[ViewVC] Diff of: cvs/AnyEvent-MP/MP.pm

Comparing AnyEvent-MP/MP.pm (file contents):
Revision 1.1 by root, Thu Jul 30 08:38:50 2009 UTC vs.
Revision 1.63 by root, Thu Aug 27 21:29:37 2009 UTC

…		…
4		4
5	=head1 SYNOPSIS	5	=head1 SYNOPSIS
6		6
7	use AnyEvent::MP;	7	use AnyEvent::MP;
8		8
		9	$NODE # contains this node's noderef
		10	NODE # returns this node's noderef
		11	NODE $port # returns the noderef of the port
		12
		13	$SELF # receiving/own port id in rcv callbacks
		14
		15	# initialise the node so it can send/receive messages
		16	initialise_node;
		17
		18	# ports are message endpoints
		19
		20	# sending messages
		21	snd $port, type => data...;
		22	snd $port, @msg;
		23	snd @msg_with_first_element_being_a_port;
		24
		25	# creating/using ports, the simple way
		26	my $simple_port = port { my @msg = @_; 0 };
		27
		28	# creating/using ports, tagged message matching
		29	my $port = port;
		30	rcv $port, ping => sub { snd $_[0], "pong"; 0 };
		31	rcv $port, pong => sub { warn "pong received\n"; 0 };
		32
		33	# create a port on another node
		34	my $port = spawn $node, $initfunc, @initdata;
		35
		36	# monitoring
		37	mon $port, $cb->(@msg) # callback is invoked on death
		38	mon $port, $otherport # kill otherport on abnormal death
		39	mon $port, $otherport, @msg # send message on death
		40
		41	=head1 CURRENT STATUS
		42
		43	AnyEvent::MP - stable API, should work
		44	AnyEvent::MP::Intro - outdated
		45	AnyEvent::MP::Kernel - WIP
		46	AnyEvent::MP::Transport - mostly stable
		47
		48	stay tuned.
		49
9	=head1 DESCRIPTION	50	=head1 DESCRIPTION
10		51
		52	This module (-family) implements a simple message passing framework.
		53
		54	Despite its simplicity, you can securely message other processes running
		55	on the same or other hosts.
		56
		57	For an introduction to this module family, see the L<AnyEvent::MP::Intro>
		58	manual page.
		59
		60	At the moment, this module family is severly broken and underdocumented,
		61	so do not use. This was uploaded mainly to reserve the CPAN namespace -
		62	stay tuned!
		63
		64	=head1 CONCEPTS
		65
		66	=over 4
		67
		68	=item port
		69
		70	A port is something you can send messages to (with the C<snd> function).
		71
		72	Ports allow you to register C<rcv> handlers that can match all or just
		73	some messages. Messages will not be queued.
		74
		75	=item port ID - C<noderef#portname>
		76
		77	A port ID is the concatenation of a noderef, a hash-mark (C<#>) as
		78	separator, and a port name (a printable string of unspecified format). An
		79	exception is the the node port, whose ID is identical to its node
		80	reference.
		81
		82	=item node
		83
		84	A node is a single process containing at least one port - the node port,
		85	which provides nodes to manage each other remotely, and to create new
		86	ports.
		87
		88	Nodes are either private (single-process only), slaves (can only talk to
		89	public nodes, but do not need an open port) or public nodes (connectable
		90	from any other node).
		91
		92	=item node ID - C<[a-za-Z0-9_\-.:]+>
		93
		94	A node ID is a string that either simply identifies the node (for
		95	private and slave nodes), or contains a recipe on how to reach a given
		96	node (for public nodes).
		97
		98	This recipe is simply a comma-separated list of C<address:port> pairs (for
		99	TCP/IP, other protocols might look different).
		100
		101	Node references come in two flavours: resolved (containing only numerical
		102	addresses) or unresolved (where hostnames are used instead of addresses).
		103
		104	Before using an unresolved node reference in a message you first have to
		105	resolve it.
