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

Comparing AnyEvent-Fork-RPC/RPC.pm (file contents):
Revision 1.9 by root, Wed Apr 17 21:48:35 2013 UTC vs.
Revision 1.12 by root, Thu Apr 18 10:40:34 2013 UTC

…		…
39	Loading this module also always loads L<AnyEvent::Fork>, so you can make a	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.	40	separate C<use AnyEvent::Fork> if you wish, but you don't have to.
41		41
42	=head1 EXAMPLES	42	=head1 EXAMPLES
43		43
44	=head2 Synchronous Backend	44	=head2 Example 1: Synchronous Backend
45		45
46	Here is a simple example that implements a backend that executes C<unlink>	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	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	48	number of requests it has processed every three requests, which is clearly
49	silly, but illustrates the use of events.	49	silly, but illustrates the use of events.
50		50
51	First the parent process:	51	First the parent process:
52		52
53	use AnyEvent;	53	use AnyEvent;
54	use AnyEvent::Fork;
55	use AnyEvent::Fork::RPC;	54	use AnyEvent::Fork::RPC;
56		55
57	my $done = AE::cv;	56	my $done = AE::cv;
58		57
59	my $rpc = AnyEvent::Fork	58	my $rpc = AnyEvent::Fork
…		…
136	ensures that perl will correctly interpret calls to it.	135	ensures that perl will correctly interpret calls to it.
137		136
138	And as a final remark, there is a fine module on CPAN that can	137	And as a final remark, there is a fine module on CPAN that can
139	asynchronously C<rmdir> and C<unlink> and a lot more, and more efficiently	138	asynchronously C<rmdir> and C<unlink> and a lot more, and more efficiently
140	than this example, namely L<IO::AIO>.	139	than this example, namely L<IO::AIO>.
		140
		141	=head3 Example 1a: the same with the asynchronous backend
		142
		143	This example only shows what needs to be changed to use the async backend
		144	instead. Doing this is not very useful, the purpose of this example is
		145	to show the minimum amount of change that is required to go from the
		146	synchronous to the asynchronous backend.
		147
		148	To use the async backend in the previous example, you need to add the
		149	C<async> parameter to the C<AnyEvent::Fork::RPC::run> call:
		150
		151	->AnyEvent::Fork::RPC::run ("MyWorker::run",
		152	async => 1,
		153	...
		154
		155	And since the function call protocol is now changed, you need to adopt
		156	C<MyWorker::run> to the async API.
		157
		158	First, you need to accept the extra initial C<$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.
141		289
142	=head1 PARENT PROCESS USAGE	290	=head1 PARENT PROCESS USAGE
143		291
144	This module exports nothing, and only implements a single function:	292	This module exports nothing, and only implements a single function:
145		293
…		…
431	See the examples section earlier in this document for some actual	579	See the examples section earlier in this document for some actual
432	examples.	580	examples.
433		581
434	=back	582	=back
435		583
		584	=head1 ADVANCED TOPICS
		585
		586	=head2 Choosing a backend
		587
		588	So how do you decide which backend to use? Well, that's your problem to
		589	solve, but here are some thoughts on the matter:
		590
		591	=over 4
		592
		593	=item Synchronous
		594
		595	The synchronous backend does not rely on any external modules (well,
		596	except L<common::sense>, which works around a bug in how perl's warning
		597	system works). This keeps the process very small, for example, on my
		598	system, an empty perl interpreter uses 1492kB RSS, which becomes 2020kB
		599	after C<use warnings; use strict> (for people who grew up with C64s around
		600	them this is probably shocking every single time they see it). The worker
		601	process in the first example in this document uses 1792kB.
		602
		603	Since the calls are done synchronously, slow jobs will keep newer jobs
		604	from executing.
		605
		606	The synchronous backend also has no overhead due to running an event loop
		607	- reading requests is therefore very efficient, while writing responses is
		608	less so, as every response results in a write syscall.
		609
		610	If the parent process is busy and a bit slow reading responses, the child
		611	waits instead of processing further requests. This also limits the amount
		612	of memory needed for buffering, as never more than one response has to be
		613	buffered.
		614
