NAME
AnyEvent::Fork::RPC - simple RPC extension for AnyEvent::Fork
SYNOPSIS
use AnyEvent::Fork;
use AnyEvent::Fork::RPC;
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 or AnyEvent::Fork::Remote, 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.
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;
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 "ERROR: $_[0]"; exit 1 },
on_event => sub { warn "$_[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 the
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;
use AnyEvent::Fork::RPC;
my $done = AE::cv;
my $rpc = AnyEvent::Fork
->new
->require ("AnyEvent::Fork::RPC::Async")
->eval (do { local $/; <DATA> })
->AnyEvent::Fork::RPC::run ("run",
async => 1,
on_error => sub { warn "ERROR: $_[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.
Example 3: Asynchronous backend with Coro
With Coro you can create a nice asynchronous backend implementation by
defining an rpc server function that creates a new Coro thread for every
request that calls a function "normally", i.e. the parameters from the
parent process are passed to it, and any return values are returned to
the parent process, e.g.:
package My::Arith;
sub add {
return $_[0] + $_[1];
}
sub mul {
return $_[0] * $_[1];
}
sub run {
my ($done, $func, @arg) = @_;
Coro::async_pool {
$done->($func->(@arg));
};
}
The "run" function creates a new thread for every invocation, using the
first argument as function name, and calls the $done callback on it's
return values. This makes it quite natural to define the "add" and "mul"
functions to add or multiply two numbers and return the result.
Since this is the asynchronous backend, it's quite possible to define
RPC function that do I/O or wait for external events - their execution
will overlap as needed.
The above could be used like this:
my $rpc = AnyEvent::Fork
->new
->require ("MyWorker")
->AnyEvent::Fork::RPC::run ("My::Arith::run",
on_error => ..., on_event => ..., on_destroy => ...,
);
$rpc->(add => 1, 3, Coro::rouse_cb); say Coro::rouse_wait;
$rpc->(mul => 3, 2, Coro::rouse_cb); say Coro::rouse_wait;
The "say"'s will print 4 and 6.
Example 4: Forward AnyEvent::Log messages using "on_event"
This partial example shows how to use the "event" function to forward
AnyEvent::Log messages to the parent.
For this, the parent needs to provide a suitable "on_event":
->AnyEvent::Fork::RPC::run (
on_event => sub {
if ($_[0] eq "ae_log") {
my (undef, $level, $message) = @_;
AE::log $level, $message;
} else {
# other event types
}
},
)
In the child, as early as possible, the following code should
reconfigure AnyEvent::Log to log via "AnyEvent::Fork::RPC::event":
$AnyEvent::Log::LOG->log_cb (sub {
my ($timestamp, $orig_ctx, $level, $message) = @{+shift};
if (defined &AnyEvent::Fork::RPC::event) {
AnyEvent::Fork::RPC::event (ae_log => $level, $message);
} else {
warn "[$$ before init] $message\n";
}
});
There is an important twist - the "AnyEvent::Fork::RPC::event" function
is only defined when the child is fully initialised. If you redirect the
log messages in your "init" function for example, then the "event"
function might not yet be available. This is why the log callback checks
whether the fucntion is there using "defined", and only then uses it to
log the message.
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, then the error is reported with
loglevel "error" via "AE::log".
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.
done => $function (default "CORE::exit")
The function to call when the asynchronous backend detects an
end of file condition when reading from the communications
socket *and* there are no outstanding requests. It's ignored by
the synchronous backend.
By overriding this you can prolong the life of a RPC process
after e.g. the parent has exited by running the event loop in
the provided function (or simply calling it, for example, when
your child process uses EV you could provide EV::run as "done"
function).
Of course, in that case you are responsible for exiting at the
appropriate time and not returning from
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 (the "AnyEvent::Fork::RPC" distribution does
not provide these extra serialiser 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 - all of them are also available as
global variables that make them easier to use.
$AnyEvent::Fork::RPC::STRING_SERIALISER - octet strings only
This serialiser (currently the default) concatenates
length-prefixes octet strings, and is the default. That
means you can only pass (and return) strings containing
character codes 0-255.