		106
		107	=back
		108
		109	=head1 VARIABLES/FUNCTIONS
		110
		111	=over 4
		112
11	=cut	113	=cut
12		114
13	package AnyEvent::MP;	115	package AnyEvent::MP;
14		116
		117	use AnyEvent::MP::Kernel;
		118
15	use common::sense;	119	use common::sense;
16		120
		121	use Carp ();
		122
17	use AE ();	123	use AE ();
18		124
19	our $VERSION = '0.0';	125	use base "Exporter";
20		126
21	sub nonce($) {	127	our $VERSION = $AnyEvent::MP::Kernel::VERSION;
22	my $nonce;
23		128
24	if (open my $fh, "</dev/urandom") {	129	our @EXPORT = qw(
25	sysread $fh, $nonce, $_[0];	130	NODE $NODE *SELF node_of after
26	} else {	131	resolve_node initialise_node
		132	snd rcv mon mon_guard kil reg psub spawn
		133	port
		134	);
		135
		136	our $SELF;
		137
		138	sub _self_die() {
		139	my $msg = $@;
		140	$msg =~ s/\n+$// unless ref $msg;
		141	kil $SELF, die => $msg;
		142	}
		143
		144	=item $thisnode = NODE / $NODE
		145
		146	The C<NODE> function returns, and the C<$NODE> variable contains the
		147	node id of the local node. The value is initialised by a call to
		148	C<initialise_node>.
		149
		150	=item $nodeid = node_of $port
		151
		152	Extracts and returns the noderef from a port ID or a node ID.
		153
		154	=item initialise_node $profile_name
		155
		156	Before a node can talk to other nodes on the network it has to initialise
		157	itself - the minimum a node needs to know is it's own name, and optionally
		158	it should know the noderefs of some other nodes in the network.
		159
		160	This function initialises a node - it must be called exactly once (or
		161	never) before calling other AnyEvent::MP functions.
		162
		163	All arguments (optionally except for the first) are noderefs, which can be
		164	either resolved or unresolved.
		165
		166	The first argument will be looked up in the configuration database first
		167	(if it is C<undef> then the current nodename will be used instead) to find
		168	the relevant configuration profile (see L<aemp>). If none is found then
		169	the default configuration is used. The configuration supplies additional
		170	seed/master nodes and can override the actual noderef.
		171
		172	There are two types of networked nodes, public nodes and slave nodes:
		173
		174	=over 4
		175
		176	=item public nodes
		177
		178	For public nodes, C<$noderef> (supplied either directly to
		179	C<initialise_node> or indirectly via a profile or the nodename) must be a
		180	noderef (possibly unresolved, in which case it will be resolved).
		181
		182	After resolving, the node will bind itself on all endpoints.
		183
		184	=item slave nodes
		185
		186	When the C<$noderef> (either as given or overriden by the config file)
		187	is the special string C<slave/>, then the node will become a slave
		188	node. Slave nodes cannot be contacted from outside, and cannot talk to
		189	each other (at least in this version of AnyEvent::MP).
		190
		191	Slave nodes work by creating connections to all public nodes, using the
		192	L<AnyEvent::MP::Global> service.
		193
		194	=back
		195
		196	After initialising itself, the node will connect to all additional
		197	C<$seednodes> that are specified diretcly or via a profile. Seednodes are
		198	optional and can be used to quickly bootstrap the node into an existing
		199	network.
		200
		201	All the seednodes will also be specially marked to automatically retry
		202	connecting to them indefinitely, so make sure that seednodes are really
		203	reliable and up (this might also change in the future).
		204
		205	Example: become a public node listening on the guessed noderef, or the one
		206	specified via C<aemp> for the current node. This should be the most common
		207	form of invocation for "daemon"-type nodes.
		208
		209	initialise_node;
		210
		211	Example: become a slave node to any of the the seednodes specified via
		212	C<aemp>. This form is often used for commandline clients.
		213
		214	initialise_node "slave/";
		215
		216	Example: become a public node, and try to contact some well-known master
		217	servers to become part of the network.
