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

Comparing AnyEvent-Fork-RPC/RPC.pm (file contents):
Revision 1.5 by root, Wed Apr 17 19:39:54 2013 UTC vs.
Revision 1.21 by root, Sun Apr 21 12:01:54 2013 UTC

…		…
1	=head1 NAME	1	=head1 NAME
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	THE API IS NOT FINISHED, CONSIDER THIS A TECHNOLOGY DEMO
		6
5	=head1 SYNOPSIS	7	=head1 SYNOPSIS
6		8
7	use AnyEvent::Fork;
8	use AnyEvent::Fork::RPC;	9	use AnyEvent::Fork::RPC;
		10	# use AnyEvent::Fork is not needed
9		11
10	my $rpc = AnyEvent::Fork	12	my $rpc = AnyEvent::Fork
11	->new	13	->new
12	->require ("MyModule")	14	->require ("MyModule")
13	->AnyEvent::Fork::RPC::run (	15	->AnyEvent::Fork::RPC::run (
14	"MyModule::server",	16	"MyModule::server",
15	);	17	);
16		18
		19	use AnyEvent;
		20
17	my $cv = AE::cv;	21	my $cv = AE::cv;
18		22
19	$rpc->(1, 2, 3, sub {	23	$rpc->(1, 2, 3, sub {
20	print "MyModule::server returned @_\n";	24	print "MyModule::server returned @_\n";
21	$cv->send;	25	$cv->send;
34	concurrently in the child, using AnyEvent.	38	concurrently in the child, using AnyEvent.
35		39
36	It also implements an asynchronous event mechanism from the child to the	40	It also implements an asynchronous event mechanism from the child to the
37	parent, that could be used for progress indications or other information.	41	parent, that could be used for progress indications or other information.
38		42
		43	Loading this module also always loads L<AnyEvent::Fork>, so you can make a
		44	separate C<use AnyEvent::Fork> if you wish, but you don't have to.
		45
39	=head1 EXAMPLES	46	=head1 EXAMPLES
40		47
41	=head2 Synchronous Backend	48	=head2 Example 1: Synchronous Backend
42		49
43	Here is a simple example that implements a backend that executes C<unlink>	50	Here is a simple example that implements a backend that executes C<unlink>
44	and C<rmdir> calls, and reports their status back. It also reports the	51	and C<rmdir> calls, and reports their status back. It also reports the
45	number of requests it has processed every three requests, which is clearly	52	number of requests it has processed every three requests, which is clearly
46	silly, but illustrates the use of events.	53	silly, but illustrates the use of events.
47		54
48	First the parent process:	55	First the parent process:
49		56
50	use AnyEvent;	57	use AnyEvent;
51	use AnyEvent::Fork;
52	use AnyEvent::Fork::RPC;	58	use AnyEvent::Fork::RPC;
53		59
54	my $done = AE::cv;	60	my $done = AE::cv;
55		61
56	my $rpc = AnyEvent::Fork	62	my $rpc = AnyEvent::Fork
…		…
103	dies with a fatal error - obviously, you must never let this happen :).	109	dies with a fatal error - obviously, you must never let this happen :).
104		110
105	Eventually it returns the status value true if the command was successful,	111	Eventually it returns the status value true if the command was successful,
106	or the status value 0 and the stringified error message.	112	or the status value 0 and the stringified error message.
107		113
108	On my system, running the first cdoe fragment with the given	114	On my system, running the first code fragment with the given
109	F<MyWorker.pm> in the current directory yields:	115	F<MyWorker.pm> in the current directory yields:
110		116
111	/tmp/somepath/1: No such file or directory	117	/tmp/somepath/1: No such file or directory
112	/tmp/somepath/2: No such file or directory	118	/tmp/somepath/2: No such file or directory
113	3 requests handled	119	3 requests handled
…		…
134		140
135	And as a final remark, there is a fine module on CPAN that can	141	And as a final remark, there is a fine module on CPAN that can
136	asynchronously C<rmdir> and C<unlink> and a lot more, and more efficiently	142	asynchronously C<rmdir> and C<unlink> and a lot more, and more efficiently
137	than this example, namely L<IO::AIO>.	143	than this example, namely L<IO::AIO>.
138		144
		145	=head3 Example 1a: the same with the asynchronous backend
		146
		147	This example only shows what needs to be changed to use the async backend
		148	instead. Doing this is not very useful, the purpose of this example is
		149	to show the minimum amount of change that is required to go from the
		150	synchronous to the asynchronous backend.
