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

Comparing AnyEvent-Fork-RPC/RPC.pm (file contents):
Revision 1.3 by root, Wed Apr 17 17:16:48 2013 UTC vs.
Revision 1.13 by root, Thu Apr 18 11:11:26 2013 UTC

…		…
2		2
3	AnyEvent::Fork::RPC - simple RPC extension for AnyEvent::Fork	3	AnyEvent::Fork::RPC - simple RPC extension for AnyEvent::Fork
4		4
5	=head1 SYNOPSIS	5	=head1 SYNOPSIS
6		6
7	use AnyEvent::Fork;
8	use AnyEvent::Fork::RPC;	7	use AnyEvent::Fork::RPC;
		8	# use AnyEvent::Fork is not needed
9		9
10	my $rpc = AnyEvent::Fork	10	my $rpc = AnyEvent::Fork
11	->new	11	->new
12	->require ("MyModule")	12	->require ("MyModule")
13	->AnyEvent::Fork::RPC::run (	13	->AnyEvent::Fork::RPC::run (
…		…
34	concurrently in the child, using AnyEvent.	34	concurrently in the child, using AnyEvent.
35		35
36	It also implements an asynchronous event mechanism from the child to the	36	It also implements an asynchronous event mechanism from the child to the
37	parent, that could be used for progress indications or other information.	37	parent, that could be used for progress indications or other information.
38		38
		39	Loading this module also always loads L<AnyEvent::Fork>, so you can make a
		40	separate C<use AnyEvent::Fork> if you wish, but you don't have to.
		41
		42	=head1 EXAMPLES
		43
		44	=head2 Example 1: Synchronous Backend
		45
		46	Here is a simple example that implements a backend that executes C<unlink>
		47	and C<rmdir> calls, and reports their status back. It also reports the
		48	number of requests it has processed every three requests, which is clearly
		49	silly, but illustrates the use of events.
		50
		51	First the parent process:
		52
		53	use AnyEvent;
		54	use AnyEvent::Fork::RPC;
		55
		56	my $done = AE::cv;
		57
		58	my $rpc = AnyEvent::Fork
		59	->new
		60	->require ("MyWorker")
		61	->AnyEvent::Fork::RPC::run ("MyWorker::run",
		62	on_error => sub { warn "FATAL: $_[0]"; exit 1 },
		63	on_event => sub { warn "$_[0] requests handled\n" },
		64	on_destroy => $done,
		65	);
		66
		67	for my $id (1..6) {
		68	$rpc->(rmdir => "/tmp/somepath/$id", sub {
		69	$_[0]
		70	or warn "/tmp/somepath/$id: $_[1]\n";
		71	});
		72	}
		73
		74	undef $rpc;
		75
		76	$done->recv;
		77
		78	The parent creates the process, queues a few rmdir's. It then forgets
		79	about the C<$rpc> object, so that the child exits after it has handled the
		80	requests, and then it waits till the requests have been handled.
		81
		82	The child is implemented using a separate module, C<MyWorker>, shown here:
		83
		84	package MyWorker;
		85
		86	my $count;
		87
		88	sub run {
		89	my ($cmd, $path) = @_;
		90
		91	AnyEvent::Fork::RPC::event ($count)
		92	unless ++$count % 3;
		93
		94	my $status = $cmd eq "rmdir" ? rmdir $path
		95	: $cmd eq "unlink" ? unlink $path
		96	: die "fatal error, illegal command '$cmd'";
		97
		98	$status or (0, "$!")
		99	}
		100
		101	1
		102
		103	The C<run> function first sends a "progress" event every three calls, and
		104	then executes C<rmdir> or C<unlink>, depending on the first parameter (or
		105	dies with a fatal error - obviously, you must never let this happen :).
		106
		107	Eventually it returns the status value true if the command was successful,
		108	or the status value 0 and the stringified error message.
		109
		110	On my system, running the first code fragment with the given
		111	F<MyWorker.pm> in the current directory yields:
		112
		113	/tmp/somepath/1: No such file or directory
		114	/tmp/somepath/2: No such file or directory
		115	3 requests handled
		116	/tmp/somepath/3: No such file or directory
		117	/tmp/somepath/4: No such file or directory
		118	/tmp/somepath/5: No such file or directory
		119	6 requests handled
		120	/tmp/somepath/6: No such file or directory
		121
		122	Obviously, none of the directories I am trying to delete even exist. Also,
		123	the events and responses are processed in exactly the same order as
		124	they were created in the child, which is true for both synchronous and
		125	asynchronous backends.