		218
		219	initialise_node undef, "master1", "master2";
		220
		221	Example: become a public node listening on port C<4041>.
		222
		223	initialise_node 4041;
		224
		225	Example: become a public node, only visible on localhost port 4044.
		226
		227	initialise_node "localhost:4044";
		228
		229	=item $cv = resolve_node $noderef
		230
		231	Takes an unresolved node reference that may contain hostnames and
		232	abbreviated IDs, resolves all of them and returns a resolved node
		233	reference.
		234
		235	In addition to C<address:port> pairs allowed in resolved noderefs, the
		236	following forms are supported:
		237
		238	=over 4
		239
		240	=item the empty string
		241
		242	An empty-string component gets resolved as if the default port (4040) was
		243	specified.
		244
		245	=item naked port numbers (e.g. C<1234>)
		246
		247	These are resolved by prepending the local nodename and a colon, to be
		248	further resolved.
		249
		250	=item hostnames (e.g. C<localhost:1234>, C<localhost>)
		251
		252	These are resolved by using AnyEvent::DNS to resolve them, optionally
		253	looking up SRV records for the C<aemp=4040> port, if no port was
		254	specified.
		255
		256	=back
		257
		258	=item $SELF
		259
		260	Contains the current port id while executing C<rcv> callbacks or C<psub>
		261	blocks.
		262
		263	=item SELF, %SELF, @SELF...
		264
		265	Due to some quirks in how perl exports variables, it is impossible to
		266	just export C<$SELF>, all the symbols called C<SELF> are exported by this
		267	module, but only C<$SELF> is currently used.
		268
		269	=item snd $port, type => @data
		270
		271	=item snd $port, @msg
		272
		273	Send the given message to the given port ID, which can identify either
		274	a local or a remote port, and must be a port ID.
		275
		276	While the message can be about anything, it is highly recommended to use a
		277	string as first element (a port ID, or some word that indicates a request
		278	type etc.).
		279
		280	The message data effectively becomes read-only after a call to this
		281	function: modifying any argument is not allowed and can cause many
		282	problems.
		283
		284	The type of data you can transfer depends on the transport protocol: when
		285	JSON is used, then only strings, numbers and arrays and hashes consisting
		286	of those are allowed (no objects). When Storable is used, then anything
		287	that Storable can serialise and deserialise is allowed, and for the local
		288	node, anything can be passed.
		289
		290	=item $local_port = port
		291
		292	Create a new local port object and returns its port ID. Initially it has
		293	no callbacks set and will throw an error when it receives messages.
		294
		295	=item $local_port = port { my @msg = @_ }
		296
		297	Creates a new local port, and returns its ID. Semantically the same as
		298	creating a port and calling C<rcv $port, $callback> on it.
		299
		300	The block will be called for every message received on the port, with the
		301	global variable C<$SELF> set to the port ID. Runtime errors will cause the
		302	port to be C<kil>ed. The message will be passed as-is, no extra argument
		303	(i.e. no port ID) will be passed to the callback.
		304
		305	If you want to stop/destroy the port, simply C<kil> it:
		306
		307	my $port = port {
		308	my @msg = @_;
27	# shit...	309	...
28	our $nonce_init;	310	kil $SELF;
29	unless ($nonce_init++) {
30	srand time ^ $$ ^ unpack "%L*", qx"ps -edalf" . qx"ipconfig /all";
31	}
32
33	$nonce = join "", map +(chr rand 256), 1 .. $_[0]
34	}	311	};
35		312
36	$nonce	313	=cut
37	}
38		314
39	our $DEFAULT_SECRET;	315	sub rcv($@);
40		316
41	sub default_secret {	317	sub _kilme {
42	unless (defined $DEFAULT_SECRET) {	318	die "received message on port without callback";
43	if (open my $fh, "<$ENV{HOME}/.aemp-secret") {	319	}
44	sysread $fh, $DEFAULT_SECRET, -s $fh;	320
		321	sub port(;&) {
		322	my $id = "$UNIQ." . $ID++;
		323	my $port = "$NODE#$id";
		324
		325	rcv $port, shift \|\| \&_kilme;
		326
		327	$port
		328	}
		329
		330	=item rcv $local_port, $callback->(@msg)
