[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 1.64 by root, Fri Aug 28 00:58:44 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		44	AnyEvent::MP::Intro - outdated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 send to ports will not be queued, regardless of
		74	anything was listening for them or not.
		75
		76	=item port ID - C<noderef#portname>
		77
		78	A port ID is the concatenation of a noderef, a hash-mark (C<#>) as
		79	separator, and a port name (a printable string of unspecified format). An
		80	exception is the the node port, whose ID is identical to its node
		81	reference.
		82
		83	=item node
		84
		85	A node is a single process containing at least one port - the node port,
		86	which provides nodes to manage each other remotely, and to create new
		87	ports.
		88
		89	Nodes are either private (single-process only), slaves (can only talk to
		90	public nodes, but do not need an open port) or public nodes (connectable
		91	from any other node).
		92
		93	=item node ID - C<[a-za-Z0-9_\-.:]+>
		94
		95	A node ID is a string that uniquely identifies the node within a
		96	network. Depending on the configuration used, node IDs can look like a
		97	hostname, a hostname and a port, or a random string. AnyEvent::MP itself
		98	doesn't interpret node IDs in any way.
		99
		100	=item binds - C<ip:port>
		101
		102	Nodes can only talk to each other by creating some kind of connection to
		103	each other. To do this, nodes should listen on one or more local transport
		104	endpoints - binds. Currently, only standard C<ip:port> specifications can
		105	be used, which specify TCP ports to listen on.
		106
		107	=item seeds - C<host:port>
		108
		109	When a node starts, it knows nothing about the network. To teach the node
		110	about the network it first has to contact some other node within the
		111	network. This node is called a seed.
		112
		113	Seeds are transport endpoint(s) of as many nodes as one wants. Those nodes
		114	are expected to be long-running, and at least one of those should always
		115	be available. When nodes run out of connections (e.g. due to a network
		116	error), they try to re-establish connections to some seednodes again to
		117	join the network.
		118
		119	=back
		120
		121	=head1 VARIABLES/FUNCTIONS
		122
		123	=over 4
		124
11	=cut	125	=cut
12		126
13	package AnyEvent::MP;	127	package AnyEvent::MP;
14		128
		129	use AnyEvent::MP::Kernel;
		130
15	use common::sense;	131	use common::sense;
16		132
		133	use Carp ();
		134
17	use AE ();	135	use AE ();
18		136
19	our $VERSION = '0.0';	137	use base "Exporter";
20		138
21	sub nonce($) {	139	our $VERSION = $AnyEvent::MP::Kernel::VERSION;
22	my $nonce;
23		140
24	if (open my $fh, "</dev/urandom") {	141	our @EXPORT = qw(
25	sysread $fh, $nonce, $_[0];	142	NODE $NODE *SELF node_of after
26	} else {	143	resolve_node initialise_node
		144	snd rcv mon mon_guard kil reg psub spawn
		145	port
		146	);
		147
		148	our $SELF;
		149
		150	sub _self_die() {
		151	my $msg = $@;
		152	$msg =~ s/\n+$// unless ref $msg;
		153	kil $SELF, die => $msg;
		154	}
		155
		156	=item $thisnode = NODE / $NODE
		157
		158	The C<NODE> function returns, and the C<$NODE> variable contains the node
		159	ID of the node running in the current process. This value is initialised by
		160	a call to C<initialise_node>.
		161
		162	=item $nodeid = node_of $port
		163
		164	Extracts and returns the node ID part from a port ID or a node ID.
		165
		166	=item initialise_node $profile_name
		167
		168	Before a node can talk to other nodes on the network (i.e. enter
		169	"distributed mode") it has to initialise itself - the minimum a node needs
		170	to know is its own name, and optionally it should know the addresses of
		171	some other nodes in the network to discover other nodes.
		172
		173	This function initialises a node - it must be called exactly once (or
		174	never) before calling other AnyEvent::MP functions.
