=head1 NAME

AnyEvent::Task - Client/server-based asynchronous worker pool

=head1 SYNOPSIS 1: PASSWORD HASHING

=head2 Server

    use AnyEvent::Task;
    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


=head2 Client

    use AnyEvent::Task::Client;

    my $client = AnyEvent::Task::Client->new(
                   connect => ['unix/', '/tmp/anyevent-task.socket'],
                 );

    my $checkout; $checkout = $client->checkout( timeout => 5, );

    $checkout->hash('secret',
      sub {
        my ($checkout, $crypted) = @_;
        die "hashing process died: $@" if defined $@;

        print "Hashed password is $crypted\n";

        $checkout->verify($crypted,
          sub {
            my ($checkout, $result) = @_;
            print "Verify result is $result\n":
          });
      });




=head1 SYNOPSIS 2: DBI


=head2 Server

    use AnyEvent::Task::Server;
    use DBI;

    my $dbh;

    my $server = AnyEvent::Task::Server->new(
                   listen => ['unix/', '/tmp/anyevent-task.socket'],
                   setup => sub {
                     $dbh = DBI->connect(...);
                   },
                   interface => sub {
                     my ($method, @args) = @_;
                     $args[0] = $dbh->prepare_cached($args[0]) if defined $args[0];
                     $dbh->$method(@args);
                   },
                 );

    $server->run; # or AE::cv->recv


=head2 Client

    use AnyEvent::Task::Client;

    my $dbh_pool = AnyEvent::Task::Client->new(
                     connect => ['unix/', '/tmp/anyevent-task.socket'],
                   );

    my $username = 'jimmy';

    my $dbh = $dbh_pool->checkout;

    $dbh->selectrow_hashref(q{ SELECT email FROM user WHERE username = ? },
                            undef, $username,
      sub {
        my ($dbh, $row) = @_;
        die "DB lookup failed: $@" if defined $dbh;
        print "User's email is $row->{email}\n";
        ## Use same $dbh here if using transactions
      });

=head1 DESCRIPTION

B<WARNING:> This module's API may change without warning. Also, the docs are somewhat incomplete and out of date. I will be fixing this soonish.

The synopsis makes this module sounds much more complicated than it actually is. L<AnyEvent::Task> is a fork-on-demand but persistent-worker server (L<AnyEvent::Task::Server>) combined with an asynchronous interface to a request queue and pooled-worker client (L<AnyEvent::Task::Client>). Both client and server are of course built with L<AnyEvent> because it's awesome. However, workers can't use AnyEvent (yet).

A server is started with C<< AnyEvent::Task::Server->new >>. This should at least be passed the C<listen> and C<interface> arguments. Keep the returned server object around for as long as you want the server to be running. C<interface> is the code that should handle each request. See the interface section below for its specification. A C<setup> coderef can be passed in to run some code when a new worker is forked. A C<checkout_done> coderef can be passed in to run some code whenever a checkout is released (see below).

A client is started with C<< AnyEvent::Task::Client->new >>. You only need to pass C<connect> to this. Keep the returned client object around as long as you wish the client to be connected.

After both the server and client are initialised, each process must enter AnyEvent's "main loop" in some way, possibly just C<< AE::cv->recv >>.

In the client process, you may call the C<checkout> method on the client object. This checkout object can be used to run code on a remote worker process in a non-blocking manner. The C<checkout> method doesn't require any arguments, but C<timeout> is recommended.

You can treat a checkout object as an object that proxies its method calls to a worker process or a function that does the same. You pass the arguments to these method calls as an argument to the checkout object, followed by a callback as the last argument. This callback will be called once the worker process has returned the results. This callback will normally be passed two arguments, the checkout object and the return value. In the event of an exception thrown inside the worker, only the checkout object will be passed in and C<$@> will be set to the error message.



=head1 INTERFACE

There are two formats possible for the C<interface> option when creating a server. 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 C<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 C<interface> is a hash ref. This is a minor short-cut for method dispatch where the method invoked on the checkout object is the key to which coderef to be 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 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.

A future backwards compatible RPC protocol may use L<Storable> or something else, although note that you can already serialise an object with Storable manually, send the resulting string over the existing protocol, and then deserialise it in the worker.




=head1 STARTING THE SERVER

Technically, running the server and the client in the same process is possible, but is highly discouraged since the server will C<fork()> when the client desires a worker process. When this happens, all descriptors in use by the client and server are duped into the worker process. This will at least interfere with cleaning up (closing) these descriptors in the client. So after a C<fork()> the worker should close all descriptors except for its connection to the client and a pipe to the server which is used in order to detect a server shutdown (and then gracefully exit). Also, forking a busy client may be memory-inefficient.

