[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 1.61 by root, Mon Aug 24 08:06:49 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; # -OR-
		17	initialise_node "localhost:4040"; # -OR-
		18	initialise_node "slave/", "localhost:4040"
		19
		20	# ports are message endpoints
		21
		22	# sending messages
		23	snd $port, type => data...;
		24	snd $port, @msg;
		25	snd @msg_with_first_element_being_a_port;
		26
		27	# creating/using ports, the simple way
		28	my $simple_port = port { my @msg = @_; 0 };
		29
		30	# creating/using ports, tagged message matching
		31	my $port = port;
		32	rcv $port, ping => sub { snd $_[0], "pong"; 0 };
		33	rcv $port, pong => sub { warn "pong received\n"; 0 };
		34
		35	# create a port on another node
		36	my $port = spawn $node, $initfunc, @initdata;
		37
		38	# monitoring
		39	mon $port, $cb->(@msg) # callback is invoked on death
		40	mon $port, $otherport # kill otherport on abnormal death
		41	mon $port, $otherport, @msg # send message on death
		42
		43	=head1 CURRENT STATUS
		44
		45	AnyEvent::MP - stable API, should work
		46	AnyEvent::MP::Intro - outdated
		47	AnyEvent::MP::Kernel - WIP
		48	AnyEvent::MP::Transport - mostly stable
		49
		50	stay tuned.
		51
9	=head1 DESCRIPTION	52	=head1 DESCRIPTION
10		53
		54	This module (-family) implements a simple message passing framework.
		55
		56	Despite its simplicity, you can securely message other processes running
		57	on the same or other hosts.
		58
		59	For an introduction to this module family, see the L<AnyEvent::MP::Intro>
		60	manual page.
		61
		62	At the moment, this module family is severly broken and underdocumented,
		63	so do not use. This was uploaded mainly to reserve the CPAN namespace -
		64	stay tuned!
		65
		66	=head1 CONCEPTS
		67
		68	=over 4
		69
		70	=item port
		71
		72	A port is something you can send messages to (with the C<snd> function).
		73
		74	Ports allow you to register C<rcv> handlers that can match all or just
		75	some messages. Messages will not be queued.
		76
		77	=item port id - C<noderef#portname>
		78
		79	A port ID is the concatenation of a noderef, a hash-mark (C<#>) as
		80	separator, and a port name (a printable string of unspecified format). An
		81	exception is the the node port, whose ID is identical to its node
		82	reference.
		83
		84	=item node
		85
		86	A node is a single process containing at least one port - the node port,
		87	which provides nodes to manage each other remotely, and to create new
		88	ports.
		89
		90	Nodes are either private (single-process only), slaves (connected to a
		91	master node only) or public nodes (connectable from unrelated nodes).
		92
		93	=item noderef - C<host:port,host:port...>, C<id@noderef>, C<id>
		94
		95	A node reference is a string that either simply identifies the node (for
		96	private and slave nodes), or contains a recipe on how to reach a given
		97	node (for public nodes).
		98
		99	This recipe is simply a comma-separated list of C<address:port> pairs (for
		100	TCP/IP, other protocols might look different).
		101
		102	Node references come in two flavours: resolved (containing only numerical
		103	addresses) or unresolved (where hostnames are used instead of addresses).
		104
		105	Before using an unresolved node reference in a message you first have to
		106	resolve it.