		175
		176	The first argument is a profile name. If it is C<undef> or missing, then
		177	the current nodename will be used instead (i.e. F<uname -n>).
		178
		179	The function then looks up the profile in the aemp configuration (see the
		180	L<aemp> commandline utility).
		181
		182	If the profile specifies a node ID, then this will become the node ID of
		183	this process. If not, then the profile name will be used as node ID. The
		184	special node ID of C<anon/> will be replaced by a random node ID.
		185
		186	The next step is to look up the binds in the profile, followed by binding
		187	aemp protocol listeners on all binds specified (it is possible and valid
		188	to have no binds, meaning that the node cannot be contacted form the
		189	outside. This means the node cannot talk to other nodes that also have no
		190	binds, but it can still talk to all "normal" nodes).
		191
		192	If the profile does not specify a binds list, then the node ID will be
		193	treated as if it were of the form C<host:port>, which will be resolved and
		194	used as binds list.
		195
		196	Lastly, the seeds list from the profile is passed to the
		197	L<AnyEvent::MP::Global> module, which will then use it to keep
		198	connectivity with at least on of those seed nodes at any point in time.
		199
		200	Example: become a distributed node listening on the guessed noderef, or
		201	the one specified via C<aemp> for the current node. This should be the
		202	most common form of invocation for "daemon"-type nodes.
		203
		204	initialise_node;
		205
		206	Example: become an anonymous node. This form is often used for commandline
		207	clients.
		208
		209	initialise_node "anon/";
		210
		211	Example: become a distributed node. If there is no profile of the given
		212	name, or no binds list was specified, resolve C<localhost:4044> and bind
		213	on the resulting addresses.
		214
		215	initialise_node "localhost:4044";
		216
		217	=item $SELF
		218
		219	Contains the current port id while executing C<rcv> callbacks or C<psub>
		220	blocks.
		221
		222	=item SELF, %SELF, @SELF...
		223
		224	Due to some quirks in how perl exports variables, it is impossible to
		225	just export C<$SELF>, all the symbols called C<SELF> are exported by this
		226	module, but only C<$SELF> is currently used.
		227
		228	=item snd $port, type => @data
		229
		230	=item snd $port, @msg
		231
		232	Send the given message to the given port ID, which can identify either
		233	a local or a remote port, and must be a port ID.
		234
		235	While the message can be about anything, it is highly recommended to use a
		236	string as first element (a port ID, or some word that indicates a request
		237	type etc.).
		238
		239	The message data effectively becomes read-only after a call to this
		240	function: modifying any argument is not allowed and can cause many
		241	problems.
		242
		243	The type of data you can transfer depends on the transport protocol: when
		244	JSON is used, then only strings, numbers and arrays and hashes consisting
		245	of those are allowed (no objects). When Storable is used, then anything
		246	that Storable can serialise and deserialise is allowed, and for the local
		247	node, anything can be passed.
		248
		249	=item $local_port = port
		250
		251	Create a new local port object and returns its port ID. Initially it has
		252	no callbacks set and will throw an error when it receives messages.
		253
		254	=item $local_port = port { my @msg = @_ }
		255
		256	Creates a new local port, and returns its ID. Semantically the same as
		257	creating a port and calling C<rcv $port, $callback> on it.
		258
		259	The block will be called for every message received on the port, with the
		260	global variable C<$SELF> set to the port ID. Runtime errors will cause the
		261	port to be C<kil>ed. The message will be passed as-is, no extra argument
		262	(i.e. no port ID) will be passed to the callback.
		263
		264	If you want to stop/destroy the port, simply C<kil> it:
		265
		266	my $port = port {
		267	my @msg = @_;
27	# shit...	268	...
