NAME
AnyEvent::Task - Client/server-based asynchronous worker pool
SYNOPSIS 1: PASSWORD HASHING
Server
use AnyEvent::Task::Server;
use Authen::Passphrase::BlowfishCrypt;
my $dev_urandom;
my $server = AnyEvent::Task::Server->new(
listen => ['unix/', '/tmp/anyevent-task.socket'],
setup => sub {
open($dev_urandom, "/dev/urandom") || die "open urandom: $!";
},
interface => {
hash => sub {
my ($plaintext) = @_;
read($dev_urandom, my $salt, 16) == 16 || die "bad read from urandom";
return Authen::Passphrase::BlowfishCrypt->new(cost => 10,
salt => $salt,
passphrase => $plaintext)
->as_crypt;
},
verify => sub {
my ($crypted, $plaintext) = @_;
return Authen::Passphrase::BlowfishCrypt->from_crypt($crypted)
->match($plaintext);
},
},
);
$server->run; # or AE::cv->recv
Client
use AnyEvent::Task::Client;
my $client = AnyEvent::Task::Client->new(
connect => ['unix/', '/tmp/anyevent-task.socket'],
);
my $checkout = $client->checkout( timeout => 5, );
my $cv = AE::cv;
$checkout->hash('secret',
sub {
my ($checkout, $crypted) = @_;
print "Hashed password is $crypted\n";
$checkout->verify($crypted, 'secret',
sub {
my ($checkout, $result) = @_;
print "Verify result is $result\n";
$cv->send;
});
});
$cv->recv;
Output
Hashed password is $2a$10$NwTOwxmTlG0Lk8YZMT29/uysC9RiZX4jtWCx.deBbb2evRjCq6ovi
Verify result is 1
SYNOPSIS 2: DBI
Server
use AnyEvent::Task::Server;
use DBI;
my $dbh;
AnyEvent::Task::Server->new(
listen => ['unix/', '/tmp/anyevent-task.socket'],
setup => sub {
$dbh = DBI->connect("dbi:SQLite:dbname=/tmp/junk.sqlite3","","",{ RaiseError => 1, });
},
interface => sub {
my ($method, @args) = @_;
$dbh->$method(@args);
},
)->run;
Client
use AnyEvent::Task::Client;
my $client = AnyEvent::Task::Client->new(
connect => ['unix/', '/tmp/anyevent-task.socket'],
);
my $dbh = $client->checkout;
my $cv = AE::cv;
$dbh->do(q{ CREATE TABLE user(username TEXT PRIMARY KEY, email TEXT); },
sub { });
## Requests will queue up on the checkout and execute in order:
$dbh->do(q{ INSERT INTO user (username, email) VALUES (?, ?) },
undef, 'jimmy',
'jimmy@example.com',
sub { });
$dbh->selectrow_hashref(q{ SELECT * FROM user }, sub {
my ($dbh, $user) = @_;
print "username: $user->{username}, email: $user->{email}\n";
$cv->send;
});
$cv->recv;
Output
username: jimmy, email: jimmy@example.com
DESCRIPTION
The synopses make this module look much more complicated than it
actually is. In a nutshell, a synchronous worker process is forked off
by a server whenever a client asks for one. The client keeps as many of
these workers around as it wants and delegates tasks to them
asynchronously.
Another way of saying that is that AnyEvent::Task is a
pre-fork-on-demand server (AnyEvent::Task::Server) combined with a
persistent worker-pooled client (AnyEvent::Task::Client).
The examples in the synopses are complete stand-alone programs. Run the
server in one window and the client in another. The server will remain
running but the client will exit after printing its output. Typically
the "client" programs would be embedded in a server program such as a
web-server.
Note that the client examples don't implement error checking (see the
"ERROR HANDLING" section).
A server is started with "AnyEvent::Task::Server->new". This constructor
should be passed in at least the "listen" and "interface" arguments.
Keep the returned server object around for as long as you want the
server to be running. "listen" is an array ref containing the host and
service options to be passed to AnyEvent::Socket's "tcp_server"
function. "interface" is the code that should handle each request. See
the INTERFACE section below for its specification. A "setup" coderef can
be passed in to run some code after a new worker is forked. A
"checkout_done" coderef can be passed in to run some code whenever a
checkout is released in order to perform any required clean-up.