		331
		332	Replaces the default callback on the specified port. There is no way to
		333	remove the default callback: use C<sub { }> to disable it, or better
		334	C<kil> the port when it is no longer needed.
		335
		336	The global C<$SELF> (exported by this module) contains C<$port> while
		337	executing the callback. Runtime errors during callback execution will
		338	result in the port being C<kil>ed.
		339
		340	The default callback received all messages not matched by a more specific
		341	C<tag> match.
		342
		343	=item rcv $local_port, tag => $callback->(@msg_without_tag), ...
		344
		345	Register (or replace) callbacks to be called on messages starting with the
		346	given tag on the given port (and return the port), or unregister it (when
		347	C<$callback> is C<$undef> or missing). There can only be one callback
		348	registered for each tag.
		349
		350	The original message will be passed to the callback, after the first
		351	element (the tag) has been removed. The callback will use the same
		352	environment as the default callback (see above).
		353
		354	Example: create a port and bind receivers on it in one go.
		355
		356	my $port = rcv port,
		357	msg1 => sub { ... },
		358	msg2 => sub { ... },
		359	;
		360
		361	Example: create a port, bind receivers and send it in a message elsewhere
		362	in one go:
		363
		364	snd $otherport, reply =>
		365	rcv port,
		366	msg1 => sub { ... },
		367	...
		368	;
		369
		370	Example: temporarily register a rcv callback for a tag matching some port
		371	(e.g. for a rpc reply) and unregister it after a message was received.
		372
		373	rcv $port, $otherport => sub {
		374	my @reply = @_;
		375
		376	rcv $SELF, $otherport;
		377	};
		378
		379	=cut
		380
		381	sub rcv($@) {
		382	my $port = shift;
		383	my ($noderef, $portid) = split /#/, $port, 2;
		384
		385	$NODE{$noderef} == $NODE{""}
		386	or Carp::croak "$port: rcv can only be called on local ports, caught";
		387
		388	while (@_) {
		389	if (ref $_[0]) {
		390	if (my $self = $PORT_DATA{$portid}) {
		391	"AnyEvent::MP::Port" eq ref $self
		392	or Carp::croak "$port: rcv can only be called on message matching ports, caught";
		393
		394	$self->[2] = shift;
45	} else {	395	} else {
46	$DEFAULT_SECRET = nonce 32;	396	my $cb = shift;
		397	$PORT{$portid} = sub {
		398	local $SELF = $port;
		399	eval { &$cb }; _self_die if $@;
		400	};
		401	}
		402	} elsif (defined $_[0]) {
		403	my $self = $PORT_DATA{$portid} \|\|= do {
		404	my $self = bless [$PORT{$port} \|\| sub { }, { }, $port], "AnyEvent::MP::Port";
		405
		406	$PORT{$portid} = sub {
		407	local $SELF = $port;
		408
		409	if (my $cb = $self->[1]{$_[0]}) {
		410	shift;
		411	eval { &$cb }; _self_die if $@;
		412	} else {
		413	&{ $self->[0] };
		414	}
		415	};
		416
		417	$self
		418	};
		419
		420	"AnyEvent::MP::Port" eq ref $self
		421	or Carp::croak "$port: rcv can only be called on message matching ports, caught";
		422
		423	my ($tag, $cb) = splice @_, 0, 2;
		424
		425	if (defined $cb) {
		426	$self->[1]{$tag} = $cb;
		427	} else {
		428	delete $self->[1]{$tag};
		429	}
47	}	430	}
48	}	431	}
49		432
50	$DEFAULT_SECRET	433	$port
51	}	434	}
		435
		436	=item $closure = psub { BLOCK }
		437
		438	Remembers C<$SELF> and creates a closure out of the BLOCK. When the
		439	closure is executed, sets up the environment in the same way as in C<rcv>
		440	callbacks, i.e. runtime errors will cause the port to get C<kil>ed.