		107
		108	=back
		109
		110	=head1 VARIABLES/FUNCTIONS
		111
		112	=over 4
		113
11	=cut	114	=cut
12		115
13	package AnyEvent::MP;	116	package AnyEvent::MP;
14		117
		118	use AnyEvent::MP::Kernel;
		119
15	use common::sense;	120	use common::sense;
16		121
		122	use Carp ();
		123
17	use AE ();	124	use AE ();
18		125
19	our $VERSION = '0.0';	126	use base "Exporter";
20		127
21	sub nonce($) {	128	our $VERSION = $AnyEvent::MP::Kernel::VERSION;
22	my $nonce;
23		129
24	if (open my $fh, "</dev/urandom") {	130	our @EXPORT = qw(
25	sysread $fh, $nonce, $_[0];	131	NODE $NODE *SELF node_of after
26	} else {	132	resolve_node initialise_node
		133	snd rcv mon mon_guard kil reg psub spawn
		134	port
		135	);
		136
		137	our $SELF;
		138
		139	sub _self_die() {
		140	my $msg = $@;
		141	$msg =~ s/\n+$// unless ref $msg;
		142	kil $SELF, die => $msg;
		143	}
		144
		145	=item $thisnode = NODE / $NODE
		146
		147	The C<NODE> function returns, and the C<$NODE> variable contains the
		148	noderef of the local node. The value is initialised by a call to
		149	C<initialise_node>.
		150
		151	=item $noderef = node_of $port
		152
		153	Extracts and returns the noderef from a port ID or a noderef.
		154
		155	=item initialise_node $noderef, $seednode, $seednode...
		156
		157	=item initialise_node "slave/", $master, $master...
		158
		159	Before a node can talk to other nodes on the network it has to initialise
		160	itself - the minimum a node needs to know is it's own name, and optionally
		161	it should know the noderefs of some other nodes in the network.
		162
		163	This function initialises a node - it must be called exactly once (or
		164	never) before calling other AnyEvent::MP functions.
		165
		166	All arguments (optionally except for the first) are noderefs, which can be
		167	either resolved or unresolved.
		168
		169	The first argument will be looked up in the configuration database first
		170	(if it is C<undef> then the current nodename will be used instead) to find
		171	the relevant configuration profile (see L<aemp>). If none is found then
		172	the default configuration is used. The configuration supplies additional
		173	seed/master nodes and can override the actual noderef.
		174
		175	There are two types of networked nodes, public nodes and slave nodes:
		176
		177	=over 4
		178
		179	=item public nodes
		180
		181	For public nodes, C<$noderef> (supplied either directly to
		182	C<initialise_node> or indirectly via a profile or the nodename) must be a
		183	noderef (possibly unresolved, in which case it will be resolved).
		184
		185	After resolving, the node will bind itself on all endpoints and try to
		186	connect to all additional C<$seednodes> that are specified. Seednodes are
		187	optional and can be used to quickly bootstrap the node into an existing
		188	network.
		189
		190	=item slave nodes
		191
		192	When the C<$noderef> (either as given or overriden by the config file)
		193	is the special string C<slave/>, then the node will become a slave
		194	node. Slave nodes cannot be contacted from outside and will route most of
		195	their traffic to the master node that they attach to.
		196
		197	At least one additional noderef is required (either by specifying it
		198	directly or because it is part of the configuration profile): The node
		199	will try to connect to all of them and will become a slave attached to the
		200	first node it can successfully connect to.
		201
		202	Note that slave nodes cannot change their name, and consequently, their
		203	master, so if the master goes down, the slave node will not function well
		204	anymore until it can re-establish conenciton to its master. This makes
		205	slave nodes unsuitable for long-term nodes or fault-tolerant networks.
		206
		207	=back
		208
		209	This function will block until all nodes have been resolved and, for slave
		210	nodes, until it has successfully established a connection to a master
		211	server.
		212
		213	All the seednodes will also be specially marked to automatically retry
		214	connecting to them infinitely.
		215
		216	Example: become a public node listening on the guessed noderef, or the one
		217	specified via C<aemp> for the current node. This should be the most common
		218	form of invocation for "daemon"-type nodes.
		219
		220	initialise_node;
		221
		222	Example: become a slave node to any of the the seednodes specified via
		223	C<aemp>. This form is often used for commandline clients.
		224
		225	initialise_node "slave/";
		226
		227	Example: become a slave node to any of the specified master servers. This
		228	form is also often used for commandline clients.
