::sense;
   use AnyEvent::MP;
   use AnyEvent::MP::Global;

   my $nick = shift;

   configure;

   $| = 1;

   my $port = port;

   my ($client, $server);

   sub server_connect {
      my $servernodes = grp_get "eg_chat_server2"
         or return after 1, \&server_connect;

      print "\rconnecting...\n";

      $client = port { print "\r  \r@_\n> " };
      mon $client, sub {
         print "\rdisconnected @_\n";
         &server_connect;
      };

      $server = spawn $servernodes->[0], "::client_connect", $client, $nick;
      mon $server, $client;
   }

   server_connect;

   my $w = AnyEvent->io (fh => 0, poll => 'r', cb => sub {
      chomp (my $line = <STDIN>);
      print "> ";
      snd $server, $line
        if $server;
   });

   print "> ";
   AnyEvent->condvar->recv;

The client is quite similar to the previous one, but instead of contacting
the server I<port> (which no longer exists), it C<spawn>s (creates) a new
the server I<port on node>:

      $server = spawn $servernodes->[0], "::client_connect", $client, $nick;
      mon $server, $client;

And of course the first thing after creating it is monitoring it.

The C<spawn> function creates a new port on a remote node and returns
its port ID. After creating the port it calls a function on the remote
node, passing any remaining arguments to it, and - most importantly -
executes the function within the context of the new port, so it can be
manipulated by referring to C<$SELF>. The init function can reside in a
module (actually it normally I<should> reside in a module) - AnyEvent::MP
will automatically load the module if the function isn't defined.

The C<spawn> function returns immediately, which means you can instantly
send messages to the port, long before the remote node has even heard
of our request to create a port on it. In fact, the remote node might
not even be running. Despite these troubling facts, everything should
work just fine: if the node isn't running (or the init function throws an
exception), then the monitor will trigger because the port doesn't exist.

If the spawn message gets delivered, but the monitoring message is not
because of network problems (extremely unlikely, but monitoring, after
all, is implemented by passing a message, and messages can get lost), then
this connection loss will eventually trigger the monitoring action. On the
remote node (which in return monitors the client) the port will also be
cleaned up on connection loss. When the remote node comes up again and our
monitoring message can be delivered, it will instantly fail because the
port has been cleaned up in the meantime.

If your head is spinning by now, that's fine - just keep in mind, after
creating a port, monitor it on the local node, and monitor "the other
side" from the remote node, and all will be cleaned up just fine.

=head2 Services

Above it was mentioned that C<spawn> automatically loads modules, and this
can be exploited in various ways.

Assume for a moment you put the server into a file called
F<mymod/chatserver.pm> reachable from the current directory. Then you
could run a node there with:

   aemp run

The other nodes could C<spawn> the server by using
C<mymod::chatserver::client_connect> as init function.

Likewise, when you have some service that starts automatically (similar to
AnyEvent::MP::Global), then you can configure this service statically:

   aemp profile mysrvnode services mymod::service::
   aemp run profile mysrvnode

And the module will automatically be loaded in the node, as specifying a
module name (with C<::>-suffix) will simply load the module, which is then
free to do whatever it wants.

Of course, you can also do it in the much more standard way by writing
a module (e.g. C<BK::Backend::IRC>), installing it as part of a module
distribution and then configure nodes, for example, if I want to run the
Bummskraut IRC backend on a machine named "ruth", I could do this:

   aemp profile ruth addservice BK::Backend::IRC::

And any F<aemp run> on that host will automatically have the Bummskraut
IRC backend running.

That's plenty of possibilities you can use - it's all up to you how you
structure your application.

=head1 PART 4: Coro::MP - selective receive

Not all problems lend themselves naturally to an event-based solution:
sometimes things are easier if you can decide in what order you want to
receive messages, irregardless of the order in which they were sent.

In these cases, L<Coro::MP> can provide a nice solution: instead of
registering callbacks for each message type, C<Coro::MP> attached a
(coro-) thread to a port. The thread can then opt to selectively receive
messages it is interested in. Other messages are not lost, but queued, and
can be received at a later time.

The C<Coro::MP> module is not part of L<AnyEvent::MP>, but a separate
module. It is, however, tightly integrated into C<AnyEvent::MP> - the
ports it creates are fully compatible to C<AnyEvent::MP> ports.

In fact, C<Coro::MP> is more of an extension than a separate module: all
functions exported by C<AnyEvent::MP> are exported by it as well.

To illustrate how programing with C<Coro::MP> looks like, consider the
following (slightly contrived) example: Let's implement a server that
accepts a C<< (write_file =>, $port, $path) >> message with a (source)
port and a filename, followed by as many C<< (data => $port, $data) >>
messages as required to fill the file, followed by an empty C<< (data =>
$port) >> message.

The server only writes a single file at a time, other requests will stay
in the queue until the current file has been finished.

