--- AnyEvent-Fork-RPC/RPC.pm 2013/04/17 21:48:35 1.9 +++ AnyEvent-Fork-RPC/RPC.pm 2013/04/18 13:11:12 1.14 @@ -41,7 +41,7 @@ =head1 EXAMPLES -=head2 Synchronous Backend +=head2 Example 1: Synchronous Backend Here is a simple example that implements a backend that executes C and C calls, and reports their status back. It also reports the @@ -51,7 +51,6 @@ First the parent process: use AnyEvent; - use AnyEvent::Fork; use AnyEvent::Fork::RPC; my $done = AE::cv; @@ -139,6 +138,155 @@ asynchronously C and C and a lot more, and more efficiently than this example, namely L. +=head3 Example 1a: the same with the asynchronous backend + +This example only shows what needs to be changed to use the async backend +instead. Doing this is not very useful, the purpose of this example is +to show the minimum amount of change that is required to go from the +synchronous to the asynchronous backend. + +To use the async backend in the previous example, you need to add the +C parameter to the C call: + + ->AnyEvent::Fork::RPC::run ("MyWorker::run", + async => 1, + ... + +And since the function call protocol is now changed, you need to adopt +C to the async API. + +First, you need to accept the extra initial C<$done> callback: + + sub run { + my ($done, $cmd, $path) = @_; + +And since a response is now generated when C<$done> is called, as opposed +to when the function returns, we need to call the C<$done> function with +the status: + + $done->($status or (0, "$!")); + +A few remarks are in order. First, it's quite pointless to use the async +backend for this example - but it I possible. Second, you can call +C<$done> before or after returning from the function. Third, having both +returned from the function and having called the C<$done> callback, the +child process may exit at any time, so you should call C<$done> only when +you really I done. + +=head2 Example 2: Asynchronous Backend + +This example implements multiple count-downs in the child, using +L timers. While this is a bit silly (one could use timers in te +parent just as well), it illustrates the ability to use AnyEvent in the +child and the fact that responses can arrive in a different order then the +requests. + +It also shows how to embed the actual child code into a C<__DATA__> +section, so it doesn't need any external files at all. + +And when your parent process is often busy, and you have stricter timing +requirements, then running timers in a child process suddenly doesn't look +so silly anymore. + +Without further ado, here is the code: + + use AnyEvent; + use AnyEvent::Fork::RPC; + + my $done = AE::cv; + + my $rpc = AnyEvent::Fork + ->new + ->require ("AnyEvent::Fork::RPC::Async") + ->eval (do { local $/; }) + ->AnyEvent::Fork::RPC::run ("run", + async => 1, + on_error => sub { warn "FATAL: $_[0]"; exit 1 }, + on_event => sub { print $_[0] }, + on_destroy => $done, + ); + + for my $count (3, 2, 1) { + $rpc->($count, sub { + warn "job $count finished\n"; + }); + } + + undef $rpc; + + $done->recv; + + __DATA__ + + # this ends up in main, as we don't use a package declaration + + use AnyEvent; + + sub run { + my ($done, $count) = @_; + + my $n; + + AnyEvent::Fork::RPC::event "starting to count up to $count\n"; + + my $w; $w = AE::timer 1, 1, sub { + ++$n; + + AnyEvent::Fork::RPC::event "count $n of $count\n"; + + if ($n == $count) { + undef $w; + $done->(); + } + }; + } + +The parent part (the one before the C<__DATA__> section) isn't very +different from the earlier examples. It sets async mode, preloads +the backend module (so the C function is +declared), uses a slightly different C handler (which we use +simply for logging purposes) and then, instead of loading a module with +the actual worker code, it C's the code from the data section in the +child process. + +It then starts three countdowns, from 3 to 1 seconds downwards, destroys +the rpc object so the example finishes eventually, and then just waits for +the stuff to trickle in. + +The worker code uses the event function to log some progress messages, but +mostly just creates a recurring one-second timer. + +The timer callback increments a counter, logs a message, and eventually, +when the count has been