A client is started with "AnyEvent::Task::Client->new". You only need to
pass "connect" to this constructor which is an array ref containing the
host and service options to be passed to AnyEvent::Socket's
"tcp_connect". Keep the returned client object around as long as you
wish the client to be connected.
After the server and client are initialised, each process must enter
AnyEvent's "main loop" in some way, possibly just "AE::cv->recv". The
"run" method on the server object is a convenient short-cut for this.
To acquire a worker process you call the "checkout" method on the client
object. The "checkout" method doesn't need any arguments, but several
optional ones such as "timeout" are described below. As long as the
checkout object is around, this checkout has exclusive access to the
worker.
The checkout object is an object that proxies its method calls to a
worker process or a function that does the same. The arguments to this
method/function are the arguments you wish to send to the worker process
followed by a callback to run when the operation completes. The callback
will be passed two arguments: the original checkout object and the value
returned by the worker process. The checkout object is passed into the
callback as a convenience just in case you no longer have the original
checkout available lexically.
In the event of an exception thrown by the worker process, a timeout, or
some other unexpected condition, an error is raised in the dynamic
context of the callback (see the "ERROR HANDLING" section).
DESIGN
Both client and server are of course built with AnyEvent. However,
workers can't use AnyEvent (yet). I've never found a need to do event
processing in the worker since if the library you wish to use is already
AnyEvent-compatible you can simply use the library in the client
process. If the client process is too over-loaded, it may make sense to
run multiple client processes.
Each client maintains a "pool" of connections to worker processes. Every
time a checkout is requested, the request is placed into a first-come,
first-serve queue. Once a worker process becomes available, it is
associated with that checkout until that checkout is garbage collected
which in perl means as soon as it is no longer needed. Each checkout
also maintains a queue of requested method-calls so that as soon as a
worker process is allocated to a checkout, any queued method calls are
filled in order.
"timeout" can be passed as a keyword argument to "checkout". Once a
request is queued up on that checkout, a timer of "timout" seconds
(default is 30, undef means infinity) is started. If the request
completes during this timeframe, the timer is cancelled. If the timer
expires, the worker connection is terminated and an exception is thrown
in the dynamic context of the callback (see the "ERROR HANDLING"
section).
Note that since timeouts are associated with a checkout, checkouts can
be created before the server is started. As long as the server is
running within "timeout" seconds, no error will be thrown and no
requests will be lost. The client will continually try to acquire worker
processes until a server is available, and once one is available it will
attempt to allocate all queued checkouts.
Because of checkout queuing, the maximum number of worker processes a
client will attempt to obtain can be limited with the "max_workers"
argument when creating a client object. If there are more live checkouts
than "max_workers", the remaining checkouts will have to wait until one
of the other workers becomes available. Because of timeouts, some
checkouts may never be serviced if the system can't handle the load (the
timeout error should be handled to indicate the service is temporarily
unavailable).
The "min_workers" argument determines how many "hot-standby" workers
should be pre-forked when creating the client. The default is 2 though
note that this may change to 0 in the future.
STARTING THE SERVER
Typically you will want to start the client and server as completely
separate processes as shown in the synopses.
Running the server and the client in the same process is technically
possible but is highly discouraged since the server will "fork()" when
the client demands a new worker process. In this case, all descriptors
in use by the client are duped into the worker process and the worker
ought to close these extra descriptors. Also, forking a busy client may
be memory-inefficient (and dangerous if it uses threads).
Since it's more of a bother than it's worth to run the server and the
client in the same process, there is an alternate server constructor,
"AnyEvent::Task::Server::fork_task_server" for when you'd like to fork a
dedicated server process. It can be passed the same arguments as the
regular "new" constructor:
## my ($keepalive_pipe, $server_pid) =
AnyEvent::Task::Server::fork_task_server(
listen => ['unix/', '/tmp/anyevent-task.socket'],
interface => sub {
return "Hello from PID $$";
},
);
The only differences between this and the regular constructor is that
"fork_task_server" will fork a process which becomes the server and will
also install a "keep-alive" pipe between the server and the client. This
keep-alive pipe will be used by the server to detect when its parent
(the client process) exits.
If "AnyEvent::Task::Server::fork_task_server" is called in a void
context then the reference to this keep-alive pipe is pushed onto
@AnyEvent::Task::Server::children_sockets. Otherwise, the keep-alive
pipe and the server's PID are returned. Closing the pipe will terminate
the server gracefully. "kill" the PID to terminate it immediately. Note
that even when the server is shutdown, existing worker processes and
checkouts may still be active in the client. The client object and all
checkout objects should be destroyed if you wish to ensure all workers
are shutdown.