28	our $nonce_init;	269	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	}	270	};
35		271
36	$nonce	272	=cut
37	}
38		273
39	our $DEFAULT_SECRET;	274	sub rcv($@);
40		275
41	sub default_secret {	276	sub _kilme {
42	unless (defined $DEFAULT_SECRET) {	277	die "received message on port without callback";
43	if (open my $fh, "<$ENV{HOME}/.aemp-secret") {	278	}
44	sysread $fh, $DEFAULT_SECRET, -s $fh;	279
		280	sub port(;&) {
		281	my $id = "$UNIQ." . $ID++;
		282	my $port = "$NODE#$id";
		283
		284	rcv $port, shift \|\| \&_kilme;
		285
		286	$port
		287	}
		288
		289	=item rcv $local_port, $callback->(@msg)
		290
		291	Replaces the default callback on the specified port. There is no way to
		292	remove the default callback: use C<sub { }> to disable it, or better
		293	C<kil> the port when it is no longer needed.
		294
		295	The global C<$SELF> (exported by this module) contains C<$port> while
		296	executing the callback. Runtime errors during callback execution will
		297	result in the port being C<kil>ed.
		298
		299	The default callback received all messages not matched by a more specific
		300	C<tag> match.
		301
		302	=item rcv $local_port, tag => $callback->(@msg_without_tag), ...
		303
		304	Register (or replace) callbacks to be called on messages starting with the
		305	given tag on the given port (and return the port), or unregister it (when
		306	C<$callback> is C<$undef> or missing). There can only be one callback
		307	registered for each tag.
		308
		309	The original message will be passed to the callback, after the first
		310	element (the tag) has been removed. The callback will use the same
		311	environment as the default callback (see above).
		312
		313	Example: create a port and bind receivers on it in one go.
		314
		315	my $port = rcv port,
		316	msg1 => sub { ... },
		317	msg2 => sub { ... },
		318	;
		319
		320	Example: create a port, bind receivers and send it in a message elsewhere
		321	in one go:
		322
		323	snd $otherport, reply =>
		324	rcv port,
		325	msg1 => sub { ... },
		326	...
		327	;
		328
		329	Example: temporarily register a rcv callback for a tag matching some port
		330	(e.g. for a rpc reply) and unregister it after a message was received.
		331
		332	rcv $port, $otherport => sub {
		333	my @reply = @_;
		334
		335	rcv $SELF, $otherport;
		336	};
		337
		338	=cut
		339
		340	sub rcv($@) {
		341	my $port = shift;
		342	my ($noderef, $portid) = split /#/, $port, 2;
		343
		344	$NODE{$noderef} == $NODE{""}
		345	or Carp::croak "$port: rcv can only be called on local ports, caught";
		346
		347	while (@_) {
		348	if (ref $_[0]) {
		349	if (my $self = $PORT_DATA{$portid}) {
		350	"AnyEvent::MP::Port" eq ref $self
		351	or Carp::croak "$port: rcv can only be called on message matching ports, caught";
		352
		353	$self->[2] = shift;
45	} else {	354	} else {
46	$DEFAULT_SECRET = nonce 32;	355	my $cb = shift;
		356	$PORT{$portid} = sub {
		357	local $SELF = $port;
		358	eval { &$cb }; _self_die if $@;
		359	};
		360	}
		361	} elsif (defined $_[0]) {
		362	my $self = $PORT_DATA{$portid} \|\|= do {
		363	my $self = bless [$PORT{$port} \|\| sub { }, { }, $port], "AnyEvent::MP::Port";
		364
		365	$PORT{$portid} = sub {
		366	local $SELF = $port;
		367
		368	if (my $cb = $self->[1]{$_[0]}) {
		369	shift;
		370	eval { &$cb }; _self_die if $@;
		371	} else {
		372	&{ $self->[0] };
		373	}
		374	};
		375
		376	$self
		377	};
		378
		379	"AnyEvent::MP::Port" eq ref $self
		380	or Carp::croak "$port: rcv can only be called on message matching ports, caught";
		381
		382	my ($tag, $cb) = splice @_, 0, 2;
		383
		384	if (defined $cb) {
		385	$self->[1]{$tag} = $cb;
		386	} else {
		387	delete $self->[1]{$tag};
		388	}
47	}	389	}
48	}	390	}
49		391
50	$DEFAULT_SECRET	392	$port
51	}	393	}
		394
		395	=item $closure = psub { BLOCK }
		396
		397	Remembers C<$SELF> and creates a closure out of the BLOCK. When the
		398	closure is executed, sets up the environment in the same way as in C<rcv>
		399	callbacks, i.e. runtime errors will cause the port to get C<kil>ed.