		126
		127	Note that the parentheses in the call to C<AnyEvent::Fork::RPC::event> are
		128	not optional. That is because the function isn't defined when the code is
		129	compiled. You can make sure it is visible by pre-loading the correct
		130	backend module in the call to C<require>:
		131
		132	->require ("AnyEvent::Fork::RPC::Sync", "MyWorker")
		133
		134	Since the backend module declares the C<event> function, loading it first
		135	ensures that perl will correctly interpret calls to it.
		136
		137	And as a final remark, there is a fine module on CPAN that can
		138	asynchronously C<rmdir> and C<unlink> and a lot more, and more efficiently
		139	than this example, namely L<IO::AIO>.
		140
		141	=head3 Example 1a: the same with the asynchronous backend
		142
		143	This example only shows what needs to be changed to use the async backend
		144	instead. Doing this is not very useful, the purpose of this example is
		145	to show the minimum amount of change that is required to go from the
		146	synchronous to the asynchronous backend.
		147
		148	To use the async backend in the previous example, you need to add the
		149	C<async> parameter to the C<AnyEvent::Fork::RPC::run> call:
		150
		151	->AnyEvent::Fork::RPC::run ("MyWorker::run",
		152	async => 1,
		153	...
		154
		155	And since the function call protocol is now changed, you need to adopt
		156	C<MyWorker::run> to the async API.
		157
		158	First, you need to accept the extra initial C<$done> callback:
		159
		160	sub run {
		161	my ($done, $cmd, $path) = @_;
		162
		163	And since a response is now generated when C<$done> is called, as opposed
		164	to when the function returns, we need to call the C<$done> function with
		165	the status:
		166
		167	$done->($status or (0, "$!"));
		168
		169	A few remarks are in order. First, it's quite pointless to use the async
		170	backend for this example - but it I<is> possible. Second, you can call
		171	C<$done> before or after returning from the function. Third, having both
		172	returned from the function and having called the C<$done> callback, the
		173	child process may exit at any time, so you should call C<$done> only when
		174	you really I<are> done.
		175
		176	=head2 Example 2: Asynchronous Backend
		177
		178	This example implements multiple count-downs in the child, using
		179	L<AnyEvent> timers. While this is a bit silly (one could use timers in te
		180	parent just as well), it illustrates the ability to use AnyEvent in the
		181	child and the fact that responses can arrive in a different order then the
		182	requests.
		183
		184	It also shows how to embed the actual child code into a C<__DATA__>
		185	section, so it doesn't need any external files at all.
		186
		187	And when your parent process is often busy, and you have stricter timing
		188	requirements, then running timers in a child process suddenly doesn't look
		189	so silly anymore.
		190
		191	Without further ado, here is the code:
		192
		193	use AnyEvent;
		194	use AnyEvent::Fork::RPC;
		195
		196	my $done = AE::cv;
		197
		198	my $rpc = AnyEvent::Fork
		199	->new
		200	->require ("AnyEvent::Fork::RPC::Async")
		201	->eval (do { local $/; <DATA> })
		202	->AnyEvent::Fork::RPC::run ("run",
		203	async => 1,
		204	on_error => sub { warn "FATAL: $_[0]"; exit 1 },
		205	on_event => sub { print $_[0] },
		206	on_destroy => $done,
		207	);
		208
		209	for my $count (3, 2, 1) {
		210	$rpc->($count, sub {
		211	warn "job $count finished\n";
		212	});
		213	}
		214
		215	undef $rpc;
		216
		217	$done->recv;
		218
		219	__DATA__
		220
		221	# this ends up in main, as we don't use a package declaration
		222
		223	use AnyEvent;
		224
		225	sub run {
		226	my ($done, $count) = @_;
		227
		228	my $n;
		229
		230	AnyEvent::Fork::RPC::event "starting to count up to $count\n";
		231
		232	my $w; $w = AE::timer 1, 1, sub {
		233	++$n;
		234
		235	AnyEvent::Fork::RPC::event "count $n of $count\n";
		236
		237	if ($n == $count) {
		238	undef $w;
		239	$done->();
		240	}
		241	};
		242	}
		243
		244	The parent part (the one before the C<__DATA__> section) isn't very
		245	different from the earlier examples. It sets async mode, preloads
		246	the backend module (so the C<AnyEvent::Fork::RPC::event> function is
		247	declared), uses a slightly different C<on_event> handler (which we use
		248	simply for logging purposes) and then, instead of loading a module with
		249	the actual worker code, it C<eval>'s the code from the data section in the
		250	child process.