Since the "fork_task_server" constructor calls fork and requires using
AnyEvent in both the parent and child processes, it is important that
you not install any AnyEvent watchers before calling it. The usual
caveats about forking AnyEvent processes apply (see the AnyEvent docs).
You should also not call "fork_task_server" after having started threads
since, again, this function calls fork. Forking a threaded process is
dangerous because the threads might have userspace data-structures in
inconsistent states at the time of the fork.
INTERFACE
When creating a server, there are two possible formats for the
"interface" option. The first and most general is a coderef. This
coderef will be passed the list of arguments that were sent when the
checkout was called in the client process (without the trailing callback
of course).
As described above, you can use a checkout object as a coderef or as an
object with methods. If the checkout is invoked as an object, the method
name is prepended to the arguments passed to "interface":
interface => sub {
my ($method, @args) = @_;
},
If the checkout is invoked as a coderef, method is omitted:
interface => sub {
my (@args) = @_;
},
The second format possible for "interface" is a hash ref. This is a
simple method dispatch feature where the method invoked on the checkout
object is the key used to lookup which coderef to run in the worker:
interface => {
method1 => sub {
my (@args) = @_;
},
method2 => sub {
my (@args) = @_;
},
},
Note that since the protocol between the client and the worker process
is currently JSON-based, all arguments and return values must be
serializable to JSON. This includes most perl scalars like strings, a
limited range of numerical types, and hash/list constructs with no
cyclical references.
Because there isn't any way for the callback to indicate the context it
desires, interface subs are always called in scalar context.
A future backwards compatible RPC protocol may use Sereal. Although it's
inefficient you can already serialise an object with Sereal manually,
send the resulting string over the existing protocol, and then
deserialise it in the worker.
LOGGING
Because workers run in a separate process, they can't directly use
logging contexts in the client process. That is why this module is
integrated with Log::Defer.
A Log::Defer object is created on demand in the worker process. Once the
worker is done an operation, any messages in the object will be
extracted and sent back to the client. The client then merges this into
its main Log::Defer object that was passed in when creating the
checkout.
In your server code, use AnyEvent::Task::Logger. It exports the function
"logger" which returns a Log::Defer object:
use AnyEvent::Task::Server;
use AnyEvent::Task::Logger;
AnyEvent::Task::Server->new(
listen => ['unix/', '/tmp/anyevent-task.socket'],
interface => sub {
logger->info('about to compute some operation');
{
my $timer = logger->timer('computing some operation');
select undef,undef,undef, 1; ## sleep for 1 second
}
},
)->run;
Note: Portable server code should never call "sleep" because on some
systems it will interfere with the recoverable worker timeout feature
implemented with "SIGALRM".
In your client code, pass a Log::Defer object in when you create a
checkout:
use AnyEvent::Task::Client;
use Log::Defer;
my $client = AnyEvent::Task::Client->new(
connect => ['unix/', '/tmp/anyevent-task.socket'],
);
my $log_defer_object = Log::Defer->new(sub {
my $msg = shift;
use Data::Dumper; ## or whatever
print Dumper($msg);
});
$log_defer_object->info('going to compute some operation in a worker');
my $checkout = $client->checkout(log_defer_object => $log_defer_object);
my $cv = AE::cv;
$checkout->(sub {
$log_defer_object->info('finished some operation');
$cv->send;
});
$cv->recv;
When run, the above client will print something like this:
$VAR1 = {
'start' => '1363232705.96839',
'end' => '1.027309',
'logs' => [
[
'0.000179',
30,
'going to compute some operation in a worker'
],
[
'0.023881061050415',
30,
'about to compute some operation'
],
[
'1.025965',
30,
'finished some operation'
]
],
'timers' => {
'computing some operation' => [
'0.024089061050415',
'1.02470206105041'
]
}
};
ERROR HANDLING
In a synchronous program, if you expected some operation to throw an
exception you might wrap it in "eval" like this:
my $crypted;
eval {
$crypted = hash('secret');
};
if ($@) {
say "hash failed: $@";
} else {
say "hashed password is $crypted";
}
But in an asynchronous program, typically "hash" would initiate some
kind of asynchronous operation and then return immediately, allowing the
program to go about other tasks while waiting for the result. Since the
error might come back at any time in the future, the program needs a way
to map the exception that is thrown back to the original context.