		151
		152	To use the async backend in the previous example, you need to add the
		153	C<async> parameter to the C<AnyEvent::Fork::RPC::run> call:
		154
		155	->AnyEvent::Fork::RPC::run ("MyWorker::run",
		156	async => 1,
		157	...
		158
		159	And since the function call protocol is now changed, you need to adopt
		160	C<MyWorker::run> to the async API.
		161
		162	First, you need to accept the extra initial C<$done> callback:
		163
		164	sub run {
		165	my ($done, $cmd, $path) = @_;
		166
		167	And since a response is now generated when C<$done> is called, as opposed
		168	to when the function returns, we need to call the C<$done> function with
		169	the status:
		170
		171	$done->($status or (0, "$!"));
		172
		173	A few remarks are in order. First, it's quite pointless to use the async
		174	backend for this example - but it I<is> possible. Second, you can call
		175	C<$done> before or after returning from the function. Third, having both
		176	returned from the function and having called the C<$done> callback, the
		177	child process may exit at any time, so you should call C<$done> only when
		178	you really I<are> done.
		179
		180	=head2 Example 2: Asynchronous Backend
		181
		182	This example implements multiple count-downs in the child, using
		183	L<AnyEvent> timers. While this is a bit silly (one could use timers in te
		184	parent just as well), it illustrates the ability to use AnyEvent in the
		185	child and the fact that responses can arrive in a different order then the
		186	requests.
		187
		188	It also shows how to embed the actual child code into a C<__DATA__>
		189	section, so it doesn't need any external files at all.
		190
		191	And when your parent process is often busy, and you have stricter timing
		192	requirements, then running timers in a child process suddenly doesn't look
		193	so silly anymore.
		194
		195	Without further ado, here is the code:
		196
		197	use AnyEvent;
		198	use AnyEvent::Fork::RPC;
		199
		200	my $done = AE::cv;
		201
		202	my $rpc = AnyEvent::Fork
		203	->new
		204	->require ("AnyEvent::Fork::RPC::Async")
		205	->eval (do { local $/; <DATA> })
		206	->AnyEvent::Fork::RPC::run ("run",
		207	async => 1,
		208	on_error => sub { warn "FATAL: $_[0]"; exit 1 },
		209	on_event => sub { print $_[0] },
		210	on_destroy => $done,
		211	);
		212
		213	for my $count (3, 2, 1) {
		214	$rpc->($count, sub {
		215	warn "job $count finished\n";
		216	});
		217	}
		218
		219	undef $rpc;
		220
		221	$done->recv;
		222
		223	__DATA__
		224
		225	# this ends up in main, as we don't use a package declaration
		226
		227	use AnyEvent;
		228
		229	sub run {
		230	my ($done, $count) = @_;
		231
		232	my $n;
		233
		234	AnyEvent::Fork::RPC::event "starting to count up to $count\n";
		235
		236	my $w; $w = AE::timer 1, 1, sub {
		237	++$n;
		238
		239	AnyEvent::Fork::RPC::event "count $n of $count\n";
		240
		241	if ($n == $count) {
		242	undef $w;
		243	$done->();
		244	}
		245	};
		246	}
		247
		248	The parent part (the one before the C<__DATA__> section) isn't very
		249	different from the earlier examples. It sets async mode, preloads
		250	the backend module (so the C<AnyEvent::Fork::RPC::event> function is
		251	declared), uses a slightly different C<on_event> handler (which we use
		252	simply for logging purposes) and then, instead of loading a module with
		253	the actual worker code, it C<eval>'s the code from the data section in the
		254	child process.
		255
		256	It then starts three countdowns, from 3 to 1 seconds downwards, destroys
		257	the rpc object so the example finishes eventually, and then just waits for
		258	the stuff to trickle in.
		259
		260	The worker code uses the event function to log some progress messages, but
		261	mostly just creates a recurring one-second timer.
		262
		263	The timer callback increments a counter, logs a message, and eventually,
		264	when the count has been reached, calls the finish callback.
		265
		266	On my system, this results in the following output. Since all timers fire
		267	at roughly the same time, the actual order isn't guaranteed, but the order
		268	shown is very likely what you would get, too.
