NAME AnyEvent::Fork::RPC - simple RPC extension for AnyEvent::Fork THE API IS NOT FINISHED, CONSIDER THIS A BETA RELEASE SYNOPSIS use AnyEvent::Fork::RPC; # use AnyEvent::Fork is not needed my $rpc = AnyEvent::Fork ->new ->require ("MyModule") ->AnyEvent::Fork::RPC::run ( "MyModule::server", ); use AnyEvent; my $cv = AE::cv; $rpc->(1, 2, 3, sub { print "MyModule::server returned @_\n"; $cv->send; }); $cv->recv; DESCRIPTION This module implements a simple RPC protocol and backend for processes created via AnyEvent::Fork, allowing you to call a function in the child process and receive its return values (up to 4GB serialised). It implements two different backends: a synchronous one that works like a normal function call, and an asynchronous one that can run multiple jobs concurrently in the child, using AnyEvent. It also implements an asynchronous event mechanism from the child to the parent, that could be used for progress indications or other information. Loading this module also always loads AnyEvent::Fork, so you can make a separate "use AnyEvent::Fork" if you wish, but you don't have to. EXAMPLES Example 1: Synchronous Backend Here is a simple example that implements a backend that executes "unlink" and "rmdir" calls, and reports their status back. It also reports the number of requests it has processed every three requests, which is clearly silly, but illustrates the use of events. First the parent process: use AnyEvent; use AnyEvent::Fork::RPC; my $done = AE::cv; my $rpc = AnyEvent::Fork ->new ->require ("MyWorker") ->AnyEvent::Fork::RPC::run ("MyWorker::run", on_error => sub { warn "FATAL: $_[0]"; exit 1 }, on_event => sub { warn "$_[0] requests handled\n" }, on_destroy => $done, ); for my $id (1..6) { $rpc->(rmdir => "/tmp/somepath/$id", sub { $_[0] or warn "/tmp/somepath/$id: $_[1]\n"; }); } undef $rpc; $done->recv; The parent creates the process, queues a few rmdir's. It then forgets about the $rpc object, so that the child exits after it has handled the requests, and then it waits till the requests have been handled. The child is implemented using a separate module, "MyWorker", shown here: package MyWorker; my $count; sub run { my ($cmd, $path) = @_; AnyEvent::Fork::RPC::event ($count) unless ++$count % 3; my $status = $cmd eq "rmdir" ? rmdir $path : $cmd eq "unlink" ? unlink $path : die "fatal error, illegal command '$cmd'"; $status or (0, "$!") } 1 The "run" function first sends a "progress" event every three calls, and then executes "rmdir" or "unlink", depending on the first parameter (or dies with a fatal error - obviously, you must never let this happen :). Eventually it returns the status value true if the command was successful, or the status value 0 and the stringified error message. On my system, running the first code fragment with the given MyWorker.pm in the current directory yields: /tmp/somepath/1: No such file or directory /tmp/somepath/2: No such file or directory 3 requests handled /tmp/somepath/3: No such file or directory /tmp/somepath/4: No such file or directory /tmp/somepath/5: No such file or directory 6 requests handled /tmp/somepath/6: No such file or directory Obviously, none of the directories I am trying to delete even exist. Also, the events and responses are processed in exactly the same order as they were created in the child, which is true for both synchronous and asynchronous backends. Note that the parentheses in the call to "AnyEvent::Fork::RPC::event" are not optional. That is because the function isn't defined when the code is compiled. You can make sure it is visible by pre-loading the correct backend module in the call to "require": ->require ("AnyEvent::Fork::RPC::Sync", "MyWorker") Since the backend module declares the "event" function, loading it first ensures that perl will correctly interpret calls to it. And as a final remark, there is a fine module on CPAN that can asynchronously "rmdir" and "unlink" and a lot more, and more efficiently than this example, namely IO::AIO. 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 "async" parameter to the "AnyEvent::Fork::RPC::run" call: ->AnyEvent::Fork::RPC::run ("MyWorker::run", async => 1, ... And since the function call protocol is now changed, you need to adopt "MyWorker::run" to the async API. First, you need to accept the extra initial $done callback: sub run { my ($done, $cmd, $path) = @_; And since a response is now generated when $done is called, as opposed to when the function returns, we need to call the $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 *is* possible. Second, you can call $done before or after returning from the function. Third, having both returned from the function and having called the $done callback, the child process may exit at any time, so you should call $done only when you really *are* done. Example 2: Asynchronous Backend This example implements multiple count-downs in the child, using AnyEvent 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 "__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 "__DATA__" section) isn't very different from the earlier examples. It sets async mode, preloads the backend module (so the "AnyEvent::Fork::RPC::event" function is declared), uses a slightly different "on_event" handler (which we use simply for logging purposes) and then, instead of loading a module with the actual worker code, it "eval"'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 *very* 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 AnyEvent::Fork does not actually fork, you are free to use about any module in the child, not just AnyEvent, but also IO::AIO, or Tk for example. PARENT PROCESS USAGE This module exports nothing, and only implements a single function: my $rpc = AnyEvent::Fork::RPC::run $fork, $function, [key => value...] The traditional way to call it. But it is way cooler to call it in the following way: my $rpc = $fork->AnyEvent::Fork::RPC::run ($function, [key => value...]) This "run" function/method can be used in place of the AnyEvent::Fork::run method. Just like that method, it takes over the AnyEvent::Fork process, but instead of calling the specified $function directly, it runs a server that accepts RPC calls and handles responses. It returns a function reference that can be used to call the function in the child process, handling serialisation and data transfers. The following key/value pairs are allowed. It is recommended to have at least an "on_error" or "on_event" handler set. on_error => $cb->($msg) Called on (fatal) errors, with a descriptive (hopefully) message. If this callback is not provided, but "on_event" is, then the "on_event" callback is called with the first argument being the string "error", followed by the error