		441
		442	This is useful when you register callbacks from C<rcv> callbacks:
		443
		444	rcv delayed_reply => sub {
		445	my ($delay, @reply) = @_;
		446	my $timer = AE::timer $delay, 0, psub {
		447	snd @reply, $SELF;
		448	};
		449	};
		450
		451	=cut
		452
		453	sub psub(&) {
		454	my $cb = shift;
		455
		456	my $port = $SELF
		457	or Carp::croak "psub can only be called from within rcv or psub callbacks, not";
		458
		459	sub {
		460	local $SELF = $port;
		461
		462	if (wantarray) {
		463	my @res = eval { &$cb };
		464	_self_die if $@;
		465	@res
		466	} else {
		467	my $res = eval { &$cb };
		468	_self_die if $@;
		469	$res
		470	}
		471	}
		472	}
		473
		474	=item $guard = mon $port, $cb->(@reason)
		475
		476	=item $guard = mon $port, $rcvport
		477
		478	=item $guard = mon $port
		479
		480	=item $guard = mon $port, $rcvport, @msg
		481
		482	Monitor the given port and do something when the port is killed or
		483	messages to it were lost, and optionally return a guard that can be used
		484	to stop monitoring again.
		485
		486	C<mon> effectively guarantees that, in the absence of hardware failures,
		487	that after starting the monitor, either all messages sent to the port
		488	will arrive, or the monitoring action will be invoked after possible
		489	message loss has been detected. No messages will be lost "in between"
		490	(after the first lost message no further messages will be received by the
		491	port). After the monitoring action was invoked, further messages might get
		492	delivered again.
		493
		494	Note that monitoring-actions are one-shot: once released, they are removed
		495	and will not trigger again.
		496
		497	In the first form (callback), the callback is simply called with any
		498	number of C<@reason> elements (no @reason means that the port was deleted
		499	"normally"). Note also that I<< the callback B<must> never die >>, so use
		500	C<eval> if unsure.
		501
		502	In the second form (another port given), the other port (C<$rcvport>)
		503	will be C<kil>'ed with C<@reason>, iff a @reason was specified, i.e. on
		504	"normal" kils nothing happens, while under all other conditions, the other
		505	port is killed with the same reason.
		506
		507	The third form (kill self) is the same as the second form, except that
		508	C<$rvport> defaults to C<$SELF>.
		509
		510	In the last form (message), a message of the form C<@msg, @reason> will be
		511	C<snd>.
		512
		513	As a rule of thumb, monitoring requests should always monitor a port from
		514	a local port (or callback). The reason is that kill messages might get
		515	lost, just like any other message. Another less obvious reason is that
		516	even monitoring requests can get lost (for exmaple, when the connection
		517	to the other node goes down permanently). When monitoring a port locally
		518	these problems do not exist.
		519
		520	Example: call a given callback when C<$port> is killed.
		521
		522	mon $port, sub { warn "port died because of <@_>\n" };
		523
		524	Example: kill ourselves when C<$port> is killed abnormally.
		525
		526	mon $port;
		527
		528	Example: send us a restart message when another C<$port> is killed.
		529
		530	mon $port, $self => "restart";
		531
		532	=cut
		533
		534	sub mon {
		535	my ($noderef, $port) = split /#/, shift, 2;
		536
		537	my $node = $NODE{$noderef} \|\| add_node $noderef;
		538
		539	my $cb = @_ ? shift : $SELF \|\| Carp::croak 'mon: called with one argument only, but $SELF not set,';
		540
		541	unless (ref $cb) {
		542	if (@_) {
		543	# send a kill info message
		544	my (@msg) = ($cb, @_);
		545	$cb = sub { snd @msg, @_ };
		546	} else {
		547	# simply kill other port
		548	my $port = $cb;
		549	$cb = sub { kil $port, @_ if @_ };
		550	}
		551	}
		552
		553	$node->monitor ($port, $cb);
		554
		555	defined wantarray
		556	and AnyEvent::Util::guard { $node->unmonitor ($port, $cb) }
		557	}
		558
		559	=item $guard = mon_guard $port, $ref, $ref...