		251
		252	It then starts three countdowns, from 3 to 1 seconds downwards, destroys
		253	the rpc object so the example finishes eventually, and then just waits for
		254	the stuff to trickle in.
		255
		256	The worker code uses the event function to log some progress messages, but
		257	mostly just creates a recurring one-second timer.
		258
		259	The timer callback increments a counter, logs a message, and eventually,
		260	when the count has been reached, calls the finish callback.
		261
		262	On my system, this results in the following output. Since all timers fire
		263	at roughly the same time, the actual order isn't guaranteed, but the order
		264	shown is very likely what you would get, too.
		265
		266	starting to count up to 3
		267	starting to count up to 2
		268	starting to count up to 1
		269	count 1 of 3
		270	count 1 of 2
		271	count 1 of 1
		272	job 1 finished
		273	count 2 of 2
		274	job 2 finished
		275	count 2 of 3
		276	count 3 of 3
		277	job 3 finished
		278
		279	While the overall ordering isn't guaranteed, the async backend still
		280	guarantees that events and responses are delivered to the parent process
		281	in the exact same ordering as they were generated in the child process.
		282
		283	And unless your system is I<very> busy, it should clearly show that the
		284	job started last will finish first, as it has the lowest count.
		285
		286	This concludes the async example. Since L<AnyEvent::Fork> does not
		287	actually fork, you are free to use about any module in the child, not just
		288	L<AnyEvent>, but also L<IO::AIO>, or L<Tk> for example.
		289
39	=head1 PARENT PROCESS USAGE	290	=head1 PARENT PROCESS USAGE
40		291
41	This module exports nothing, and only implements a single function:	292	This module exports nothing, and only implements a single function:
42		293
43	=over 4	294	=over 4
…		…
50		301
51	use Errno ();	302	use Errno ();
52	use Guard ();	303	use Guard ();
53		304
54	use AnyEvent;	305	use AnyEvent;
55	#use AnyEvent::Fork;	306	use AnyEvent::Fork; # we don't actually depend on it, this is for convenience
56		307
57	our $VERSION = 0.1;	308	our $VERSION = 0.1;
58		309
59	=item my $rpc = AnyEvent::Fork::RPC::run $fork, $function, [key => value...]	310	=item my $rpc = AnyEvent::Fork::RPC::run $fork, $function, [key => value...]
60		311
…		…
92	Called for every call to the C<AnyEvent::Fork::RPC::event> function in the	343	Called for every call to the C<AnyEvent::Fork::RPC::event> function in the
93	child, with the arguments of that function passed to the callback.	344	child, with the arguments of that function passed to the callback.
94		345
95	Also called on errors when no C<on_error> handler is provided.	346	Also called on errors when no C<on_error> handler is provided.
96		347
		348	=item on_destroy => $cb->()
		349
		350	Called when the C<$rpc> object has been destroyed and all requests have
		351	been successfully handled. This is useful when you queue some requests and
		352	want the child to go away after it has handled them. The problem is that
		353	the parent must not exit either until all requests have been handled, and
		354	this can be accomplished by waiting for this callback.
		355
97	=item init => $function (default none)	356	=item init => $function (default none)
98		357
99	When specified (by name), this function is called in the child as the very	358	When specified (by name), this function is called in the child as the very
100	first thing when taking over the process, with all the arguments normally	359	first thing when taking over the process, with all the arguments normally
101	passed to the C<AnyEvent::Fork::run> function, except the communications	360	passed to the C<AnyEvent::Fork::run> function, except the communications
102	socket.	361	socket.
103		362
104	It can be used to do one-time things in the child such as storing passed	363	It can be used to do one-time things in the child such as storing passed
105	parameters or opening database connections.	364	parameters or opening database connections.