		400
		401	This is useful when you register callbacks from C<rcv> callbacks:
		402
		403	rcv delayed_reply => sub {
		404	my ($delay, @reply) = @_;
		405	my $timer = AE::timer $delay, 0, psub {
		406	snd @reply, $SELF;
		407	};
		408	};
		409
		410	=cut
		411
		412	sub psub(&) {
		413	my $cb = shift;
		414
		415	my $port = $SELF
		416	or Carp::croak "psub can only be called from within rcv or psub callbacks, not";
		417
		418	sub {
		419	local $SELF = $port;
		420
		421	if (wantarray) {
		422	my @res = eval { &$cb };
		423	_self_die if $@;
		424	@res
		425	} else {
		426	my $res = eval { &$cb };
		427	_self_die if $@;
		428	$res
		429	}
		430	}
		431	}
		432
		433	=item $guard = mon $port, $cb->(@reason)
		434
		435	=item $guard = mon $port, $rcvport
		436
		437	=item $guard = mon $port
		438
		439	=item $guard = mon $port, $rcvport, @msg
		440
		441	Monitor the given port and do something when the port is killed or
		442	messages to it were lost, and optionally return a guard that can be used
		443	to stop monitoring again.
		444
		445	C<mon> effectively guarantees that, in the absence of hardware failures,
		446	that after starting the monitor, either all messages sent to the port
		447	will arrive, or the monitoring action will be invoked after possible
		448	message loss has been detected. No messages will be lost "in between"
		449	(after the first lost message no further messages will be received by the
		450	port). After the monitoring action was invoked, further messages might get
		451	delivered again.
		452
		453	Note that monitoring-actions are one-shot: once released, they are removed
		454	and will not trigger again.
		455
		456	In the first form (callback), the callback is simply called with any
		457	number of C<@reason> elements (no @reason means that the port was deleted
		458	"normally"). Note also that I<< the callback B<must> never die >>, so use
		459	C<eval> if unsure.
		460
		461	In the second form (another port given), the other port (C<$rcvport>)
		462	will be C<kil>'ed with C<@reason>, iff a @reason was specified, i.e. on
		463	"normal" kils nothing happens, while under all other conditions, the other
		464	port is killed with the same reason.
		465
		466	The third form (kill self) is the same as the second form, except that
		467	C<$rvport> defaults to C<$SELF>.
		468
		469	In the last form (message), a message of the form C<@msg, @reason> will be
		470	C<snd>.
		471
		472	As a rule of thumb, monitoring requests should always monitor a port from
		473	a local port (or callback). The reason is that kill messages might get
		474	lost, just like any other message. Another less obvious reason is that
		475	even monitoring requests can get lost (for exmaple, when the connection
		476	to the other node goes down permanently). When monitoring a port locally
		477	these problems do not exist.
		478
		479	Example: call a given callback when C<$port> is killed.
		480
		481	mon $port, sub { warn "port died because of <@_>\n" };
		482
		483	Example: kill ourselves when C<$port> is killed abnormally.
		484
		485	mon $port;