		229
		230	initialise_node "slave/", "master1", "192.168.13.17", "mp.example.net";
		231
		232	Example: become a public node, and try to contact some well-known master
		233	servers to become part of the network.
		234
		235	initialise_node undef, "master1", "master2";
		236
		237	Example: become a public node listening on port C<4041>.
		238
		239	initialise_node 4041;
		240
		241	Example: become a public node, only visible on localhost port 4044.
		242
		243	initialise_node "localhost:4044";
		244
		245	=item $cv = resolve_node $noderef
		246
		247	Takes an unresolved node reference that may contain hostnames and
		248	abbreviated IDs, resolves all of them and returns a resolved node
		249	reference.
		250
		251	In addition to C<address:port> pairs allowed in resolved noderefs, the
		252	following forms are supported:
		253
		254	=over 4
		255
		256	=item the empty string
		257
		258	An empty-string component gets resolved as if the default port (4040) was
		259	specified.
		260
		261	=item naked port numbers (e.g. C<1234>)
		262
		263	These are resolved by prepending the local nodename and a colon, to be
		264	further resolved.
		265
		266	=item hostnames (e.g. C<localhost:1234>, C<localhost>)
		267
		268	These are resolved by using AnyEvent::DNS to resolve them, optionally
		269	looking up SRV records for the C<aemp=4040> port, if no port was
		270	specified.
		271
		272	=back
		273
		274	=item $SELF
		275
		276	Contains the current port id while executing C<rcv> callbacks or C<psub>
		277	blocks.
		278
		279	=item SELF, %SELF, @SELF...
		280
		281	Due to some quirks in how perl exports variables, it is impossible to
		282	just export C<$SELF>, all the symbols called C<SELF> are exported by this
		283	module, but only C<$SELF> is currently used.
		284
		285	=item snd $port, type => @data
		286
		287	=item snd $port, @msg
		288
		289	Send the given message to the given port ID, which can identify either
		290	a local or a remote port, and must be a port ID.
		291
		292	While the message can be about anything, it is highly recommended to use a
		293	string as first element (a port ID, or some word that indicates a request
		294	type etc.).
		295
		296	The message data effectively becomes read-only after a call to this
		297	function: modifying any argument is not allowed and can cause many
		298	problems.
		299
		300	The type of data you can transfer depends on the transport protocol: when
		301	JSON is used, then only strings, numbers and arrays and hashes consisting
		302	of those are allowed (no objects). When Storable is used, then anything
		303	that Storable can serialise and deserialise is allowed, and for the local
		304	node, anything can be passed.
		305
		306	=item $local_port = port
		307
		308	Create a new local port object and returns its port ID. Initially it has
		309	no callbacks set and will throw an error when it receives messages.
		310
		311	=item $local_port = port { my @msg = @_ }
		312
		313	Creates a new local port, and returns its ID. Semantically the same as
		314	creating a port and calling C<rcv $port, $callback> on it.
		315
		316	The block will be called for every message received on the port, with the
		317	global variable C<$SELF> set to the port ID. Runtime errors will cause the
		318	port to be C<kil>ed. The message will be passed as-is, no extra argument
		319	(i.e. no port ID) will be passed to the callback.
		320
		321	If you want to stop/destroy the port, simply C<kil> it:
		322
		323	my $port = port {
		324	my @msg = @_;
27	# shit...	325	...