106		365
		366	It is called very early - before the serialisers are created or the
		367	C<$function> name is resolved into a function reference, so it could be
		368	used to load any modules that provide the serialiser or function. It can
		369	not, however, create events.
		370
107	=item async => $boolean (default: 0)	371	=item async => $boolean (default: 0)
108		372
109	The default server used in the child does all I/O blockingly, and only	373	The default server used in the child does all I/O blockingly, and only
110	allows a single RPC call to execute concurrently.	374	allows a single RPC call to execute concurrently.
111		375
…		…
116	the calling semantics of the returned C<$rpc> function.	380	the calling semantics of the returned C<$rpc> function.
117		381
118	If you want to pre-load the actual back-end modules to enable memory	382	If you want to pre-load the actual back-end modules to enable memory
119	sharing, then you should load C<AnyEvent::Fork::RPC::Sync> for	383	sharing, then you should load C<AnyEvent::Fork::RPC::Sync> for
120	synchronous, and C<AnyEvent::Fork::RPC::Async> for asynchronous mode.	384	synchronous, and C<AnyEvent::Fork::RPC::Async> for asynchronous mode.
		385
		386	If you use a template process and want to fork both sync and async
		387	children, then it is permissible to load both modules.
121		388
122	=item serialiser => $string (default: '(sub { pack "(w/a)", @_ }, sub { unpack "(w/a)", shift })')	389	=item serialiser => $string (default: '(sub { pack "(w/a)", @_ }, sub { unpack "(w/a)", shift })')
123		390
124	All arguments, result data and event data have to be serialised to be	391	All arguments, result data and event data have to be serialised to be
125	transferred between the processes. For this, they have to be frozen and	392	transferred between the processes. For this, they have to be frozen and
…		…
138	pre-load it into your L<AnyEvent::Fork> process, or you can add a C<use>	405	pre-load it into your L<AnyEvent::Fork> process, or you can add a C<use>
139	or C<require> statement into the serialiser string. Or both.	406	or C<require> statement into the serialiser string. Or both.
140		407
141	=back	408	=back
142		409
		410	See the examples section earlier in this document for some actual
		411	examples.
		412
143	=cut	413	=cut
144		414
145	our $STRING_SERIALISER = '(sub { pack "(w/a)", @_ }, sub { unpack "(w/a)", shift })';	415	our $STRING_SERIALISER = '(sub { pack "(w/a)", @_ }, sub { unpack "(w/a)", shift })';
146
147	# ideally, we want (SvLEN - SvCUR) \|\| 1024 or somesuch...
148	sub rlen($) { ($_[0] < 384 ? 512 + 16 : 2 << int +(log $_[0] + 512) / log 2) - $_[0] - 16 }
149		416
150	sub run {	417	sub run {
151	my ($self, $function, %arg) = @_;	418	my ($self, $function, %arg) = @_;
152		419
153	my $serialiser = delete $arg{serialiser} \|\| $STRING_SERIALISER;	420	my $serialiser = delete $arg{serialiser} \|\| $STRING_SERIALISER;
154	my $on_event = delete $arg{on_event};	421	my $on_event = delete $arg{on_event};
155	my $on_error = delete $arg{on_error};	422	my $on_error = delete $arg{on_error};
		423	my $on_destroy = delete $arg{on_destroy};
156		424
157	# default for on_error is to on_event, if specified	425	# default for on_error is to on_event, if specified
158	$on_error \|\|= $on_event	426	$on_error \|\|= $on_event
159	? sub { $on_event->(error => shift) }	427	? sub { $on_event->(error => shift) }
160	: sub { die "AnyEvent::Fork::RPC: uncaught error: $_[0].\n" };	428	: sub { die "AnyEvent::Fork::RPC: uncaught error: $_[0].\n" };
…		…
162	# default for on_event is to raise an error	430	# default for on_event is to raise an error
163	$on_event \|\|= sub { $on_error->("event received, but no on_event handler") };	431	$on_event \|\|= sub { $on_error->("event received, but no on_event handler") };