		560
		561	Monitors the given C<$port> and keeps the passed references. When the port
		562	is killed, the references will be freed.
		563
		564	Optionally returns a guard that will stop the monitoring.
		565
		566	This function is useful when you create e.g. timers or other watchers and
		567	want to free them when the port gets killed:
		568
		569	$port->rcv (start => sub {
		570	my $timer; $timer = mon_guard $port, AE::timer 1, 1, sub {
		571	undef $timer if 0.9 < rand;
		572	});
		573	});
		574
		575	=cut
		576
		577	sub mon_guard {
		578	my ($port, @refs) = @_;
		579
		580	#TODO: mon-less form?
		581
		582	mon $port, sub { 0 && @refs }
		583	}
		584
		585	=item kil $port[, @reason]
		586
		587	Kill the specified port with the given C<@reason>.
		588
		589	If no C<@reason> is specified, then the port is killed "normally" (linked
		590	ports will not be kileld, or even notified).
		591
		592	Otherwise, linked ports get killed with the same reason (second form of
		593	C<mon>, see below).
		594
		595	Runtime errors while evaluating C<rcv> callbacks or inside C<psub> blocks
		596	will be reported as reason C<< die => $@ >>.
		597
		598	Transport/communication errors are reported as C<< transport_error =>
		599	$message >>.
		600
		601	=cut
		602
		603	=item $port = spawn $node, $initfunc[, @initdata]
		604
		605	Creates a port on the node C<$node> (which can also be a port ID, in which
		606	case it's the node where that port resides).
		607
		608	The port ID of the newly created port is return immediately, and it is
		609	permissible to immediately start sending messages or monitor the port.
		610
		611	After the port has been created, the init function is
		612	called. This function must be a fully-qualified function name
		613	(e.g. C<MyApp::Chat::Server::init>). To specify a function in the main
		614	program, use C<::name>.
		615
		616	If the function doesn't exist, then the node tries to C<require>
		617	the package, then the package above the package and so on (e.g.
		618	C<MyApp::Chat::Server>, C<MyApp::Chat>, C<MyApp>) until the function
		619	exists or it runs out of package names.
		620
		621	The init function is then called with the newly-created port as context
		622	object (C<$SELF>) and the C<@initdata> values as arguments.
		623
		624	A common idiom is to pass your own port, monitor the spawned port, and
		625	in the init function, monitor the original port. This two-way monitoring
		626	ensures that both ports get cleaned up when there is a problem.
		627
		628	Example: spawn a chat server port on C<$othernode>.
		629
		630	# this node, executed from within a port context:
		631	my $server = spawn $othernode, "MyApp::Chat::Server::connect", $SELF;
		632	mon $server;
		633
		634	# init function on C<$othernode>
		635	sub connect {
		636	my ($srcport) = @_;
		637
		638	mon $srcport;
		639
		640	rcv $SELF, sub {
		641	...
		642	};
		643	}
		644
		645	=cut
		646
		647	sub _spawn {
		648	my $port = shift;
		649	my $init = shift;
		650
		651	local $SELF = "$NODE#$port";
		652	eval {
		653	&{ load_func $init }
		654	};
		655	_self_die if $@;
		656	}
		657
		658	sub spawn(@) {
		659	my ($noderef, undef) = split /#/, shift, 2;
		660
		661	my $id = "$RUNIQ." . $ID++;
		662
		663	$_[0] =~ /::/
		664	or Carp::croak "spawn init function must be a fully-qualified name, caught";
		665
		666	snd_to_func $noderef, "AnyEvent::MP::_spawn" => $id, @_;
		667
		668	"$noderef#$id"
		669	}
		670
		671	=item after $timeout, @msg
		672
		673	=item after $timeout, $callback
		674
		675	Either sends the given message, or call the given callback, after the
		676	specified number of seconds.