28	our $nonce_init;	326	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	}	327	};
35		328
36	$nonce	329	=cut
37	}
38		330
39	our $DEFAULT_SECRET;	331	sub rcv($@);
40		332
41	sub default_secret {	333	sub _kilme {
42	unless (defined $DEFAULT_SECRET) {	334	die "received message on port without callback";
43	if (open my $fh, "<$ENV{HOME}/.aemp-secret") {	335	}
44	sysread $fh, $DEFAULT_SECRET, -s $fh;	336
		337	sub port(;&) {
		338	my $id = "$UNIQ." . $ID++;
		339	my $port = "$NODE#$id";
		340
		341	rcv $port, shift \|\| \&_kilme;
		342
		343	$port
		344	}
		345
		346	=item rcv $local_port, $callback->(@msg)
		347
		348	Replaces the default callback on the specified port. There is no way to
		349	remove the default callback: use C<sub { }> to disable it, or better
		350	C<kil> the port when it is no longer needed.
		351
		352	The global C<$SELF> (exported by this module) contains C<$port> while
		353	executing the callback. Runtime errors during callback execution will
		354	result in the port being C<kil>ed.
		355
		356	The default callback received all messages not matched by a more specific
		357	C<tag> match.
		358
		359	=item rcv $local_port, tag => $callback->(@msg_without_tag), ...
		360
		361	Register (or replace) callbacks to be called on messages starting with the
		362	given tag on the given port (and return the port), or unregister it (when
		363	C<$callback> is C<$undef> or missing). There can only be one callback
		364	registered for each tag.
		365
		366	The original message will be passed to the callback, after the first
		367	element (the tag) has been removed. The callback will use the same
		368	environment as the default callback (see above).
		369
		370	Example: create a port and bind receivers on it in one go.
		371
		372	my $port = rcv port,
		373	msg1 => sub { ... },
		374	msg2 => sub { ... },
		375	;
		376
		377	Example: create a port, bind receivers and send it in a message elsewhere
		378	in one go:
		379
		380	snd $otherport, reply =>
		381	rcv port,
		382	msg1 => sub { ... },
		383	...
		384	;
		385
		386	Example: temporarily register a rcv callback for a tag matching some port
		387	(e.g. for a rpc reply) and unregister it after a message was received.
		388
		389	rcv $port, $otherport => sub {
		390	my @reply = @_;
		391
		392	rcv $SELF, $otherport;
		393	};
		394
		395	=cut
		396
		397	sub rcv($@) {
		398	my $port = shift;
		399	my ($noderef, $portid) = split /#/, $port, 2;
		400
		401	$NODE{$noderef} == $NODE{""}
		402	or Carp::croak "$port: rcv can only be called on local ports, caught";
		403
		404	while (@_) {
		405	if (ref $_[0]) {
		406	if (my $self = $PORT_DATA{$portid}) {
		407	"AnyEvent::MP::Port" eq ref $self
		408	or Carp::croak "$port: rcv can only be called on message matching ports, caught";
		409
		410	$self->[2] = shift;
45	} else {	411	} else {
46	$DEFAULT_SECRET = nonce 32;	412	my $cb = shift;
		413	$PORT{$portid} = sub {
		414	local $SELF = $port;
		415	eval { &$cb }; _self_die if $@;
		416	};
		417	}
		418	} elsif (defined $_[0]) {
		419	my $self = $PORT_DATA{$portid} \|\|= do {
		420	my $self = bless [$PORT{$port} \|\| sub { }, { }, $port], "AnyEvent::MP::Port";
		421
		422	$PORT{$portid} = sub {
		423	local $SELF = $port;
		424
		425	if (my $cb = $self->[1]{$_[0]}) {
		426	shift;
		427	eval { &$cb }; _self_die if $@;
		428	} else {
		429	&{ $self->[0] };
		430	}
		431	};
		432
		433	$self
		434	};
		435
		436	"AnyEvent::MP::Port" eq ref $self
		437	or Carp::croak "$port: rcv can only be called on message matching ports, caught";
		438
		439	my ($tag, $cb) = splice @_, 0, 2;
		440
		441	if (defined $cb) {
		442	$self->[1]{$tag} = $cb;
		443	} else {
		444	delete $self->[1]{$tag};
		445	}
47	}	446	}
48	}	447	}
49		448
50	$DEFAULT_SECRET	449	$port
51	}	450	}
		451
		452	=item $closure = psub { BLOCK }
		453
		454	Remembers C<$SELF> and creates a closure out of the BLOCK. When the
		455	closure is executed, sets up the environment in the same way as in C<rcv>
		456	callbacks, i.e. runtime errors will cause the port to get C<kil>ed.