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, C<AnyEvent::Task::Server::fork_task_server>. It can be passed the same arguments as the regular C<new> constructor:

    ## my ($keepalive_pipe, $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 this will fork a process which becomes the server, and that it will install a "keep-alive" pipe between the server and the client. This keep-alive pipe will be used by the server to detect when the client/parent process exits.

If C<AnyEvent::Task::Server::fork_task_server> is called in a void context, then the reference to this keep-alive pipe is pushed onto C<@AnyEvent::Task::Server::children_sockets>. Otherwise, the keep-alive pipe and the server's PID are returned. Closing the pipe will terminate the worker gracefully. Killing the PID will attempt to terminate the worker immediately.

Since this constructor forks 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 applications apply (see AnyEvent docs).


=head1 DESIGN

The first thing to realise is that each client maintains a "pool" of connections to worker processes. Every time a checkout is issued, it 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. Each checkout also maintains a queue of requests, so that as soon as this worker process is allocated, the requests are filled also on a first-come, first-server basis.

C<timeout> can be passed as a keyword argument to C<checkout>. Once a request is queued up on that checkout, a timer of C<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 however, the worker connection is terminated and an exception is thrown in the dynamic context of the callback (see L<Callback::Frame>). FIXME: document this better.

Note that since timeouts are associated with a checkout, the client process can be started before the server and as long as the server is started within C<timeout> seconds, 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 fill all queued checkouts. Because of this, you should usually install a L<Callback::Frame> C<catch> block to handle timeout errors gracefully (log something and send error message to the client if applicable).

Additionally, because of checkout queuing the maximum number of worker processes a client should attempt to obtain can be limited with the C<max_workers> argument when creating a client object. If there are more live checkouts than C<max_workers>, the remaining checkouts will have to wait until one of the other checkouts becomes available. Note that typically a request is issued as soon as the checkout is created and in this case the timer starts then, meaning that some checkouts may never be serviced if the system can't handle the load.

The C<min_workers> argument can be used to "pre-fork" some "hot-standby" worker processes when creating the client. The default is 2 though note that this may change (FIXME: consider if the default should be 0).



=head1 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, discrete 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.

AnyEvent::Task servers (currently) all obey a very specific implementation policy: They are kind of like CGI servers in that each process is guaranteed to be handling only one connection at once so it can perform blocking operations without worrying about holding up other connections.

Actually, since a single process can handle many requests in a row, the AnyEvent::Task server is more like a FastCGI server, except that while a client holds a checkout, it is guaranteed an exclusive lock on that process. 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.

Probably the most 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 the worker process should terminate. In the normal case, a client will just return the worker to its worker pool. A worker can request a shutdown when its parent server dies but can't outright refuse to accept commands until the client is good and ready.

Client process can be started and checkouts can be obtained before the server is even started. The client will continue to try to obtain worker processes until either the server starts or the checkout in question times out.

The client decides the timeout for each checkout and different clients can have different timeouts while connecting to the same server.

The client even decides how many minimum and maximum workers it will run at once. The server is really just a simple on-demand-forking server and most of the sophistication is in the asynchronous client.



=head1 SEE ALSO

L<The AnyEvent::Task github repo|https://github.com/hoytech/AnyEvent-Task>

There's about a million CPAN modules that do similar things.

This module is designed to be used in a non-blocking, process-based program on unix. Depending on your exact requirements you might find something else useful: L<Parallel::ForkManager>, L<Thread::Pool>, an HTTP server of some kind, &c.

If you're into AnyEvent, L<AnyEvent::DBI> and L<AnyEvent::Worker> (based on AnyEvent::DBI), and L<AnyEvent::ForkObject> send and receive commands from worker processes similar to this module. L<AnyEvent::Worker::Pool> also has an implementation of a worker pool.

If you're into POE there is L<POE::Component::Pool::DBI>, L<POEx::WorkerPool>, L<POE::Component::ResourcePool>, L<POE::Component::PreforkDispatch>, L<Cantella::Worker>, &c.



=head1 BUGS

This module is still being developed and there are still some important FIXMEs remaining. Please sit tight.




=head1 AUTHOR

Doug Hoyte, C<< <doug@hcsw.org> >>

=head1 COPYRIGHT & LICENSE

Copyright 2012 Doug Hoyte.

This module is licensed under the same terms as perl itself.







=cut