164		432
165	my ($f, $t) = eval $serialiser; die $@ if $@;	433	my ($f, $t) = eval $serialiser; die $@ if $@;
166		434
167	my (@rcb, $fh, $shutdown, $wbuf, $ww, $rbuf, $rw);	435	my (@rcb, %rcb, $fh, $shutdown, $wbuf, $ww);
		436	my ($rlen, $rbuf, $rw) = 512 - 16;
168		437
169	my $wcb = sub {	438	my $wcb = sub {
170	my $len = syswrite $fh, $wbuf;	439	my $len = syswrite $fh, $wbuf;
171		440
172	if (!defined $len) {	441	unless (defined $len) {
173	if ($! != Errno::EAGAIN && $! != Errno::EWOULDBLOCK) {	442	if ($! != Errno::EAGAIN && $! != Errno::EWOULDBLOCK) {
174	undef $rw; undef $ww; # it ends here	443	undef $rw; undef $ww; # it ends here
175	$on_error->("$!");	444	$on_error->("$!");
176	}	445	}
177	}	446	}
…		…
188		457
189	$self->require ($module)	458	$self->require ($module)
190	->send_arg ($function, $arg{init}, $serialiser)	459	->send_arg ($function, $arg{init}, $serialiser)
191	->run ("$module\::run", sub {	460	->run ("$module\::run", sub {
192	$fh = shift;	461	$fh = shift;
		462
		463	my ($id, $len);
193	$rw = AE::io $fh, 0, sub {	464	$rw = AE::io $fh, 0, sub {
		465	$rlen = $rlen * 2 + 16 if $rlen - 128 < length $rbuf;
194	my $len = sysread $fh, $rbuf, rlen length $rbuf, length $rbuf;	466	$len = sysread $fh, $rbuf, $rlen - length $rbuf, length $rbuf;
195		467
196	if ($len) {	468	if ($len) {
197	while (5 <= length $rbuf) {	469	while (8 <= length $rbuf) {
198	$len = unpack "L", $rbuf;	470	($id, $len) = unpack "LL", $rbuf;
199	4 + $len <= length $rbuf	471	8 + $len <= length $rbuf
200	or last;	472	or last;
201		473
202	my @r = $t->(substr $rbuf, 4, $len);	474	my @r = $t->(substr $rbuf, 8, $len);
203	substr $rbuf, 0, $len + 4, "";	475	substr $rbuf, 0, 8 + $len, "";
		476
		477	if ($id) {
		478	if (@rcb) {
		479	(shift @rcb)->(@r);
		480	} elsif (my $cb = delete $rcb{$id}) {
		481	$cb->(@r);
		482	} else {
		483	undef $rw; undef $ww;
		484	$on_error->("unexpected data from child");
204		485	}
205	if (pop @r) {	486	} else {
206	$on_event->(@r);	487	$on_event->(@r);
207	} elsif (@rcb) {
208	(shift @rcb)->(@r);
209	} else {
210	undef $rw; undef $ww;
211	$on_error->("unexpected data from child");
212	}	488	}
213	}	489	}
214	} elsif (defined $len) {	490	} elsif (defined $len) {
215	undef $rw; undef $ww; # it ends here	491	undef $rw; undef $ww; # it ends here
		492
		493	if (@rcb \|\| %rcb) {
216	$on_error->("unexpected eof")	494	$on_error->("unexpected eof");
217	if @rcb;	495	} else {
		496	$on_destroy->();
		497	}
218	} elsif ($! != Errno::EAGAIN && $! != Errno::EWOULDBLOCK) {	498	} elsif ($! != Errno::EAGAIN && $! != Errno::EWOULDBLOCK) {
219	undef $rw; undef $ww; # it ends here	499	undef $rw; undef $ww; # it ends here
220	$on_error->("read: $!");	500	$on_error->("read: $!");
221	}	501	}
222	};	502	};
…		…
227	my $guard = Guard::guard {	507	my $guard = Guard::guard {
228	$shutdown = 1;	508	$shutdown = 1;
229	$ww \|\|= $fh && AE::io $fh, 1, $wcb;	509	$ww \|\|= $fh && AE::io $fh, 1, $wcb;
230	};	510	};
231		511
		512	my $id;
		513
		514	$arg{async}
232	sub {	515	? sub {
233	push @rcb, pop;	516	$id = ($id == 0xffffffff ? 0 : $id) + 1;
		517	$id = ($id == 0xffffffff ? 0 : $id) + 1 while exists $rcb{$id}; # rarely loops
234		518
		519	$rcb{$id} = pop;
		520
235	$guard; # keep it alive	521	$guard; # keep it alive
236		522
237	$wbuf .= pack "L/a*", &$f;	523	$wbuf .= pack "LL/a*", $id, &$f;
238	$ww \|\|= $fh && AE::io $fh, 1, $wcb;	524	$ww \|\|= $fh && AE::io $fh, 1, $wcb;
239	}	525	}
		526	: sub {
		527	push @rcb, pop;
		528
		529	$guard; # keep it alive
		530
		531	$wbuf .= pack "L/a*", &$f;
		532	$ww \|\|= $fh && AE::io $fh, 1, $wcb;
		533	}
240	}	534	}
241		535
		536	=item $rpc->(..., $cb->(...))