		457
		458	This is useful when you register callbacks from C<rcv> callbacks:
		459
		460	rcv delayed_reply => sub {
		461	my ($delay, @reply) = @_;
		462	my $timer = AE::timer $delay, 0, psub {
		463	snd @reply, $SELF;
		464	};
		465	};
		466
		467	=cut
		468
		469	sub psub(&) {
		470	my $cb = shift;
		471
		472	my $port = $SELF
		473	or Carp::croak "psub can only be called from within rcv or psub callbacks, not";
		474
		475	sub {
		476	local $SELF = $port;
		477
		478	if (wantarray) {
		479	my @res = eval { &$cb };
		480	_self_die if $@;
		481	@res
		482	} else {
		483	my $res = eval { &$cb };
		484	_self_die if $@;
		485	$res
		486	}
		487	}
		488	}
		489
		490	=item $guard = mon $port, $cb->(@reason)
		491
		492	=item $guard = mon $port, $rcvport
		493
		494	=item $guard = mon $port
		495
		496	=item $guard = mon $port, $rcvport, @msg
		497
		498	Monitor the given port and do something when the port is killed or
		499	messages to it were lost, and optionally return a guard that can be used
		500	to stop monitoring again.
		501
		502	C<mon> effectively guarantees that, in the absence of hardware failures,
		503	that after starting the monitor, either all messages sent to the port
		504	will arrive, or the monitoring action will be invoked after possible
		505	message loss has been detected. No messages will be lost "in between"
		506	(after the first lost message no further messages will be received by the
		507	port). After the monitoring action was invoked, further messages might get
		508	delivered again.
		509
		510	Note that monitoring-actions are one-shot: once released, they are removed
		511	and will not trigger again.
		512
		513	In the first form (callback), the callback is simply called with any
		514	number of C<@reason> elements (no @reason means that the port was deleted
		515	"normally"). Note also that I<< the callback B<must> never die >>, so use
		516	C<eval> if unsure.
		517
		518	In the second form (another port given), the other port (C<$rcvport>)
		519	will be C<kil>'ed with C<@reason>, iff a @reason was specified, i.e. on
		520	"normal" kils nothing happens, while under all other conditions, the other
		521	port is killed with the same reason.
		522
		523	The third form (kill self) is the same as the second form, except that
		524	C<$rvport> defaults to C<$SELF>.
		525
		526	In the last form (message), a message of the form C<@msg, @reason> will be
		527	C<snd>.
		528
		529	As a rule of thumb, monitoring requests should always monitor a port from
		530	a local port (or callback). The reason is that kill messages might get
		531	lost, just like any other message. Another less obvious reason is that
		532	even monitoring requests can get lost (for exmaple, when the connection
		533	to the other node goes down permanently). When monitoring a port locally
		534	these problems do not exist.
		535
		536	Example: call a given callback when C<$port> is killed.
		537
		538	mon $port, sub { warn "port died because of <@_>\n" };
		539
		540	Example: kill ourselves when C<$port> is killed abnormally.
		541
		542	mon $port;
		543
		544	Example: send us a restart message when another C<$port> is killed.
		545
		546	mon $port, $self => "restart";
		547
		548	=cut
		549
		550	sub mon {
		551	my ($noderef, $port) = split /#/, shift, 2;
		552
		553	my $node = $NODE{$noderef} \|\| add_node $noderef;
		554
		555	my $cb = @_ ? shift : $SELF \|\| Carp::croak 'mon: called with one argument only, but $SELF not set,';
		556
		557	unless (ref $cb) {
		558	if (@_) {
		559	# send a kill info message
		560	my (@msg) = ($cb, @_);
		561	$cb = sub { snd @msg, @_ };
		562	} else {
		563	# simply kill other port
		564	my $port = $cb;
		565	$cb = sub { kil $port, @_ if @_ };
		566	}
		567	}
		568
		569	$node->monitor ($port, $cb);
		570
		571	defined wantarray
		572	and AnyEvent::Util::guard { $node->unmonitor ($port, $cb) }
		573	}
		574
		575	=item $guard = mon_guard $port, $ref, $ref...