		269
		270	starting to count up to 3
		271	starting to count up to 2
		272	starting to count up to 1
		273	count 1 of 3
		274	count 1 of 2
		275	count 1 of 1
		276	job 1 finished
		277	count 2 of 2
		278	job 2 finished
		279	count 2 of 3
		280	count 3 of 3
		281	job 3 finished
		282
		283	While the overall ordering isn't guaranteed, the async backend still
		284	guarantees that events and responses are delivered to the parent process
		285	in the exact same ordering as they were generated in the child process.
		286
		287	And unless your system is I<very> busy, it should clearly show that the
		288	job started last will finish first, as it has the lowest count.
		289
		290	This concludes the async example. Since L<AnyEvent::Fork> does not
		291	actually fork, you are free to use about any module in the child, not just
		292	L<AnyEvent>, but also L<IO::AIO>, or L<Tk> for example.
		293
139	=head1 PARENT PROCESS USAGE	294	=head1 PARENT PROCESS USAGE
140		295
141	This module exports nothing, and only implements a single function:	296	This module exports nothing, and only implements a single function:
142		297
143	=over 4	298	=over 4
…		…
150		305
151	use Errno ();	306	use Errno ();
152	use Guard ();	307	use Guard ();
153		308
154	use AnyEvent;	309	use AnyEvent;
155	#use AnyEvent::Fork;	310	use AnyEvent::Fork; # we don't actually depend on it, this is for convenience
156		311
157	our $VERSION = 0.1;	312	our $VERSION = 0.1;
158		313
159	=item my $rpc = AnyEvent::Fork::RPC::run $fork, $function, [key => value...]	314	=item my $rpc = AnyEvent::Fork::RPC::run $fork, $function, [key => value...]
160		315
…		…
198		353
199	Called when the C<$rpc> object has been destroyed and all requests have	354	Called when the C<$rpc> object has been destroyed and all requests have
200	been successfully handled. This is useful when you queue some requests and	355	been successfully handled. This is useful when you queue some requests and
201	want the child to go away after it has handled them. The problem is that	356	want the child to go away after it has handled them. The problem is that
202	the parent must not exit either until all requests have been handled, and	357	the parent must not exit either until all requests have been handled, and
203	this cna be accomplished by waiting for this callback.	358	this can be accomplished by waiting for this callback.
204		359
205	=item init => $function (default none)	360	=item init => $function (default none)
206		361
207	When specified (by name), this function is called in the child as the very	362	When specified (by name), this function is called in the child as the very
208	first thing when taking over the process, with all the arguments normally	363	first thing when taking over the process, with all the arguments normally
…		…
221		376
222	The default server used in the child does all I/O blockingly, and only	377	The default server used in the child does all I/O blockingly, and only
223	allows a single RPC call to execute concurrently.	378	allows a single RPC call to execute concurrently.
224		379
225	Setting C<async> to a true value switches to another implementation that	380	Setting C<async> to a true value switches to another implementation that
226	uses L<AnyEvent> in the child and allows multiple concurrent RPC calls.	381	uses L<AnyEvent> in the child and allows multiple concurrent RPC calls (it
		382	does not support recursion in the event loop however, blocking condvar
		383	calls will fail).
227		384
228	The actual API in the child is documented in the section that describes	385	The actual API in the child is documented in the section that describes
229	the calling semantics of the returned C<$rpc> function.	386	the calling semantics of the returned C<$rpc> function.
230		387
231	If you want to pre-load the actual back-end modules to enable memory	388	If you want to pre-load the actual back-end modules to enable memory
232	sharing, then you should load C<AnyEvent::Fork::RPC::Sync> for	389	sharing, then you should load C<AnyEvent::Fork::RPC::Sync> for
233	synchronous, and C<AnyEvent::Fork::RPC::Async> for asynchronous mode.	390	synchronous, and C<AnyEvent::Fork::RPC::Async> for asynchronous mode.
234		391
235	If you use a template process and want to fork both sync and async	392	If you use a template process and want to fork both sync and async
236	children, then it is permissible to laod both modules.	393	children, then it is permissible to load both modules.
237		394
238	=item serialiser => $string (default: '(sub { pack "(w/a)", @_ }, sub { unpack "(w/a)", shift })')	395	=item serialiser => $string (default: $AnyEvent::Fork::RPC::STRING_SERIALISER)
239		396
240	All arguments, result data and event data have to be serialised to be	397	All arguments, result data and event data have to be serialised to be
241	transferred between the processes. For this, they have to be frozen and	398	transferred between the processes. For this, they have to be frozen and
242	thawed in both parent and child processes.	399	thawed in both parent and child processes.