		615	The API in the child is simple - you just have to define a function that
		616	does something and returns something.
		617
		618	It's hard to use modules or code that relies on an event loop, as the
		619	child cannot execute anything while it waits for more input.
		620
		621	=item Asynchronous
		622
		623	The asynchronous backend relies on L<AnyEvent>, which tries to be small,
		624	but still comes at a price: On my system, the worker from example 1a uses
		625	3420kB RSS (for L<AnyEvent>, which loads L<EV>, which needs L<XSLoader>
		626	which in turn loads a lot of other modules such as L<warnings>, L<strict>,
		627	L<vars>, L<Exporter>...).
		628
		629	It batches requests and responses reasonably efficiently, doing only as
		630	few reads and writes as needed, but needs to poll for events via the event
		631	loop.
		632
		633	Responses are queued when the parent process is busy. This means the child
		634	can continue to execute any queued requests. It also means that a child
		635	might queue a lot of responses in memory when it generates them and the
		636	parent process is slow accepting them.
		637
		638	The API is not a straightforward RPC pattern - you have to call a
		639	"done" callback to pass return values and signal completion. Also, more
		640	importantly, the API starts jobs as fast as possible - when 1000 jobs
		641	are queued and the jobs are slow, they will all run concurrently. The
		642	child must implement some queueing/limiting mechanism if this causes
		643	problems. Alternatively, the parent could limit the amount of rpc calls
		644	that are outstanding.
		645
		646	Using event-based modules such as L<IO::AIO>, L<Gtk2>, L<Tk> and so on is
		647	easy.
		648
		649	=back
		650
		651	=head2 Passing file descriptors
		652
		653	Unlike L<AnyEvent::Fork>, this module has no in-built file handle or file
		654	descriptor passing abilities.
		655
		656	The reason is that passing file descriptors is extraordinary tricky
		657	business, and conflicts with efficient batching of messages.
		658
		659	There still is a method you can use: Create a
		660	C<AnyEvent::Util::portable_socketpair> and C<send_fh> one half of it to
		661	the process before you pass control to C<AnyEvent::Fork::RPC::run>.
		662
		663	Whenever you want to pass a file descriptor, send an rpc request to the
		664	child process (so it expects the descriptor), then send it over the other
		665	half of the socketpair. The child should fetch the descriptor from the
		666	half it has passed earlier.
		667
		668	Here is some (untested) pseudocode to that effect:
		669
		670	use AnyEvent::Util;
		671	use AnyEvent::Fork::RPC;
		672	use IO::FDPass;
		673
		674	my ($s1, $s2) = AnyEvent::Util::portable_socketpair;
		675
		676	my $rpc = AnyEvent::Fork
		677	->new
		678	->send_fh ($s2)
		679	->require ("MyWorker")
		680	->AnyEvent::Fork::RPC::run ("MyWorker::run"
		681	init => "MyWorker::init",
		682	);
		683
		684	undef $s2; # no need to keep it around
		685
		686	# pass an fd
		687	$rpc->("i'll send some fd now, please expect it!", my $cv = AE::cv);
		688
		689	IO::FDPass fileno $s1, fileno $handle_to_pass;
		690
		691	$cv->recv;
		692
		693	The MyWorker module could look like this:
		694
		695	package MyWorker;
		696
		697	use IO::FDPass;
		698
		699	my $s2;
		700
		701	sub init {
		702	$s2 = $_[0];
		703	}
		704
		705	sub run {
		706	if ($_[0] eq "i'll send some fd now, please expect it!") {
		707	my $fd = IO::FDPass::recv fileno $s2;
		708	...
		709	}
		710	}
		711
		712	Of course, this might be blocking if you pass a lot of file descriptors,
		713	so you might want to look into L<AnyEvent::FDpasser> which can handle the
		714	gory details.
		715
436	=head1 SEE ALSO	716	=head1 SEE ALSO
437		717
438	L<AnyEvent::Fork> (to create the processes in the first place),	718	L<AnyEvent::Fork> (to create the processes in the first place),
439	L<AnyEvent::Fork::Pool> (to manage whole pools of processes).	719	L<AnyEvent::Fork::Pool> (to manage whole pools of processes).
440		720

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Comparing AnyEvent-Fork-RPC/RPC.pm (file contents): Revision 1.9 by root, Wed Apr 17 21:48:35 2013 UTC vs. Revision 1.12 by root, Thu Apr 18 10:40:34 2013 UTC

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Comparing AnyEvent-Fork-RPC/RPC.pm (file contents):
Revision 1.9 by root, Wed Apr 17 21:48:35 2013 UTC vs.
Revision 1.12 by root, Thu Apr 18 10:40:34 2013 UTC