		576
		577	Monitors the given C<$port> and keeps the passed references. When the port
		578	is killed, the references will be freed.
		579
		580	Optionally returns a guard that will stop the monitoring.
		581
		582	This function is useful when you create e.g. timers or other watchers and
		583	want to free them when the port gets killed:
		584
		585	$port->rcv (start => sub {
		586	my $timer; $timer = mon_guard $port, AE::timer 1, 1, sub {
		587	undef $timer if 0.9 < rand;
		588	});
		589	});
		590
		591	=cut
		592
		593	sub mon_guard {
		594	my ($port, @refs) = @_;
		595
		596	#TODO: mon-less form?
		597
		598	mon $port, sub { 0 && @refs }
		599	}
		600
		601	=item kil $port[, @reason]
		602
		603	Kill the specified port with the given C<@reason>.
		604
		605	If no C<@reason> is specified, then the port is killed "normally" (linked
		606	ports will not be kileld, or even notified).
		607
		608	Otherwise, linked ports get killed with the same reason (second form of
		609	C<mon>, see below).
		610
		611	Runtime errors while evaluating C<rcv> callbacks or inside C<psub> blocks
		612	will be reported as reason C<< die => $@ >>.
		613
		614	Transport/communication errors are reported as C<< transport_error =>
		615	$message >>.
		616
		617	=cut
		618
		619	=item $port = spawn $node, $initfunc[, @initdata]
		620
		621	Creates a port on the node C<$node> (which can also be a port ID, in which
		622	case it's the node where that port resides).
		623
		624	The port ID of the newly created port is return immediately, and it is
		625	permissible to immediately start sending messages or monitor the port.
		626
		627	After the port has been created, the init function is
		628	called. This function must be a fully-qualified function name
		629	(e.g. C<MyApp::Chat::Server::init>). To specify a function in the main
		630	program, use C<::name>.
		631
		632	If the function doesn't exist, then the node tries to C<require>
		633	the package, then the package above the package and so on (e.g.
		634	C<MyApp::Chat::Server>, C<MyApp::Chat>, C<MyApp>) until the function
		635	exists or it runs out of package names.
		636
		637	The init function is then called with the newly-created port as context
		638	object (C<$SELF>) and the C<@initdata> values as arguments.
		639
		640	A common idiom is to pass your own port, monitor the spawned port, and
		641	in the init function, monitor the original port. This two-way monitoring
		642	ensures that both ports get cleaned up when there is a problem.
		643
		644	Example: spawn a chat server port on C<$othernode>.
		645
		646	# this node, executed from within a port context:
		647	my $server = spawn $othernode, "MyApp::Chat::Server::connect", $SELF;
		648	mon $server;
		649
		650	# init function on C<$othernode>
		651	sub connect {
		652	my ($srcport) = @_;
		653
		654	mon $srcport;
		655
		656	rcv $SELF, sub {
		657	...
		658	};
		659	}
		660
		661	=cut
		662
		663	sub _spawn {
		664	my $port = shift;
		665	my $init = shift;
		666
		667	local $SELF = "$NODE#$port";
		668	eval {
		669	&{ load_func $init }
		670	};
		671	_self_die if $@;
		672	}
		673
		674	sub spawn(@) {
		675	my ($noderef, undef) = split /#/, shift, 2;
		676
		677	my $id = "$RUNIQ." . $ID++;
		678
		679	$_[0] =~ /::/
		680	or Carp::croak "spawn init function must be a fully-qualified name, caught";
		681
		682	snd_to_func $noderef, "AnyEvent::MP::_spawn" => $id, @_;
		683
		684	"$noderef#$id"
		685	}
		686
		687	=item after $timeout, @msg
		688
		689	=item after $timeout, $callback
		690
		691	Either sends the given message, or call the given callback, after the
		692	specified number of seconds.