		677
		678	This is simply a utility function that come sin handy at times.
		679
		680	=cut
		681
		682	sub after($@) {
		683	my ($timeout, @action) = @_;
		684
		685	my $t; $t = AE::timer $timeout, 0, sub {
		686	undef $t;
		687	ref $action[0]
		688	? $action[0]()
		689	: snd @action;
		690	};
		691	}
		692
		693	=back
		694
		695	=head1 AnyEvent::MP vs. Distributed Erlang
		696
		697	AnyEvent::MP got lots of its ideas from distributed Erlang (Erlang node
		698	== aemp node, Erlang process == aemp port), so many of the documents and
		699	programming techniques employed by Erlang apply to AnyEvent::MP. Here is a
		700	sample:
		701
		702	http://www.Erlang.se/doc/programming_rules.shtml
		703	http://Erlang.org/doc/getting_started/part_frame.html # chapters 3 and 4
		704	http://Erlang.org/download/Erlang-book-part1.pdf # chapters 5 and 6
		705	http://Erlang.org/download/armstrong_thesis_2003.pdf # chapters 4 and 5
		706
		707	Despite the similarities, there are also some important differences:
		708
		709	=over 4
		710
		711	=item * Node references contain the recipe on how to contact them.
		712
		713	Erlang relies on special naming and DNS to work everywhere in the
		714	same way. AEMP relies on each node knowing it's own address(es), with
		715	convenience functionality.
		716
		717	This means that AEMP requires a less tightly controlled environment at the
		718	cost of longer node references and a slightly higher management overhead.
		719
		720	=item * Erlang has a "remote ports are like local ports" philosophy, AEMP
		721	uses "local ports are like remote ports".
		722
		723	The failure modes for local ports are quite different (runtime errors
		724	only) then for remote ports - when a local port dies, you I<know> it dies,
		725	when a connection to another node dies, you know nothing about the other
		726	port.
		727
		728	Erlang pretends remote ports are as reliable as local ports, even when
		729	they are not.
		730
		731	AEMP encourages a "treat remote ports differently" philosophy, with local
		732	ports being the special case/exception, where transport errors cannot
		733	occur.
		734
		735	=item * Erlang uses processes and a mailbox, AEMP does not queue.
		736
		737	Erlang uses processes that selectively receive messages, and therefore
		738	needs a queue. AEMP is event based, queuing messages would serve no
		739	useful purpose. For the same reason the pattern-matching abilities of
		740	AnyEvent::MP are more limited, as there is little need to be able to
		741	filter messages without dequeing them.
		742
		743	(But see L<Coro::MP> for a more Erlang-like process model on top of AEMP).
		744
		745	=item * Erlang sends are synchronous, AEMP sends are asynchronous.
		746
		747	Sending messages in Erlang is synchronous and blocks the process (and
		748	so does not need a queue that can overflow). AEMP sends are immediate,
		749	connection establishment is handled in the background.
		750
		751	=item * Erlang suffers from silent message loss, AEMP does not.
		752
		753	Erlang makes few guarantees on messages delivery - messages can get lost
		754	without any of the processes realising it (i.e. you send messages a, b,
		755	and c, and the other side only receives messages a and c).
		756
		757	AEMP guarantees correct ordering, and the guarantee that there are no
		758	holes in the message sequence.
		759
		760	=item * In Erlang, processes can be declared dead and later be found to be
		761	alive.
		762
		763	In Erlang it can happen that a monitored process is declared dead and
		764	linked processes get killed, but later it turns out that the process is
		765	still alive - and can receive messages.