AnyEvent::Task accomplishes this mapping with Callback::Frame.
Callback::Frame lets you preserve error handlers (and "local" variables)
across asynchronous callbacks. Callback::Frame is not tied to
AnyEvent::Task, AnyEvent or any other async framework and can be used
with almost all callback-based libraries.
However, when using AnyEvent::Task, libraries that you use in the client
must be AnyEvent compatible. This restriction obviously does not apply
to your server code, that being the main purpose of this module:
accessing blocking resources from an asynchronous program. In your
server code, when there is an error condition you should simply "die" or
"croak" as in a synchronous program.
As an example usage of Callback::Frame, here is how we would handle
errors thrown from a worker process running the "hash" method in an
asychronous client program:
use Callback::Frame;
frame(code => sub {
$client->checkout->hash('secret', sub {
my ($checkout, $crypted) = @_;
say "Hashed password is $crypted";
});
}, catch => sub {
my $back_trace = shift;
say "Error is: $@";
say "Full back-trace: $back_trace";
})->(); ## <-- frame is created and then immediately executed
Of course if "hash" is something like a bcrypt hash function it is
unlikely to raise an exception so maybe that's a bad example. On the
other hand, maybe it's a really good example: In addition to errors that
occur while running your callbacks, AnyEvent::Task uses Callback::Frame
to throw errors if the worker process times out, so if the bcrypt "cost"
is really cranked up it might hit the default 30 second time limit.
Rationale for Callback::Frame
Why not just call the callback but set $@ and indicate an error has
occurred? This is the approach taken with AnyEvent::DBI for example. I
believe the Callback::Frame interface is superior to this method. In a
synchronous program, exceptions are out-of-band messages and code
doesn't need to locally handle them. It can let them "bubble up" the
stack, perhaps to a top-level error handler. Invoking the callback when
an error occurs forces exceptions to be handled in-band.
How about having AnyEvent::Task expose an error callback? This is the
approach taken by AnyEvent::Handle for example. I believe
Callback::Frame is superior to this method also. Although separate
callbacks are (sort of) out-of-band, you still have to write error
handler callbacks and do something relevant locally instead of allowing
the exception to bubble up to an error handler.
In servers, Callback::Frame helps you maintain the "dynamic state"
(error handlers and dynamic variables) installed for a single
connection. In other words, any errors that occur while servicing that
connection will be able to be caught by an error handler specific to
that connection. This lets you send an error response to the client and
collect associated log messages in a Log::Defer object specific to that
connection.
Callback::Frame provides an error handler stack so you can have a
top-level handler as well as nested handlers (similar to nested
"eval"s). This is useful when you wish to have a top-level "bail-out"
error handler and also nested error handlers that know how to retry or
recover from an error in an async sub-operation.
Callback::Frame is designed to be easily used with callback-based
libraries that don't know about Callback::Frame. "fub" is a shortcut for
"frame" with just the "code" argument. Instead of passing "sub { ... }"
into libraries you can pass in "fub { ... }". When invoked, this wrapped
callback will first re-establish any error handlers that you installed
with "frame" and then run your provided code. Libraries that force
in-band error signalling can be handled with callbacks such as "fub {
die $@ if $@; ... }". Separate error callbacks should simply be "fub {
die "failed becase ..." }".
It's important that all callbacks be created with "fub" (or "frame")
even if you don't expect them to fail so that the dynamic context is
preserved for nested callbacks that may. An exception is the callbacks
provided to AnyEvent::Task checkouts: These are automatically wrapped in
frames for you (although explicitly passing in fubs is fine too).
The Callback::Frame documentation explains how this works in much more
detail.
Reforking of workers after errors
If a worker throws an error, the client receives the error but the
worker process stays running. As long as the client has a reference to
the checkout (and as long as the exception wasn't "fatal" -- see below),
it can still be used to communicate with that worker so you can access
error states, rollback transactions, or do any sort of required
clean-up.
However, once the checkout object is destroyed, by default the worker
will be shutdown instead of returning to the client's worker pool as in
the normal case where no errors were thrown. This is a "safe-by-default"
behaviour that may help in the event that an exception thrown by a
worker leaves the worker process in a broken/inconsistent state for some
reason (for example a DBI connection died). This can be overridden by
setting the "dont_refork_after_error" option to 1 in the client
constructor. This will only matter if errors are being thrown frequently
and your "setup" routines take a long time (aside from the setup
routine, creating new workers is quite fast since the server has already
compiled all the application code and just has to fork).