		693
		694	This is simply a utility function that come sin handy at times.
		695
		696	=cut
		697
		698	sub after($@) {
		699	my ($timeout, @action) = @_;
		700
		701	my $t; $t = AE::timer $timeout, 0, sub {
		702	undef $t;
		703	ref $action[0]
		704	? $action[0]()
		705	: snd @action;
		706	};
		707	}
		708
		709	=back
		710
		711	=head1 AnyEvent::MP vs. Distributed Erlang
		712
		713	AnyEvent::MP got lots of its ideas from distributed Erlang (Erlang node
		714	== aemp node, Erlang process == aemp port), so many of the documents and
		715	programming techniques employed by Erlang apply to AnyEvent::MP. Here is a
		716	sample:
		717
		718	http://www.Erlang.se/doc/programming_rules.shtml
		719	http://Erlang.org/doc/getting_started/part_frame.html # chapters 3 and 4
		720	http://Erlang.org/download/Erlang-book-part1.pdf # chapters 5 and 6
		721	http://Erlang.org/download/armstrong_thesis_2003.pdf # chapters 4 and 5
		722
		723	Despite the similarities, there are also some important differences:
		724
		725	=over 4
		726
		727	=item * Node references contain the recipe on how to contact them.
		728
		729	Erlang relies on special naming and DNS to work everywhere in the
		730	same way. AEMP relies on each node knowing it's own address(es), with
		731	convenience functionality.
		732
		733	This means that AEMP requires a less tightly controlled environment at the
		734	cost of longer node references and a slightly higher management overhead.
		735
		736	=item * Erlang has a "remote ports are like local ports" philosophy, AEMP
		737	uses "local ports are like remote ports".
		738
		739	The failure modes for local ports are quite different (runtime errors
		740	only) then for remote ports - when a local port dies, you I<know> it dies,
		741	when a connection to another node dies, you know nothing about the other
		742	port.
		743
		744	Erlang pretends remote ports are as reliable as local ports, even when
		745	they are not.
		746
		747	AEMP encourages a "treat remote ports differently" philosophy, with local
		748	ports being the special case/exception, where transport errors cannot
		749	occur.
		750
		751	=item * Erlang uses processes and a mailbox, AEMP does not queue.
		752
		753	Erlang uses processes that selectively receive messages, and therefore
		754	needs a queue. AEMP is event based, queuing messages would serve no
		755	useful purpose. For the same reason the pattern-matching abilities of
		756	AnyEvent::MP are more limited, as there is little need to be able to
		757	filter messages without dequeing them.
		758
		759	(But see L<Coro::MP> for a more Erlang-like process model on top of AEMP).
		760
		761	=item * Erlang sends are synchronous, AEMP sends are asynchronous.
		762
		763	Sending messages in Erlang is synchronous and blocks the process (and
		764	so does not need a queue that can overflow). AEMP sends are immediate,
		765	connection establishment is handled in the background.
		766
		767	=item * Erlang suffers from silent message loss, AEMP does not.
		768
		769	Erlang makes few guarantees on messages delivery - messages can get lost
		770	without any of the processes realising it (i.e. you send messages a, b,
		771	and c, and the other side only receives messages a and c).
		772
		773	AEMP guarantees correct ordering, and the guarantee that there are no
		774	holes in the message sequence.
		775
		776	=item * In Erlang, processes can be declared dead and later be found to be
		777	alive.
		778
		779	In Erlang it can happen that a monitored process is declared dead and
		780	linked processes get killed, but later it turns out that the process is
		781	still alive - and can receive messages.