message. If neither handler is provided it prints the error to STDERR and will start failing badly. on_event => $cb->(...) Called for every call to the "AnyEvent::Fork::RPC::event" function in the child, with the arguments of that function passed to the callback. Also called on errors when no "on_error" handler is provided. on_destroy => $cb->() Called when the $rpc object has been destroyed and all requests have been successfully handled. This is useful when you queue some requests and want the child to go away after it has handled them. The problem is that the parent must not exit either until all requests have been handled, and this can be accomplished by waiting for this callback. init => $function (default none) When specified (by name), this function is called in the child as the very first thing when taking over the process, with all the arguments normally passed to the "AnyEvent::Fork::run" function, except the communications socket. It can be used to do one-time things in the child such as storing passed parameters or opening database connections. It is called very early - before the serialisers are created or the $function name is resolved into a function reference, so it could be used to load any modules that provide the serialiser or function. It can not, however, create events. async => $boolean (default: 0) The default server used in the child does all I/O blockingly, and only allows a single RPC call to execute concurrently. Setting "async" to a true value switches to another implementation that uses AnyEvent in the child and allows multiple concurrent RPC calls (it does not support recursion in the event loop however, blocking condvar calls will fail). The actual API in the child is documented in the section that describes the calling semantics of the returned $rpc function. If you want to pre-load the actual back-end modules to enable memory sharing, then you should load "AnyEvent::Fork::RPC::Sync" for synchronous, and "AnyEvent::Fork::RPC::Async" for asynchronous mode. If you use a template process and want to fork both sync and async children, then it is permissible to load both modules. 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 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 return two code references when evaluated: the first receives a list of perl values and must return an octet string. The second receives the octet string and must return the original list of values. If you need an external module for serialisation, then you can either pre-load it into your AnyEvent::Fork process, or you can add a "use" or "require" 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. octet strings - $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 } ) json - $AnyEvent::Fork::RPC::JSON_SERIALISER This serialiser creates JSON arrays - you have to make sure the JSON module is installed for this serialiser to work. It can be beneficial for sharing when you preload the JSON module in a template process. JSON (with JSON::XS installed) is slower than the octet string serialiser, but usually much faster than Storable, unless big chunks of binary data need to be transferred. Implementation: use JSON (); ( sub { JSON::encode_json \@_ }, sub { @{ JSON::decode_json shift } } ) storable - $AnyEvent::Fork::RPC::STORABLE_SERIALISER This serialiser uses Storable, 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 } } ) See the examples section earlier in this document for some actual examples. $rpc->(..., $cb->(...)) The RPC object returned by "AnyEvent::Fork::RPC::run" is actually a code reference. There are two things you can do with it: call it, and let it go out of scope (let it get destroyed). If "async" was false when $rpc was created (the default), then, if you call $rpc, the $function is invoked with all arguments passed to $rpc except the last one (the callback). When the function returns, the callback will be invoked with all the return values. If "async" was true, then the $function receives an additional initial argument, the result callback. In this case, returning from $function does nothing - the function only counts as "done" when the result callback is called, and any arguments passed to it are considered the return values. This makes it possible to "return" from event handlers or e.g. Coro threads. The other thing that can be done with the RPC object is to destroy it. In this case, the child process will execute all remaining RPC calls, report their results, and then exit. See the examples section earlier in this document for some actual examples. CHILD PROCESS USAGE The following function is not available in this module. They are only available in the namespace of this module when the child is running, without having to load any extra modules. They are part of the child-side API of AnyEvent::Fork::RPC. AnyEvent::Fork::RPC::event ... Send an event to the parent. Events are a bit like RPC calls made by the child process to the parent, except that there is no notion of return values. See the examples section earlier in this document for some actual examples. ADVANCED TOPICS 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: Synchronous The synchronous backend does not rely on any external modules (well, except common::sense, 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 "use warnings; use strict" (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. Asynchronous The asynchronous backend relies on AnyEvent, which tries to be small, but still comes at a price: On my system, the worker from example 1a uses 3420kB RSS (for AnyEvent, which loads EV, which needs XSLoader which in turn loads a lot of other modules such as warnings, strict, vars, Exporter...). 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. Blocking use of condvars is not supported. Using event-based modules such as IO::AIO, Gtk2, Tk and so on is easy. Passing file descriptors Unlike AnyEvent::Fork, 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 "AnyEvent::Util::portable_socketpair" and "send_fh" one half of it to the process before you pass control to "AnyEvent::Fork::RPC::run". 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 AnyEvent::FDpasser which can handle the gory details. EXCEPTIONS There are no provisions whatsoever for catching exceptions at this time - in the child, exeptions might kill the process, causing calls to be lost and the parent encountering a fatal error. In the parent, exceptions in the result callback will not be caught and cause undefined behaviour. SEE ALSO AnyEvent::Fork, to create the processes in the first place. AnyEvent::Fork::Pool, to manage whole pools of processes. AUTHOR AND CONTACT INFORMATION Marc Lehmann http://software.schmorp.de/pkg/AnyEvent-Fork-RPC