Re: Mixing asynchronous data feed with synchronous program flow control (in Raku)

[holli]

Only FWIW and good measure.

Server:

use Cro::HTTP::Router;
use Cro::HTTP::Server;
use Cro::HTTP::Router::WebSocket;

my $application = route 
{
    get -> 
    {
        web-socket -> $incoming 
        {
            supply 
            {
                my Bool $Running;

                multi sub process-command( "RUN" ) 
                {
                    True, ( $Running ?? "Already running" !! "Starting
+ to run…" );
                }

                multi sub process-command( "STOP" ) 
                {
                    False, ( $Running ?? "Stopping…" !! "Not running" 
+);
                }

                multi sub process-command( $command ) 
                {
                    $Running, "Unknown command: $command";
                }

                emit "Connected… RUN|STOP\n" ~
                     "Service is { $Running ?? "already" !! "not" } ru
+nning"; 

                whenever $incoming -> $message 
                {
                    my $command = await $message.body-text;
                    say "Command: $command";
                    ($Running, my $answer) = process-command( $command
+ );
                    emit $answer;
                }
            }
        }
    }
}

my Cro::Service $service = Cro::HTTP::Server.new(
    :host<localhost>, :port<8080>, :$application
);

$service.start;

say "Started";

react whenever signal(SIGINT) {
    say "Killed";
    $service.stop;
    exit;
}

Client:

use Cro::WebSocket::Client;

say "Connecting...";

my $conn = try await Cro::WebSocket::Client.connect( 'ws://localhost:8
+080' );

say "WebSocket handshake failed!" and exit
    unless $conn;

say "Connected";

react 
{
    # It is guarantueed, only one whenever block in this react block c
+an be running at any given time
    # There are no race conditions, for $conn

    whenever $conn.messages -> $message 
    {
        say "Incoming: ", await $message.body;
    }

    # Currently this part only works for me since I patched in the clo
+sing - supply.
    # But I am confident my pull request will be accecpted in some for
+m
    # Maybe not with that exact name
    whenever $conn.closing -> $reason 
    {
        say "Closing: $reason";
        done;
    }

    # Without the above, one would have to check here for 
    # $conn.closed != True before calling `send`
    whenever Supply.interval(5) -> $tick 
    {
        my $command = ("RUN", "STOP").pick;
        say "Sending: $command";
        
        $conn.send( $command );

        CATCH { say "Error sending <$command>: {.message}" }
    }
};

[Your Mother]

Thank you very much. I’m interested in all (clean) approaches to this. What I’ve got, and haukex corrected, is working well but it’s a prototype still that is going to grow into quite a lot of code so I’m going to look into anything I have time to learn.

[jcb]

Have you considered POE? (Learning POE is on my to-do list.)

Or if other languages are an option and this needs to scale out to a large number of connections, have you considered Erlang/OTP? (Which, as I currently understand, effectively hides a multi-threaded event loop in the language runtime, but more-or-less forces you into a message-passing functional programming paradigm as a result. I have heard good things about it but have not yet used it for a non-trivial program myself.)

[Your Mother]

Those are good thoughts. I did quite a bit of POE back in the day. I don’t think I’ve used it in… 15 years? Which, strangely enough, was the same time I tried to pick up some Erlang. I failed. :P Partly because that was exactly when Perl got back up with Catalyst, DBIC, Plack… So I stopped needing to branch out.

POE isn’t a good match for the project. Javascript is probably the best fit for the stuff I’m trying to do right now with the explosion of promise/wait/async stuff in the core and Typescript and such. In fact, I rewrote a bunch of JS in Perl for my newest client because there is no Perl lib but plenty in other languages… :( The code is doing a LOT of other things though and I’m quite solid in Perl and a bit of a behind-the-times dilettante in JS. I’ve already prototyped something like 10 clients and a few dozen approaches and can jump in and out of testing approaches and such because of my familiarity with Perl… and I think it still is the best munging/prototyping language, maybe by a lot.