243		400
244	By default, only octet strings can be passed between the processes, which	401	By default, only octet strings can be passed between the processes, which
245	is reasonably fast and efficient.	402	is reasonably fast and efficient and requires no extra modules.
246		403
247	For more complicated use cases, you can provide your own freeze and thaw	404	For more complicated use cases, you can provide your own freeze and thaw
248	functions, by specifying a string with perl source code. It's supposed to	405	functions, by specifying a string with perl source code. It's supposed to
249	return two code references when evaluated: the first receives a list of	406	return two code references when evaluated: the first receives a list of
250	perl values and must return an octet string. The second receives the octet	407	perl values and must return an octet string. The second receives the octet
…		…
252		409
253	If you need an external module for serialisation, then you can either	410	If you need an external module for serialisation, then you can either
254	pre-load it into your L<AnyEvent::Fork> process, or you can add a C<use>	411	pre-load it into your L<AnyEvent::Fork> process, or you can add a C<use>
255	or C<require> statement into the serialiser string. Or both.	412	or C<require> statement into the serialiser string. Or both.
256		413
		414	Here are some examples - some of them are also available as global
		415	variables that make them easier to use.
		416
		417	=over 4
		418
		419	=item octet strings - C<$AnyEvent::Fork::RPC::STRING_SERIALISER>
		420
		421	This serialiser concatenates length-prefixes octet strings, and is the
		422	default.
		423
		424	Implementation:
		425
		426	(
		427	sub { pack "(w/a)", @_ },
		428	sub { unpack "(w/a)", shift }
		429	)
		430
		431	=item json - C<$AnyEvent::Fork::RPC::JSON_SERIALISER>
		432
		433	This serialiser creates JSON arrays - you have to make sure the L<JSON>
		434	module is installed for this serialiser to work. It can be beneficial for
		435	sharing when you preload the L<JSON> module in a template process.
		436
		437	L<JSON> (with L<JSON::XS> installed) is slower than the octet string
		438	serialiser, but usually much faster than L<Storable>, unless big chunks of
		439	binary data need to be transferred.
		440
		441	Implementation:
		442
		443	use JSON ();
		444	(
		445	sub { JSON::encode_json \@_ },
		446	sub { @{ JSON::decode_json shift } }
		447	)
		448
		449	=item storable - C<$AnyEvent::Fork::RPC::STORABLE_SERIALISER>
		450
		451	This serialiser uses L<Storable>, which means it has high chance of
		452	serialising just about anything you throw at it, at the cost of having
		453	very high overhead per operation. It also comes with perl.
		454
		455	Implementation:
		456
		457	use Storable ();
		458	(
		459	sub { Storable::freeze \@_ },
		460	sub { @{ Storable::thaw shift } }
		461	)
		462
257	=back	463	=back
258		464
		465	=back
		466
		467	See the examples section earlier in this document for some actual
		468	examples.