		766
		767	In AEMP, when port monitoring detects a port as dead, then that port will
		768	eventually be killed - it cannot happen that a node detects a port as dead
		769	and then later sends messages to it, finding it is still alive.
		770
		771	=item * Erlang can send messages to the wrong port, AEMP does not.
		772
		773	In Erlang it is quite likely that a node that restarts reuses a process ID
		774	known to other nodes for a completely different process, causing messages
		775	destined for that process to end up in an unrelated process.
		776
		777	AEMP never reuses port IDs, so old messages or old port IDs floating
		778	around in the network will not be sent to an unrelated port.
		779
		780	=item * Erlang uses unprotected connections, AEMP uses secure
		781	authentication and can use TLS.
		782
		783	AEMP can use a proven protocol - SSL/TLS - to protect connections and
		784	securely authenticate nodes.
		785
		786	=item * The AEMP protocol is optimised for both text-based and binary
		787	communications.
		788
		789	The AEMP protocol, unlike the Erlang protocol, supports both
		790	language-independent text-only protocols (good for debugging) and binary,
		791	language-specific serialisers (e.g. Storable).
		792
		793	It has also been carefully designed to be implementable in other languages
		794	with a minimum of work while gracefully degrading fucntionality to make the
		795	protocol simple.
		796
		797	=item * AEMP has more flexible monitoring options than Erlang.
		798
		799	In Erlang, you can chose to receive I<all> exit signals as messages
		800	or I<none>, there is no in-between, so monitoring single processes is
		801	difficult to implement. Monitoring in AEMP is more flexible than in
		802	Erlang, as one can choose between automatic kill, exit message or callback
		803	on a per-process basis.
		804
		805	=item * Erlang tries to hide remote/local connections, AEMP does not.
		806
		807	Monitoring in Erlang is not an indicator of process death/crashes,
		808	as linking is (except linking is unreliable in Erlang).
		809
		810	In AEMP, you don't "look up" registered port names or send to named ports
		811	that might or might not be persistent. Instead, you normally spawn a port
		812	on the remote node. The init function monitors the you, and you monitor
		813	the remote port. Since both monitors are local to the node, they are much
		814	more reliable.
		815
		816	This also saves round-trips and avoids sending messages to the wrong port
		817	(hard to do in Erlang).
		818
		819	=back
		820
		821	=head1 RATIONALE
		822
		823	=over 4
		824
		825	=item Why strings for ports and noderefs, why not objects?
		826
		827	We considered "objects", but found that the actual number of methods
		828	thatc an be called are very low. Since port IDs and noderefs travel over
		829	the network frequently, the serialising/deserialising would add lots of
		830	overhead, as well as having to keep a proxy object.
		831
		832	Strings can easily be printed, easily serialised etc. and need no special
		833	procedures to be "valid".
		834
		835	And a a miniport consists of a single closure stored in a global hash - it
		836	can't become much cheaper.
		837
		838	=item Why favour JSON, why not real serialising format such as Storable?
		839
		840	In fact, any AnyEvent::MP node will happily accept Storable as framing
		841	format, but currently there is no way to make a node use Storable by
		842	default.
		843
		844	The default framing protocol is JSON because a) JSON::XS is many times
		845	faster for small messages and b) most importantly, after years of
		846	experience we found that object serialisation is causing more problems
		847	than it gains: Just like function calls, objects simply do not travel
		848	easily over the network, mostly because they will always be a copy, so you
		849	always have to re-think your design.
		850
		851	Keeping your messages simple, concentrating on data structures rather than
		852	objects, will keep your messages clean, tidy and efficient.
		853
		854	=back
52		855
53	=head1 SEE ALSO	856	=head1 SEE ALSO
54		857
55	L<AnyEvent>.	858	L<AnyEvent>.
56		859

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Revision 1.63 by root, Thu Aug 27 21:29:37 2009 UTC