The main advantages of this serialiser are the high speed
and that it doesn't need another module. The main
disadvantage is that you are very limited in what you can
pass - only octet strings.
Implementation:
(
sub { pack "(w/a*)*", @_ },
sub { unpack "(w/a*)*", shift }
)
$AnyEvent::Fork::RPC::CBOR_XS_SERIALISER - uses CBOR::XS
This serialiser creates CBOR::XS arrays - you have to make
sure the CBOR::XS module is installed for this serialiser to
work. It can be beneficial for sharing when you preload the
CBOR::XS module in a template process.
CBOR::XS is about as fast as the octet string serialiser,
but supports complex data structures (similar to JSON) and
is faster than any of the other serialisers. If you have the
CBOR::XS module available, it's the best choice.
The encoder enables "allow_sharing" (so this serialisation
method can encode cyclic and self-referencing data
structures).
Implementation:
use CBOR::XS ();
(
sub { CBOR::XS::encode_cbor_sharing \@_ },
sub { @{ CBOR::XS::decode_cbor shift } }
)
$AnyEvent::Fork::RPC::JSON_SERIALISER - uses JSON::XS or JSON
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 } }
)
$AnyEvent::Fork::RPC::STORABLE_SERIALISER - Storable
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. It should be used when you need to
serialise complex data structures.
Implementation:
use Storable ();
(
sub { Storable::freeze \@_ },
sub { @{ Storable::thaw shift } }
)
$AnyEvent::Fork::RPC::NSTORABLE_SERIALISER - portable Storable
This serialiser also uses Storable, but uses it's "network"
format to serialise data, which makes it possible to talk to
different perl binaries (for example, when talking to a
process created with AnyEvent::Fork::Remote).
Implementation:
use Storable ();
(
sub { Storable::nfreeze \@_ },
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.
Note: the event data, like any data send to the parent, might not be
sent immediatelly but queued for later sending, so there is no
guarantee that the event has been sent to the parent when the call
returns - when you e.g. exit directly after calling this function,
the parent might never receive the event.
PROCESS EXIT
If and when the child process exits depends on the backend and
configuration. Apart from explicit exits (e.g. by calling "exit") or
runtime conditions (uncaught exceptions, signals etc.), the backends
exit under these conditions:
Synchronous Backend
The synchronous backend is very simple: when the process waits for
another request to arrive and the writing side (usually in the
parent) is closed, it will exit normally, i.e. as if your main
program reached the end of the file.
That means that if your parent process exits, the RPC process will
usually exit as well, either because it is idle anyway, or because
it executes a request. In the latter case, you will likely get an
error when the RPc process tries to send the results to the parent
(because agruably, you shouldn't exit your parent while there are
still outstanding requests).
The process is usually quiescent when it happens, so it should
rarely be a problem, and "END" handlers can be used to clean up.
Asynchronous Backend
For the asynchronous backend, things are more complicated: Whenever
it listens for another request by the parent, it might detect that
the socket was closed (e.g. because the parent exited). It will sotp
listening for new requests and instead try to write out any
remaining data (if any) or simply check whether the socket can be
written to. After this, the RPC process is effectively done - no new
requests are incoming, no outstanding request data can be written
back.
Since chances are high that there are event watchers that the RPC
server knows nothing about (why else would one use the async backend
if not for the ability to register watchers?), the event loop would
often happily continue.
This is why the asynchronous backend explicitly calls "CORE::exit"
when it is done (under other circumstances, such as when there is an
I/O error and there is outstanding data to write, it will log a
fatal message via AnyEvent::Log, also causing the program to exit).
You can override this by specifying a function name to call via the
"done" parameter instead.
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 (in the main thread,
outside of e.g. Coro threads).
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;
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, exceptions 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::Remote, likewise, but helpful for remote processes.
AnyEvent::Fork::Pool, to manage whole pools of processes.
AUTHOR AND CONTACT INFORMATION
Marc Lehmann <
[email protected]>
http://software.schmorp.de/pkg/AnyEvent-Fork-RPC