		486
		487	Example: send us a restart message when another C<$port> is killed.
		488
		489	mon $port, $self => "restart";
		490
		491	=cut
		492
		493	sub mon {
		494	my ($noderef, $port) = split /#/, shift, 2;
		495
		496	my $node = $NODE{$noderef} \|\| add_node $noderef;
		497
		498	my $cb = @_ ? shift : $SELF \|\| Carp::croak 'mon: called with one argument only, but $SELF not set,';
		499
		500	unless (ref $cb) {
		501	if (@_) {
		502	# send a kill info message
		503	my (@msg) = ($cb, @_);
		504	$cb = sub { snd @msg, @_ };
		505	} else {
		506	# simply kill other port
		507	my $port = $cb;
		508	$cb = sub { kil $port, @_ if @_ };
		509	}
		510	}
		511
		512	$node->monitor ($port, $cb);
		513
		514	defined wantarray
		515	and AnyEvent::Util::guard { $node->unmonitor ($port, $cb) }
		516	}
		517
		518	=item $guard = mon_guard $port, $ref, $ref...
		519
		520	Monitors the given C<$port> and keeps the passed references. When the port
		521	is killed, the references will be freed.
		522
		523	Optionally returns a guard that will stop the monitoring.
		524
		525	This function is useful when you create e.g. timers or other watchers and
		526	want to free them when the port gets killed:
		527
		528	$port->rcv (start => sub {
		529	my $timer; $timer = mon_guard $port, AE::timer 1, 1, sub {
		530	undef $timer if 0.9 < rand;
		531	});
		532	});
		533
		534	=cut
		535
		536	sub mon_guard {
		537	my ($port, @refs) = @_;
		538
		539	#TODO: mon-less form?
		540
		541	mon $port, sub { 0 && @refs }
		542	}
		543
		544	=item kil $port[, @reason]
		545
		546	Kill the specified port with the given C<@reason>.
		547
		548	If no C<@reason> is specified, then the port is killed "normally" (linked
		549	ports will not be kileld, or even notified).
		550
		551	Otherwise, linked ports get killed with the same reason (second form of
		552	C<mon>, see below).
		553
		554	Runtime errors while evaluating C<rcv> callbacks or inside C<psub> blocks
		555	will be reported as reason C<< die => $@ >>.
		556
		557	Transport/communication errors are reported as C<< transport_error =>
		558	$message >>.
		559
		560	=cut
		561
		562	=item $port = spawn $node, $initfunc[, @initdata]
		563
		564	Creates a port on the node C<$node> (which can also be a port ID, in which
		565	case it's the node where that port resides).
		566
		567	The port ID of the newly created port is return immediately, and it is
		568	permissible to immediately start sending messages or monitor the port.
		569
		570	After the port has been created, the init function is
		571	called. This function must be a fully-qualified function name
		572	(e.g. C<MyApp::Chat::Server::init>). To specify a function in the main
		573	program, use C<::name>.
		574
		575	If the function doesn't exist, then the node tries to C<require>
		576	the package, then the package above the package and so on (e.g.
		577	C<MyApp::Chat::Server>, C<MyApp::Chat>, C<MyApp>) until the function
		578	exists or it runs out of package names.
		579
		580	The init function is then called with the newly-created port as context
		581	object (C<$SELF>) and the C<@initdata> values as arguments.
		582
		583	A common idiom is to pass your own port, monitor the spawned port, and
		584	in the init function, monitor the original port. This two-way monitoring
		585	ensures that both ports get cleaned up when there is a problem.
		586
		587	Example: spawn a chat server port on C<$othernode>.
		588
		589	# this node, executed from within a port context:
		590	my $server = spawn $othernode, "MyApp::Chat::Server::connect", $SELF;
		591	mon $server;
		592
		593	# init function on C<$othernode>
		594	sub connect {
		595	my ($srcport) = @_;
		596
		597	mon $srcport;
		598
		599	rcv $SELF, sub {
		600	...