		782
		783	In AEMP, when port monitoring detects a port as dead, then that port will
		784	eventually be killed - it cannot happen that a node detects a port as dead
		785	and then later sends messages to it, finding it is still alive.
		786
		787	=item * Erlang can send messages to the wrong port, AEMP does not.
		788
		789	In Erlang it is quite likely that a node that restarts reuses a process ID
		790	known to other nodes for a completely different process, causing messages
		791	destined for that process to end up in an unrelated process.
		792
		793	AEMP never reuses port IDs, so old messages or old port IDs floating
		794	around in the network will not be sent to an unrelated port.
		795
		796	=item * Erlang uses unprotected connections, AEMP uses secure
		797	authentication and can use TLS.
		798
		799	AEMP can use a proven protocol - SSL/TLS - to protect connections and
		800	securely authenticate nodes.
		801
		802	=item * The AEMP protocol is optimised for both text-based and binary
		803	communications.
		804
		805	The AEMP protocol, unlike the Erlang protocol, supports both
		806	language-independent text-only protocols (good for debugging) and binary,
		807	language-specific serialisers (e.g. Storable).
		808
		809	It has also been carefully designed to be implementable in other languages
		810	with a minimum of work while gracefully degrading fucntionality to make the
		811	protocol simple.
		812
		813	=item * AEMP has more flexible monitoring options than Erlang.
		814
		815	In Erlang, you can chose to receive I<all> exit signals as messages
		816	or I<none>, there is no in-between, so monitoring single processes is
		817	difficult to implement. Monitoring in AEMP is more flexible than in
		818	Erlang, as one can choose between automatic kill, exit message or callback
		819	on a per-process basis.
		820
		821	=item * Erlang tries to hide remote/local connections, AEMP does not.
		822
		823	Monitoring in Erlang is not an indicator of process death/crashes,
		824	as linking is (except linking is unreliable in Erlang).
		825
		826	In AEMP, you don't "look up" registered port names or send to named ports
		827	that might or might not be persistent. Instead, you normally spawn a port
		828	on the remote node. The init function monitors the you, and you monitor
		829	the remote port. Since both monitors are local to the node, they are much
		830	more reliable.
		831
		832	This also saves round-trips and avoids sending messages to the wrong port
		833	(hard to do in Erlang).
		834
		835	=back
		836
		837	=head1 RATIONALE
		838
		839	=over 4
		840
		841	=item Why strings for ports and noderefs, why not objects?
		842
		843	We considered "objects", but found that the actual number of methods
		844	thatc an be called are very low. Since port IDs and noderefs travel over
		845	the network frequently, the serialising/deserialising would add lots of
		846	overhead, as well as having to keep a proxy object.
		847
		848	Strings can easily be printed, easily serialised etc. and need no special
		849	procedures to be "valid".
		850
		851	And a a miniport consists of a single closure stored in a global hash - it
		852	can't become much cheaper.
		853
		854	=item Why favour JSON, why not real serialising format such as Storable?
		855
		856	In fact, any AnyEvent::MP node will happily accept Storable as framing
		857	format, but currently there is no way to make a node use Storable by
		858	default.
		859
		860	The default framing protocol is JSON because a) JSON::XS is many times
		861	faster for small messages and b) most importantly, after years of
		862	experience we found that object serialisation is causing more problems
		863	than it gains: Just like function calls, objects simply do not travel
		864	easily over the network, mostly because they will always be a copy, so you
		865	always have to re-think your design.
		866
		867	Keeping your messages simple, concentrating on data structures rather than
		868	objects, will keep your messages clean, tidy and efficient.
		869
		870	=back
52		871
53	=head1 SEE ALSO	872	=head1 SEE ALSO
54		873
55	L<AnyEvent>.	874	L<AnyEvent>.
56		875

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Revision 1.61 by root, Mon Aug 24 08:06:49 2009 UTC