		469
259	=cut	470	=cut
260		471
261	our $STRING_SERIALISER = '(sub { pack "(w/a)", @_ }, sub { unpack "(w/a)", shift })';	472	our $STRING_SERIALISER = '(sub { pack "(w/a)", @_ }, sub { unpack "(w/a)", shift })';
		473	our $JSON_SERIALISER = 'use JSON (); (sub { JSON::encode_json \@_ }, sub { @{ JSON::decode_json shift } })';
		474	our $STORABLE_SERIALISER = 'use Storable (); (sub { Storable::freeze \@_ }, sub { @{ Storable::thaw shift } })';
262		475
263	sub run {	476	sub run {
264	my ($self, $function, %arg) = @_;	477	my ($self, $function, %arg) = @_;
265		478
266	my $serialiser = delete $arg{serialiser} \|\| $STRING_SERIALISER;	479	my $serialiser = delete $arg{serialiser} \|\| $STRING_SERIALISER;
…		…
276	# default for on_event is to raise an error	489	# default for on_event is to raise an error
277	$on_event \|\|= sub { $on_error->("event received, but no on_event handler") };	490	$on_event \|\|= sub { $on_error->("event received, but no on_event handler") };
278		491
279	my ($f, $t) = eval $serialiser; die $@ if $@;	492	my ($f, $t) = eval $serialiser; die $@ if $@;
280		493
281	my (@rcb, $fh, $shutdown, $wbuf, $ww, $rw);	494	my (@rcb, %rcb, $fh, $shutdown, $wbuf, $ww);
282	my ($rlen, $rbuf) = 512 - 16;	495	my ($rlen, $rbuf, $rw) = 512 - 16;
283		496
284	my $wcb = sub {	497	my $wcb = sub {
285	my $len = syswrite $fh, $wbuf;	498	my $len = syswrite $fh, $wbuf;
286		499
287	if (!defined $len) {	500	unless (defined $len) {
288	if ($! != Errno::EAGAIN && $! != Errno::EWOULDBLOCK) {	501	if ($! != Errno::EAGAIN && $! != Errno::EWOULDBLOCK) {
289	undef $rw; undef $ww; # it ends here	502	undef $rw; undef $ww; # it ends here
290	$on_error->("$!");	503	$on_error->("$!");
291	}	504	}
292	}	505	}
…		…
303		516
304	$self->require ($module)	517	$self->require ($module)
305	->send_arg ($function, $arg{init}, $serialiser)	518	->send_arg ($function, $arg{init}, $serialiser)
306	->run ("$module\::run", sub {	519	->run ("$module\::run", sub {
307	$fh = shift;	520	$fh = shift;
		521
		522	my ($id, $len);
308	$rw = AE::io $fh, 0, sub {	523	$rw = AE::io $fh, 0, sub {
309	$rlen = $rlen * 2 + 16 if $rlen - 128 < length $rbuf;	524	$rlen = $rlen * 2 + 16 if $rlen - 128 < length $rbuf;
310	my $len = sysread $fh, $rbuf, $rlen - length $rbuf, length $rbuf;	525	$len = sysread $fh, $rbuf, $rlen - length $rbuf, length $rbuf;
311		526
312	if ($len) {	527	if ($len) {
313	while (5 <= length $rbuf) {	528	while (8 <= length $rbuf) {
314	$len = unpack "L", $rbuf;	529	($id, $len) = unpack "LL", $rbuf;
315	4 + $len <= length $rbuf	530	8 + $len <= length $rbuf
316	or last;	531	or last;
317		532
318	my @r = $t->(substr $rbuf, 4, $len);	533	my @r = $t->(substr $rbuf, 8, $len);
319	substr $rbuf, 0, $len + 4, "";	534	substr $rbuf, 0, 8 + $len, "";
		535
		536	if ($id) {
		537	if (@rcb) {
		538	(shift @rcb)->(@r);
		539	} elsif (my $cb = delete $rcb{$id}) {
		540	$cb->(@r);
		541	} else {
		542	undef $rw; undef $ww;
		543	$on_error->("unexpected data from child");
320		544	}
321	if (pop @r) {	545	} else {
322	$on_event->(@r);	546	$on_event->(@r);
323	} elsif (@rcb) {
324	(shift @rcb)->(@r);
325	} else {
326	undef $rw; undef $ww;
327	$on_error->("unexpected data from child");
328	}	547	}
329	}	548	}
330	} elsif (defined $len) {	549	} elsif (defined $len) {
331	undef $rw; undef $ww; # it ends here	550	undef $rw; undef $ww; # it ends here
332		551
333	if (@rcb) {	552	if (@rcb \|\| %rcb) {
334	$on_error->("unexpected eof");	553	$on_error->("unexpected eof");
335	} else {	554	} else {
336	$on_destroy->();	555	$on_destroy->()
		556	if $on_destroy;
337	}	557	}
338	} elsif ($! != Errno::EAGAIN && $! != Errno::EWOULDBLOCK) {	558	} elsif ($! != Errno::EAGAIN && $! != Errno::EWOULDBLOCK) {
339	undef $rw; undef $ww; # it ends here	559	undef $rw; undef $ww; # it ends here
340	$on_error->("read: $!");	560	$on_error->("read: $!");
341	}	561	}
…		…
344	$ww \|\|= AE::io $fh, 1, $wcb;	564	$ww \|\|= AE::io $fh, 1, $wcb;
345	});	565	});
346		566
347	my $guard = Guard::guard {	567	my $guard = Guard::guard {
348	$shutdown = 1;	568	$shutdown = 1;
349	$ww \|\|= $fh && AE::io $fh, 1, $wcb;	569
		570	shutdown $fh, 1 if $fh && !$ww;
350	};	571	};
351		572
		573	my $id;
		574
		575	$arg{async}
352	sub {	576	? sub {
353	push @rcb, pop;	577	$id = ($id == 0xffffffff ? 0 : $id) + 1;
		578	$id = ($id == 0xffffffff ? 0 : $id) + 1 while exists $rcb{$id}; # rarely loops
354		579
		580	$rcb{$id} = pop;
		581
355	$guard; # keep it alive	582	$guard if 0; # keep it alive
356		583
357	$wbuf .= pack "L/a*", &$f;	584	$wbuf .= pack "LL/a*", $id, &$f;
358	$ww \|\|= $fh && AE::io $fh, 1, $wcb;	585	$ww \|\|= $fh && AE::io $fh, 1, $wcb;
359	}	586	}
		587	: sub {
		588	push @rcb, pop;
		589
		590	$guard; # keep it alive
		591
		592	$wbuf .= pack "L/a*", &$f;
		593	$ww \|\|= $fh && AE::io $fh, 1, $wcb;
		594	}
360	}	595	}
361		596
362	=item $rpc->(..., $cb->(...))	597	=item $rpc->(..., $cb->(...))