		601	};
		602	}
		603
		604	=cut
		605
		606	sub _spawn {
		607	my $port = shift;
		608	my $init = shift;
		609
		610	local $SELF = "$NODE#$port";
		611	eval {
		612	&{ load_func $init }
		613	};
		614	_self_die if $@;
		615	}
		616
		617	sub spawn(@) {
		618	my ($noderef, undef) = split /#/, shift, 2;
		619
		620	my $id = "$RUNIQ." . $ID++;
		621
		622	$_[0] =~ /::/
		623	or Carp::croak "spawn init function must be a fully-qualified name, caught";
		624
		625	snd_to_func $noderef, "AnyEvent::MP::_spawn" => $id, @_;
		626
		627	"$noderef#$id"
		628	}
		629
		630	=item after $timeout, @msg
		631
		632	=item after $timeout, $callback
		633
		634	Either sends the given message, or call the given callback, after the
		635	specified number of seconds.
		636
		637	This is simply a utility function that come sin handy at times.
		638
		639	=cut
		640
		641	sub after($@) {
		642	my ($timeout, @action) = @_;
		643
		644	my $t; $t = AE::timer $timeout, 0, sub {
		645	undef $t;
		646	ref $action[0]
		647	? $action[0]()
		648	: snd @action;
		649	};
		650	}
		651
		652	=back
		653
		654	=head1 AnyEvent::MP vs. Distributed Erlang
		655
		656	AnyEvent::MP got lots of its ideas from distributed Erlang (Erlang node
		657	== aemp node, Erlang process == aemp port), so many of the documents and
		658	programming techniques employed by Erlang apply to AnyEvent::MP. Here is a
		659	sample:
		660
		661	http://www.Erlang.se/doc/programming_rules.shtml
		662	http://Erlang.org/doc/getting_started/part_frame.html # chapters 3 and 4
		663	http://Erlang.org/download/Erlang-book-part1.pdf # chapters 5 and 6
		664	http://Erlang.org/download/armstrong_thesis_2003.pdf # chapters 4 and 5
		665
		666	Despite the similarities, there are also some important differences:
		667
		668	=over 4
		669
		670	=item * Node references contain the recipe on how to contact them.
		671
		672	Erlang relies on special naming and DNS to work everywhere in the
		673	same way. AEMP relies on each node knowing it's own address(es), with
		674	convenience functionality.
		675
		676	This means that AEMP requires a less tightly controlled environment at the
		677	cost of longer node references and a slightly higher management overhead.
		678
		679	=item * Erlang has a "remote ports are like local ports" philosophy, AEMP
		680	uses "local ports are like remote ports".
		681
		682	The failure modes for local ports are quite different (runtime errors
		683	only) then for remote ports - when a local port dies, you I<know> it dies,
		684	when a connection to another node dies, you know nothing about the other
		685	port.
		686
		687	Erlang pretends remote ports are as reliable as local ports, even when
		688	they are not.
		689
		690	AEMP encourages a "treat remote ports differently" philosophy, with local
		691	ports being the special case/exception, where transport errors cannot
		692	occur.
		693
		694	=item * Erlang uses processes and a mailbox, AEMP does not queue.
		695
		696	Erlang uses processes that selectively receive messages, and therefore
		697	needs a queue. AEMP is event based, queuing messages would serve no
		698	useful purpose. For the same reason the pattern-matching abilities of
		699	AnyEvent::MP are more limited, as there is little need to be able to
		700	filter messages without dequeing them.
		701
		702	(But see L<Coro::MP> for a more Erlang-like process model on top of AEMP).
		703
		704	=item * Erlang sends are synchronous, AEMP sends are asynchronous.
		705
		706	Sending messages in Erlang is synchronous and blocks the process (and
		707	so does not need a queue that can overflow). AEMP sends are immediate,
		708	connection establishment is handled in the background.
		709
		710	=item * Erlang suffers from silent message loss, AEMP does not.
		711
		712	Erlang makes few guarantees on messages delivery - messages can get lost
		713	without any of the processes realising it (i.e. you send messages a, b,
		714	and c, and the other side only receives messages a and c).
		715
		716	AEMP guarantees correct ordering, and the guarantee that there are no
		717	holes in the message sequence.
		718
		719	=item * In Erlang, processes can be declared dead and later be found to be
		720	alive.
		721
		722	In Erlang it can happen that a monitored process is declared dead and
		723	linked processes get killed, but later it turns out that the process is
		724	still alive - and can receive messages.