reached, calls the finish callback. + +On my system, this results in the following output. Since all timers fire +at roughly the same time, the actual order isn't guaranteed, but the order +shown is very likely what you would get, too. + + starting to count up to 3 + starting to count up to 2 + starting to count up to 1 + count 1 of 3 + count 1 of 2 + count 1 of 1 + job 1 finished + count 2 of 2 + job 2 finished + count 2 of 3 + count 3 of 3 + job 3 finished + +While the overall ordering isn't guaranteed, the async backend still +guarantees that events and responses are delivered to the parent process +in the exact same ordering as they were generated in the child process. + +And unless your system is I busy, it should clearly show that the +job started last will finish first, as it has the lowest count. + +This concludes the async example. Since L does not +actually fork, you are free to use about any module in the child, not just +L, but also L, or L for example. + =head1 PARENT PROCESS USAGE This module exports nothing, and only implements a single function: @@ -238,14 +386,14 @@ If you use a template process and want to fork both sync and async children, then it is permissible to load both modules. -=item serialiser => $string (default: '(sub { pack "(w/a*)*", @_ }, sub { unpack "(w/a*)*", shift })') +=item serialiser => $string (default: $AnyEvent::Fork::RPC::STRING_SERIALISER) All arguments, result data and event data have to be serialised to be transferred between the processes. For this, they have to be frozen and thawed in both parent and child processes. By default, only octet strings can be passed between the processes, which -is reasonably fast and efficient. +is reasonably fast and efficient and requires no extra modules. For more complicated use cases, you can provide your own freeze and thaw functions, by specifying a string with perl source code. It's supposed to @@ -257,6 +405,57 @@ pre-load it into your L process, or you can add a C or C statement into the serialiser string. Or both. +Here are some examples - some of them are also available as global +variables that make them easier to use. + +=over 4 + +=item octet strings - C<$AnyEvent::Fork::RPC::STRING_SERIALISER> + +This serialiser concatenates length-prefixes octet strings, and is the +default. + +Implementation: + + ( + sub { pack "(w/a*)*", @_ }, + sub { unpack "(w/a*)*", shift } + ) + +=item json - C<$AnyEvent::Fork::RPC::JSON_SERIALISER> + +This serialiser creates JSON arrays - you have to make sure the L +module is installed for this serialiser to work. It can be beneficial for +sharing when you preload the L module in a template process. + +L (with L installed) is slower than the octet string +serialiser, but usually much faster than L, unless big chunks of +binary data need to be transferred. + +Implementation: + + use JSON (); + ( + sub { JSON::encode_json \@_ }, + sub { @{ JSON::decode_json shift } } + ) + +=item storable - C<$AnyEvent::Fork::RPC::STORABLE_SERIALISER> + +This serialiser uses L, which means it has high chance of +serialising just about anything you throw at it, at the cost of having +very high overhead per operation. It also comes with perl. + +Implementation: + + use Storable (); + ( + sub { Storable::freeze \@_ }, + sub { @{ Storable::thaw shift } } + ) + +=back + =back See the examples section earlier in this document for some actual @@ -264,7 +463,9 @@ =cut -our $STRING_SERIALISER = '(sub { pack "(w/a*)*", @_ }, sub { unpack "(w/a*)*", shift })'; +our $STRING_SERIALISER = '(sub { pack "(w/a*)*", @_ }, sub { unpack "(w/a*)*", shift })'; +our $JSON_SERIALISER = 'use JSON (); (sub { JSON::encode_json \@_ }, sub { @{ JSON::decode_json shift } })'; +our $STORABLE_SERIALISER = 'use Storable (); (sub { Storable::freeze \@_ }, sub { @{ Storable::thaw shift } })'; sub run { my ($self, $function, %arg) = @_; @@ -343,7 +544,6 @@ undef $rw; undef $ww; # it ends here if (@rcb || %rcb) { - use Data::Dump;ddx[\@rcb,\%rcb];#d# $on_error->("unexpected eof"); } else { $on_destroy->(); @@ -433,6 +633,138 @@ =back +=head1 ADVANCED TOPICS + +=head2 Choosing a backend + +So how do you decide which backend to use? Well, that's your problem to +solve, but here are some thoughts on the matter: + +=over 4 + +=item Synchronous + +The synchronous backend does not rely on any external modules (well, +except L, which works around a bug in how perl's warning +system works). This keeps the process very small, for example, on my +system, an empty perl interpreter uses 1492kB RSS, which becomes 2020kB +after C (for people who grew up with C64s around +them this is probably shocking every single time they see it). The worker +process in the first example in this document uses 1792kB. + +Since the calls are done synchronously, slow jobs will keep newer jobs +from executing. + +The synchronous backend also has no overhead due to running an event loop +- reading requests is therefore very efficient, while writing responses is +less so, as every response results in a write syscall. + +If the parent process is busy and a bit slow reading responses, the child +waits instead of processing further requests. This also limits the amount +of memory needed for buffering, as never more than one response has to be +buffered. + +The API in the child is simple - you just have to define a function that +does something and returns something. + +It's hard to use modules or code that relies on an event loop, as the +child cannot execute anything while it waits for more input. + +=item Asynchronous + +The asynchronous backend relies on L, which tries to be small, +but still comes at a price: On my system, the worker from example 1a uses +3420kB RSS (for L, which loads L, which needs L +which in turn loads a lot of other modules such as L, L, +L, L...). + +It batches requests and responses reasonably efficiently, doing only as +few reads and writes as needed, but needs to poll for events via the event +loop. + +Responses are queued when the parent process is busy. This means the child +can continue to execute any queued requests. It also means that a child +might queue a lot of responses in memory when it generates them and the +parent process is slow accepting them. + +The API is not a straightforward RPC pattern - you have to call a +"done" callback to pass return values and signal completion. Also, more +importantly, the API starts jobs as fast as possible - when 1000 jobs +are queued and the jobs are slow, they will all run concurrently. The +child must implement some queueing/limiting mechanism if this causes +problems. Alternatively, the parent could limit the amount of rpc calls +that are outstanding. + +Using event-based modules such as L, L, L and so on is +easy. + +=back + +=head2 Passing file descriptors + +Unlike L, this module has no in-built file handle or file +descriptor passing abilities. + +The reason is that passing file descriptors is extraordinary tricky +business, and conflicts with efficient batching of messages. + +There still is a method you can use: Create a +C and C one half of it to +the process before you pass control to C. + +Whenever you want to pass a file descriptor, send an rpc request to the +child process (so it expects the descriptor), then send it over the other +half of the socketpair. The child should fetch the descriptor from the +half it has passed earlier. + +Here is some (untested) pseudocode to that effect: + + use AnyEvent::Util; + use AnyEvent::Fork::RPC; + use IO::FDPass; + + my ($s1, $s2) = AnyEvent::Util::portable_socketpair; + + my $rpc = AnyEvent::Fork + ->new + ->send_fh ($s2) + ->require ("MyWorker") + ->AnyEvent::Fork::RPC::run ("MyWorker::run" + init => "MyWorker::init", + ); + + undef $s2; # no need to keep it around + + # pass an fd + $rpc->("i'll send some fd now, please expect it!", my $cv = AE::cv); + + IO::FDPass fileno $s1, fileno $handle_to_pass; + + $cv->recv; + +The MyWorker module could look like this: + + package MyWorker; + + use IO::FDPass; + + my $s2; + + sub init { + $s2 = $_[0]; + } + + sub run { + if ($_[0] eq "i'll send some fd now, please expect it!") { + my $fd = IO::FDPass::recv fileno $s2; + ... + } + } + +Of course, this might be blocking if you pass a lot of file descriptors, +so you might want to look into L which can handle the +gory details. + =head1 SEE ALSO L (to create the processes in the first place),