There are cases where workers will never be returned to the worker pool:
workers that have thrown fatal errors such as loss of worker connection
or hung worker timeout errors. These errors are stored in the checkout
and for as long as the checkout exists any methods on the checkout will
immediately return the stored fatal error. Your client process can
invoke this behaviour manually by calling the "throw_fatal_error" method
on a checkout object to cancel an operation and force-terminate a
worker.
Another reason that a worker might not be returned to the worker pool is
if it has been checked out "max_checkouts" times. If "max_checkouts" is
specified as an argument to the Client constructor, then workers will be
destroyed and reforked after being checked out this number of times.
When not specified, workers are never re-forked for this reason. This
parameter is useful for coping with libraries that leak memory or
otherwise become slower/more resource-hungry over time.
COMPARISON WITH HTTP
Why a custom protocol, client, and server? Can't we just use something
like HTTP?
It depends.
AnyEvent::Task clients send discrete messages and receive ordered
replies from workers, much like HTTP. The AnyEvent::Task protocol can be
extended in a backwards-compatible manner like HTTP. AnyEvent::Task
communication can be pipelined and possibly in the future even
compressed like HTTP.
The current AnyEvent::Task server obeys a very specific implementation
policy: It is like a CGI server in that each process it forks is
guaranteed to be handling only one connection at once so it can perform
blocking operations without worrying about holding up other connections.
But since a single process can handle many requests in a row without
exiting, they are more like persistent FastCGI processes. The difference
however is that while a client holds a checkout it is guaranteed an
exclusive lock on that process (useful for supporting DB transactions
for example). With a FastCGI server it is assumed that requests are
stateless so you can't necessarily be sure you'll get the same process
for two consecutive requests. In fact, if an error is thrown in the
FastCGI handler you may never get the same process back again,
preventing you from being able to recover from the error, retry, or at
least collect process state for logging reasons.
The fundamental difference between the AnyEvent::Task protocol and HTTP
is that in AnyEvent::Task the client is the dominant protocol
orchestrator whereas in HTTP it is the server.
In AnyEvent::Task, the client manages the worker pool and the client
decides if/when worker processes should terminate. In the normal case, a
client will just return the worker to its worker pool. A worker is
supposed to accept commands for as long as possible until the client
dismisses it.
The client decides the timeout for each checkout and different clients
can have different timeouts while connecting to the same server.
Client processes can be started and checkouts can be obtained before the
server is even started. The client will continue trying to connect to
the server to obtain worker processes until either the server starts or
the checkout's timeout period lapses. As well as freeing you from having
to start your services in the "right" order, this also means servers can
be restarted without throwing any errors (aka "zero-downtime restarts").
The client even decides how many minimum workers should be in the pool
upon start-up and how many maximum workers to acquire before checkout
creation requests are queued. The server is really just a dumb
fork-on-demand server and most of the sophistication is in the
asynchronous client.
SEE ALSO
The AnyEvent::Task github repo
<https://github.com/hoytech/AnyEvent-Task>
In order to handle exceptions in a meaningful way with this module, you
must use Callback::Frame. In order to maintain seamless request logging
across clients and workers, you should use Log::Defer.
There are many modules on CPAN similar to AnyEvent::Task.
This module is designed to be used in a non-blocking, process-based unix
program. Depending on your exact requirements you might find something
else useful: Parallel::ForkManager, Thread::Pool, or an HTTP server of
some kind.
If you're into AnyEvent, AnyEvent::DBI and AnyEvent::Worker (based on
AnyEvent::DBI), AnyEvent::ForkObject, and AnyEvent::Fork::RPC send and
receive commands from worker processes similar to this module.
AnyEvent::Worker::Pool also has an implementation of a worker pool.
AnyEvent::Gearman can interface with Gearman services.
If you're into POE there is POE::Component::Pool::DBI, POEx::WorkerPool,
POE::Component::ResourcePool, POE::Component::PreforkDispatch,
Cantella::Worker.
BUGS
Although this module's interface is now stable and has been in
production use for some time, there are few remaining TODO items (see
the bottom of Task.pm).
AUTHOR
Doug Hoyte, "<doug@hcsw.org>"
COPYRIGHT & LICENSE
Copyright 2012-2015 Doug Hoyte.
This module is licensed under the same terms as perl itself.