		537
		538	The RPC object returned by C<AnyEvent::Fork::RPC::run> is actually a code
		539	reference. There are two things you can do with it: call it, and let it go
		540	out of scope (let it get destroyed).
		541
		542	If C<async> was false when C<$rpc> was created (the default), then, if you
		543	call C<$rpc>, the C<$function> is invoked with all arguments passed to
		544	C<$rpc> except the last one (the callback). When the function returns, the
		545	callback will be invoked with all the return values.
		546
		547	If C<async> was true, then the C<$function> receives an additional
		548	initial argument, the result callback. In this case, returning from
		549	C<$function> does nothing - the function only counts as "done" when the
		550	result callback is called, and any arguments passed to it are considered
		551	the return values. This makes it possible to "return" from event handlers
		552	or e.g. Coro threads.
		553
		554	The other thing that can be done with the RPC object is to destroy it. In
		555	this case, the child process will execute all remaining RPC calls, report
		556	their results, and then exit.
		557
		558	See the examples section earlier in this document for some actual
		559	examples.
		560
242	=back	561	=back
243		562
244	=head1 CHILD PROCESS USAGE	563	=head1 CHILD PROCESS USAGE
245		564
246	These functions are not available in this module. They are only available	565	The following function is not available in this module. They are only
247	in the namespace of this module when the child is running, without	566	available in the namespace of this module when the child is running,
248	having to load any extra module. They are part of the child-side API of	567	without having to load any extra modules. They are part of the child-side
249	L<AnyEvent::Fork::RPC>.	568	API of L<AnyEvent::Fork::RPC>.
250		569
251	=over 4	570	=over 4
252		571
253	=item AnyEvent::Fork::RPC::event ...	572	=item AnyEvent::Fork::RPC::event ...
254		573
255	Send an event to the parent. Events are a bit like RPC calls made by the	574	Send an event to the parent. Events are a bit like RPC calls made by the
256	child process to the parent, except that there is no notion of return	575	child process to the parent, except that there is no notion of return
257	values.	576	values.
258		577
		578	See the examples section earlier in this document for some actual
		579	examples.
		580
259	=back	581	=back
		582
		583	=head1 ADVANCED TOPICS
		584
		585	=head2 Choosing a backend
		586
		587	So how do you decide which backend to use? Well, that's your problem to
		588	solve, but here are some thoughts on the matter:
		589
		590	=over 4
		591
		592	=item Synchronous
		593
		594	The synchronous backend does not rely on any external modules (well,
		595	except L<common::sense>, which works around a bug in how perl's warning
		596	system works). This keeps the process very small, for example, on my
		597	system, an empty perl interpreter uses 1492kB RSS, which becomes 2020kB
		598	after C<use warnings; use strict> (for people who grew up with C64s around
		599	them this is probably shocking every single time they see it). The worker
		600	process in the first example in this document uses 1792kB.
		601
		602	Since the calls are done synchronously, slow jobs will keep newer jobs
		603	from executing.
		604
		605	The synchronous backend also has no overhead due to running an event loop
		606	- reading requests is therefore very efficient, while writing responses is
		607	less so, as every response results in a write syscall.
		608
		609	If the parent process is busy and a bit slow reading responses, the child
		610	waits instead of processing further requests. This also limits the amount
		611	of memory needed for buffering, as never more than one response has to be
		612	buffered.