363		598
364	The RPC object returned by C<AnyEvent::Fork::RPC::run> is actually a code	599	The RPC object returned by C<AnyEvent::Fork::RPC::run> is actually a code
…		…
379		614
380	The other thing that can be done with the RPC object is to destroy it. In	615	The other thing that can be done with the RPC object is to destroy it. In
381	this case, the child process will execute all remaining RPC calls, report	616	this case, the child process will execute all remaining RPC calls, report
382	their results, and then exit.	617	their results, and then exit.
383		618
		619	See the examples section earlier in this document for some actual
		620	examples.
		621
384	=back	622	=back
385		623
386	=head1 CHILD PROCESS USAGE	624	=head1 CHILD PROCESS USAGE
387		625
388	The following function is not available in this module. They are only	626	The following function is not available in this module. They are only
…		…
396		634
397	Send an event to the parent. Events are a bit like RPC calls made by the	635	Send an event to the parent. Events are a bit like RPC calls made by the
398	child process to the parent, except that there is no notion of return	636	child process to the parent, except that there is no notion of return
399	values.	637	values.
400		638
		639	See the examples section earlier in this document for some actual
		640	examples.
		641
401	=back	642	=back
402		643
		644	=head1 ADVANCED TOPICS
		645
		646	=head2 Choosing a backend
		647
		648	So how do you decide which backend to use? Well, that's your problem to
		649	solve, but here are some thoughts on the matter:
		650
		651	=over 4
		652
		653	=item Synchronous
		654
		655	The synchronous backend does not rely on any external modules (well,
		656	except L<common::sense>, which works around a bug in how perl's warning
		657	system works). This keeps the process very small, for example, on my
		658	system, an empty perl interpreter uses 1492kB RSS, which becomes 2020kB
		659	after C<use warnings; use strict> (for people who grew up with C64s around
		660	them this is probably shocking every single time they see it). The worker
		661	process in the first example in this document uses 1792kB.
		662
		663	Since the calls are done synchronously, slow jobs will keep newer jobs
		664	from executing.
		665
		666	The synchronous backend also has no overhead due to running an event loop
		667	- reading requests is therefore very efficient, while writing responses is
		668	less so, as every response results in a write syscall.
		669
		670	If the parent process is busy and a bit slow reading responses, the child
		671	waits instead of processing further requests. This also limits the amount
		672	of memory needed for buffering, as never more than one response has to be
		673	buffered.
		674
		675	The API in the child is simple - you just have to define a function that
		676	does something and returns something.
		677
		678	It's hard to use modules or code that relies on an event loop, as the
		679	child cannot execute anything while it waits for more input.
		680
		681	=item Asynchronous
		682
		683	The asynchronous backend relies on L<AnyEvent>, which tries to be small,
		684	but still comes at a price: On my system, the worker from example 1a uses
		685	3420kB RSS (for L<AnyEvent>, which loads L<EV>, which needs L<XSLoader>
		686	which in turn loads a lot of other modules such as L<warnings>, L<strict>,
		687	L<vars>, L<Exporter>...).
		688
		689	It batches requests and responses reasonably efficiently, doing only as
		690	few reads and writes as needed, but needs to poll for events via the event
		691	loop.