		725
		726	In AEMP, when port monitoring detects a port as dead, then that port will
		727	eventually be killed - it cannot happen that a node detects a port as dead
		728	and then later sends messages to it, finding it is still alive.
		729
		730	=item * Erlang can send messages to the wrong port, AEMP does not.
		731
		732	In Erlang it is quite likely that a node that restarts reuses a process ID
		733	known to other nodes for a completely different process, causing messages
		734	destined for that process to end up in an unrelated process.
		735
		736	AEMP never reuses port IDs, so old messages or old port IDs floating
		737	around in the network will not be sent to an unrelated port.
		738
		739	=item * Erlang uses unprotected connections, AEMP uses secure
		740	authentication and can use TLS.
		741
		742	AEMP can use a proven protocol - SSL/TLS - to protect connections and
		743	securely authenticate nodes.
		744
		745	=item * The AEMP protocol is optimised for both text-based and binary
		746	communications.
		747
		748	The AEMP protocol, unlike the Erlang protocol, supports both
		749	language-independent text-only protocols (good for debugging) and binary,
		750	language-specific serialisers (e.g. Storable).
		751
		752	It has also been carefully designed to be implementable in other languages
		753	with a minimum of work while gracefully degrading fucntionality to make the
		754	protocol simple.
		755
		756	=item * AEMP has more flexible monitoring options than Erlang.
		757
		758	In Erlang, you can chose to receive I<all> exit signals as messages
		759	or I<none>, there is no in-between, so monitoring single processes is
		760	difficult to implement. Monitoring in AEMP is more flexible than in
		761	Erlang, as one can choose between automatic kill, exit message or callback
		762	on a per-process basis.
		763
		764	=item * Erlang tries to hide remote/local connections, AEMP does not.
		765
		766	Monitoring in Erlang is not an indicator of process death/crashes,
		767	as linking is (except linking is unreliable in Erlang).
		768
		769	In AEMP, you don't "look up" registered port names or send to named ports
		770	that might or might not be persistent. Instead, you normally spawn a port
		771	on the remote node. The init function monitors the you, and you monitor
		772	the remote port. Since both monitors are local to the node, they are much
		773	more reliable.
		774
		775	This also saves round-trips and avoids sending messages to the wrong port
		776	(hard to do in Erlang).
		777
		778	=back
		779
		780	=head1 RATIONALE
		781
		782	=over 4
		783
		784	=item Why strings for ports and noderefs, why not objects?
		785
		786	We considered "objects", but found that the actual number of methods
		787	thatc an be called are very low. Since port IDs and noderefs travel over
		788	the network frequently, the serialising/deserialising would add lots of
		789	overhead, as well as having to keep a proxy object.
		790
		791	Strings can easily be printed, easily serialised etc. and need no special
		792	procedures to be "valid".
		793
		794	And a a miniport consists of a single closure stored in a global hash - it
		795	can't become much cheaper.
		796
		797	=item Why favour JSON, why not real serialising format such as Storable?
		798
		799	In fact, any AnyEvent::MP node will happily accept Storable as framing
		800	format, but currently there is no way to make a node use Storable by
		801	default.
		802
		803	The default framing protocol is JSON because a) JSON::XS is many times
		804	faster for small messages and b) most importantly, after years of
		805	experience we found that object serialisation is causing more problems
		806	than it gains: Just like function calls, objects simply do not travel
		807	easily over the network, mostly because they will always be a copy, so you
		808	always have to re-think your design.
		809
		810	Keeping your messages simple, concentrating on data structures rather than
		811	objects, will keep your messages clean, tidy and efficient.
		812
		813	=back
52		814
53	=head1 SEE ALSO	815	=head1 SEE ALSO
54		816
55	L<AnyEvent>.	817	L<AnyEvent>.
56		818

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Revision 1.64 by root, Fri Aug 28 00:58:44 2009 UTC