		613
		614	The API in the child is simple - you just have to define a function that
		615	does something and returns something.
		616
		617	It's hard to use modules or code that relies on an event loop, as the
		618	child cannot execute anything while it waits for more input.
		619
		620	=item Asynchronous
		621
		622	The asynchronous backend relies on L<AnyEvent>, which tries to be small,
		623	but still comes at a price: On my system, the worker from example 1a uses
		624	3420kB RSS (for L<AnyEvent>, which loads L<EV>, which needs L<XSLoader>
		625	which in turn loads a lot of other modules such as L<warnings>, L<strict>,
		626	L<vars>, L<Exporter>...).
		627
		628	It batches requests and responses reasonably efficiently, doing only as
		629	few reads and writes as needed, but needs to poll for events via the event
		630	loop.
		631
		632	Responses are queued when the parent process is busy. This means the child
		633	can continue to execute any queued requests. It also means that a child
		634	might queue a lot of responses in memory when it generates them and the
		635	parent process is slow accepting them.
		636
		637	The API is not a straightforward RPC pattern - you have to call a
		638	"done" callback to pass return values and signal completion. Also, more
		639	importantly, the API starts jobs as fast as possible - when 1000 jobs
		640	are queued and the jobs are slow, they will all run concurrently. The
		641	child must implement some queueing/limiting mechanism if this causes
		642	problems. Alternatively, the parent could limit the amount of rpc calls
		643	that are outstanding.
		644
		645	Using event-based modules such as L<IO::AIO>, L<Gtk2>, L<Tk> and so on is
		646	easy.
		647
		648	=back
		649
		650	=head2 Passing file descriptors
		651
		652	Unlike L<AnyEvent::Fork>, this module has no in-built file handle or file
		653	descriptor passing abilities.
		654
		655	The reason is that passing file descriptors is extraordinary tricky
		656	business, and conflicts with efficient batching of messages.
		657
		658	There still is a method you can use: Create a
		659	C<AnyEvent::Util::portable_socketpair> and C<send_fh> one half of it to
		660	the process before you pass control to C<AnyEvent::Fork::RPC::run>.
		661
		662	Whenever you want to pass a file descriptor, send an rpc request to the
		663	child process (so it expects the descriptor), then send it over the other
		664	half of the socketpair. The child should fetch the descriptor from the
		665	half it has passed earlier.
		666
		667	Here is some (untested) pseudocode to that effect:
		668
		669	use AnyEvent::Util;
		670	use AnyEvent::Fork::RPC;
		671	use IO::FDPass;
		672
		673	my ($s1, $s2) = AnyEvent::Util::portable_socketpair;
		674
		675	my $rpc = AnyEvent::Fork
		676	->new
		677	->send_fh ($s2)
		678	->require ("MyWorker")
		679	->AnyEvent::Fork::RPC::run ("MyWorker::run"
		680	init => "MyWorker::init",
		681	);
		682
		683	undef $s2; # no need to keep it around
		684
		685	# pass an fd
		686	$rpc->("i'll send some fd now, please expect it!", my $cv = AE::cv);
		687
		688	IO::FDPass fileno $s1, fileno $handle_to_pass;
		689
		690	$cv->recv;
		691
		692	The MyWorker module could look like this:
		693
		694	package MyWorker;
		695
		696	use IO::FDPass;
		697
		698	my $s2;
		699
		700	sub init {
		701	$s2 = $_[0];
		702	}
		703
		704	sub run {
		705	if ($_[0] eq "i'll send some fd now, please expect it!") {
		706	my $fd = IO::FDPass::recv fileno $s2;
		707	...
		708	}
		709	}
		710
		711	Of course, this might be blocking if you pass a lot of file descriptors,
		712	so you might want to look into L<AnyEvent::FDpasser> which can handle the
		713	gory details.
260		714
261	=head1 SEE ALSO	715	=head1 SEE ALSO
262		716
263	L<AnyEvent::Fork> (to create the processes in the first place),	717	L<AnyEvent::Fork> (to create the processes in the first place),
264	L<AnyEvent::Fork::Pool> (to manage whole pools of processes).	718	L<AnyEvent::Fork::Pool> (to manage whole pools of processes).

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Revision 1.13 by root, Thu Apr 18 11:11:26 2013 UTC