		692
		693	Responses are queued when the parent process is busy. This means the child
		694	can continue to execute any queued requests. It also means that a child
		695	might queue a lot of responses in memory when it generates them and the
		696	parent process is slow accepting them.
		697
		698	The API is not a straightforward RPC pattern - you have to call a
		699	"done" callback to pass return values and signal completion. Also, more
		700	importantly, the API starts jobs as fast as possible - when 1000 jobs
		701	are queued and the jobs are slow, they will all run concurrently. The
		702	child must implement some queueing/limiting mechanism if this causes
		703	problems. Alternatively, the parent could limit the amount of rpc calls
		704	that are outstanding.
		705
		706	Blocking use of condvars is not supported.
		707
		708	Using event-based modules such as L<IO::AIO>, L<Gtk2>, L<Tk> and so on is
		709	easy.
		710
		711	=back
		712
		713	=head2 Passing file descriptors
		714
		715	Unlike L<AnyEvent::Fork>, this module has no in-built file handle or file
		716	descriptor passing abilities.
		717
		718	The reason is that passing file descriptors is extraordinary tricky
		719	business, and conflicts with efficient batching of messages.
		720
		721	There still is a method you can use: Create a
		722	C<AnyEvent::Util::portable_socketpair> and C<send_fh> one half of it to
		723	the process before you pass control to C<AnyEvent::Fork::RPC::run>.
		724
		725	Whenever you want to pass a file descriptor, send an rpc request to the
		726	child process (so it expects the descriptor), then send it over the other
		727	half of the socketpair. The child should fetch the descriptor from the
		728	half it has passed earlier.
		729
		730	Here is some (untested) pseudocode to that effect:
		731
		732	use AnyEvent::Util;
		733	use AnyEvent::Fork::RPC;
		734	use IO::FDPass;
		735
		736	my ($s1, $s2) = AnyEvent::Util::portable_socketpair;
		737
		738	my $rpc = AnyEvent::Fork
		739	->new
		740	->send_fh ($s2)
		741	->require ("MyWorker")
		742	->AnyEvent::Fork::RPC::run ("MyWorker::run"
		743	init => "MyWorker::init",
		744	);
		745
		746	undef $s2; # no need to keep it around
		747
		748	# pass an fd
		749	$rpc->("i'll send some fd now, please expect it!", my $cv = AE::cv);
		750
		751	IO::FDPass fileno $s1, fileno $handle_to_pass;
		752
		753	$cv->recv;
		754
		755	The MyWorker module could look like this:
		756
		757	package MyWorker;
		758
		759	use IO::FDPass;
		760
		761	my $s2;
		762
		763	sub init {
		764	$s2 = $_[0];
		765	}
		766
		767	sub run {
		768	if ($_[0] eq "i'll send some fd now, please expect it!") {
		769	my $fd = IO::FDPass::recv fileno $s2;
		770	...
		771	}
		772	}
		773
		774	Of course, this might be blocking if you pass a lot of file descriptors,
		775	so you might want to look into L<AnyEvent::FDpasser> which can handle the
		776	gory details.
		777
		778	=head1 EXCEPTIONS
		779
		780	There are no provisions whatsoever for catching exceptions at this time -
		781	in the child, exeptions might kill the process, causing calls to be lost
		782	and the parent encountering a fatal error. In the parent, exceptions in
		783	the result callback will not be caught and cause undefined behaviour.
		784
403	=head1 SEE ALSO	785	=head1 SEE ALSO
404		786
405	L<AnyEvent::Fork> (to create the processes in the first place),	787	L<AnyEvent::Fork>, to create the processes in the first place.
		788
406	L<AnyEvent::Fork::Pool> (to manage whole pools of processes).	789	L<AnyEvent::Fork::Pool>, to manage whole pools of processes.
407		790
408	=head1 AUTHOR AND CONTACT INFORMATION	791	=head1 AUTHOR AND CONTACT INFORMATION
409		792
410	Marc Lehmann <schmorp@schmorp.de>	793	Marc Lehmann <schmorp@schmorp.de>
411	http://software.schmorp.de/pkg/AnyEvent-Fork-RPC	794	http://software.schmorp.de/pkg/AnyEvent-Fork-RPC

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Revision 1.21 by root, Sun Apr 21 12:01:54 2013 UTC