Here is an example implementation that uses L<Coro::AIO> and largely
ignores error handling:

   my $ioserver = port_async {
      while () {
         my ($tag, $port, $path) = get_cond;

         $tag eq "write_file"
            or die "only write_file messages expected";

         my $fh = aio_open $path, O_WRONLY|O_CREAT, 0666
            or die "$path: $!";

         while () {
            my (undef, undef, $data) = get_cond {
               $_[0] eq "data" && $_[1] eq $port
            } 5
               or die "timeout waiting for data message from $port\n";

            length $data or last;

            aio_write $fh, undef, undef, $data, 0;
         };
      }
   };

   mon $ioserver, sub {
      warn "ioserver was killed: @_\n";
   }; 

Let's go through it part by part.

   my $ioserver = port_async {

Ports can be created by attaching a thread to an existing port via
C<rcv_async>, or as here by calling C<port_async> with the code to execute
as a thread. The C<async> component comes from the fact that threads are
created using the C<Coro::async> function.

The thread runs in a normal port context (so C<$SELF> is set). In
addition, when the thread returns, it will be C<kil> I<normally>, i.e.
without a reason argument.

      while () {
         my ($tag, $port, $path) = get_cond;
            or die "only write_file messages expected";

The thread is supposed to serve many file writes, which is why it executes
in a loop. The first thing it does is fetch the next message, using
C<get_cond>, the "conditional message get". Without a condition, it simply
fetches the next message from the queue, which I<must> be a C<write_file>
message.

The message contains the C<$path> to the file, which is then created:

         my $fh = aio_open $path, O_WRONLY|O_CREAT, 0666
            or die "$path: $!";

Then we enter a loop again, to serve as many C<data> messages as
necessary:

         while () {
            my (undef, undef, $data) = get_cond {
               $_[0] eq "data" && $_[1] eq $port
            } 5
               or die "timeout waiting for data message from $port\n";

This time, the condition is not empty, but instead a code block: similarly
to grep, the code block will be called with C<@_> set to each message in
the queue, and it has to return whether it wants to receive the message or
not.

In this case we are interested in C<data> messages (C<< $_[0] eq "data"
>>), whose first element is the source port (C<< $_[1] eq $port >>).

The condition must be this strict, as it is possible to receive both
C<write_file> messages and C<data> messages from other ports while we
handle the file writing.

The lone C<5> at the end is a timeout - when no matching message is
received within C<5> seconds, we assume an error and C<die>.

When an empty C<data> message is received we are done and can close the
file (which is done automatically as C<$fh> goes out of scope):

            length $data or last;

Otherwise we need to write the data:

            aio_write $fh, undef, undef, $data, 0;

That's basically it. Note that every process should have some kind of
supervisor. In our case, the supervisor simply prints any error message:

   mon $ioserver, sub {
      warn "ioserver was killed: @_\n";
   }; 

Here is a usage example:

   port_async {
      snd $ioserver, write_file => $SELF, "/tmp/unsafe";
      snd $ioserver, data => $SELF, "abc\n";
      snd $ioserver, data => $SELF, "def\n";
      snd $ioserver, data => $SELF;
   }; 

The messages are sent without any flow control or acknowledgement (feel
free to improve). Also, the source port does not actually need to be a
port - any unique ID will do - but port identifiers happen to be a simple
source of network-wide unique IDs.

Apart from C<get_cond> as seen above, there are other ways to receive
messages. The C<write_file> message above could also selectively be
received using a C<get> call:

   my ($port, $path) = get "write_file";

This is simpler, but when some other code part sends an unexpected message
to the C<$ioserver> it will stay in the queue forever. As a rule of thumb,
every threaded port should have a "fetch next message unconditionally"
somewhere, to avoid filling up the queue.

It is also possible to switch-like C<get_conds>:

  get_cond {
     $_[0] eq "msg1" and return sub {
        my (undef, @msg1_data) = @_;
        ...;
     };

     $_[0] eq "msg2" and return sub {
        my (undef, @msg2_data) = @_;
        ...;
     };

     die "unexpected message $_[0] received";
  };

=head1 THE END

This is the end of this introduction, but hopefully not the end of
your career as AEMP user. I hope the tutorial was enough to make the
basic concepts clear. Keep in mind that distributed programming is not
completely trivial, that AnyEvent::MP is still in it's infancy, and I hope
it will be useful to create exciting new applications.

=head1 SEE ALSO

L<AnyEvent::MP>

L<AnyEvent::MP::Global>

L<Coro::MP>

L<AnyEvent>

=head1 AUTHOR

  Robin Redeker <elmex@ta-sa.org>
  Marc Lehmann <schmorp@schmorp.de>

Revision:	1.46
Committed:	Sun Mar 4 18:48:27 2012 UTC (12 years, 2 months ago) by root
Branch:	MAIN
Changes since 1.45:	+17 -20 lines
Log Message:	* empty log message *