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Dancer2::Plugin::WebSocket - add a websocket interface to your Dancers app


version 0.3.1



    #!/usr/bin/env perl

    use strict;
    use warnings;

    use FindBin;
    use lib "$FindBin::Bin/../lib";

    use Plack::Builder;

    use MyApp;

    builder {
        mount( MyApp->websocket_mount );
        mount '/' => MyApp->to_app;


            # default values
            serializer: 0
            login: 0
            mount_path: /ws

  package MyApp;

  use Dancer2;
  use Dancer2::Plugin::WebSocket;

  websocket_on_message sub {
    my( $conn, $message ) = @_;
    $conn->send( $message . ' world!' );

  get '/' => sub {
    my $ws_url = websocket_url;
    return <<"END";
            var urlMySocket = "$ws_url";

            var mySocket = new WebSocket(urlMySocket);

            mySocket.onmessage = function (evt) {
              console.log( "Got message " + );

            mySocket.onopen = function(evt) {
              setTimeout( function() {
                mySocket.send('hello'); }, 2000 );

      <body><h1>WebSocket client</h1></body>

  get '/say_hi' => sub {
    $_->send([ "Hello!" ]) for websocket_connections;



Dancer2::Plugin::WebSocket provides an interface to Plack::App::WebSocket and allows to interact with the webSocket connections within the Dancer app.

Plack::App::WebSocket, and thus this plugin, requires a plack server that supports the psgi streaming, nonblocking and io. Twiggy is the most popular server fitting the bill.



If serializer is set to a true value, messages will be assumed to be JSON objects and will be automatically encoded/decoded using a JSON::MaybeXS serializer. If the value of serializer is a hash, it'll be passed as arguments to the JSON::MaybeXS constructor.

                utf8:         1
                allow_nonref: 1

By the way, if you want the connection to automatically serialize data structures to JSON on the client side, you can do something like

    var mySocket = new WebSocket(urlMySocket);
    mySocket.sendJSON = function(message) {
        return this.send(JSON.stringify(message))

    // then later...
    mySocket.sendJSON({ whoa: "auto-serialization ftw!" });

Path for the websocket mountpoint. Defaults to /ws.


In the various callbacks, the connection object $conn is a Plack::App::WebSocket::Connection object augmented with the Dancer2::Plugin::WebSocket::Connection role.

websocket_on_open sub { ... }

    websocket_on_open sub {
        my( $conn, $env ) = @_;

Code invoked when a new socket is opened. Gets the new connection object and the Plack $env hash as arguments.

websocket_on_login sub { ... }

    websocket_on_login sub {
        my( $conn, $env ) = @_;

Code invoked when a new socket is opened. Gets the connection object and the Plack $env hash as arguments.

Example: return true if user is logged in and the webapp http_cookie is the same as the websocket.

    my $login_conn;
    my $cookie_name = 'example.session';

    hook before => sub {
        if (defined cookies->{$cookie_name}) {
            $login_conn->{'cookie_id'} = cookies->{$cookie_name}->value;
        $login_conn->{'login'} = logged_in_user ? 1 : 0;

    websocket_on_login sub {
        my( $conn, $env ) = @_;

        my ($cookie_id) = ($env->{'HTTP_COOKIE'} =~ /$cookie_name=(.*);?/g);
        if (($login_conn->{'login'}) and ($login_conn->{'cookie_id'} eq $cookie_id)) {
            return 1;
        } else {
            warn "require login";
            return 0;

websocket_on_close sub { ... }

    websocket_on_close sub {
        my( $conn ) = @_;

Code invoked when a new socket is opened. Gets the connection object as argument.

websocket_on_error sub { ... }

    websocket_on_error sub {
        my( $env ) = @_;

Code invoked when an error is detected. Gets the Plack $env hash as argument and is expected to return a Plack triplet.

If not explicitly set, defaults to

    websocket_on_error sub {
        my $env = shift;
        return [
            ["Content-Type" => "text/plain"],
            ["Error: " . $env->{""}]

websocket_on_message sub { ... }

    websocket_on_message sub {
        my( $conn, $message ) = @_;

Code invoked when a message is received. Gets the connection object and the message as arguments.

Note that while websocket_on_message fires for all messages receives, you can also be a little more selective. Indeed, each connection, being a Plack::App::WebSocket::Connection object, can have its own (multiple) handlers. So you can do things like

  websocket_on_open sub {
    my( $conn, $env ) = @_;
    $conn->on( message => sub {
      my( $conn, $message ) = @_;
      warn "I'm only being executed for messages sent via this connection";


Returns the list of currently open websocket connections.


Returns the full url of the websocket mountpoint.

    # assuming host is 'localhost:5000'
    # and the mountpoint is '/ws'
    print websocket_url;  # => ws://localhost:5000/ws


Returns the mountpoint and the Plack app coderef to be used for mount in app.psgi. See the SYNOPSIS.


It seems that the closing the socket causes Google's chrome to burp the following to the console:

    WebSocket connection to 'ws://...' failed: Received a broken close frame containing a reserved status code.

Firefox seems to be happy, though. The issue is probably somewhere deep in AnyEvent::WebSocket::Server. Since the socket is being closed anyway, I am not overly worried about it.


This plugin is nothing much than a sugar topping atop Plack::App::WebSocket, which is itself AnyEvent::WebSocket::Server wrapped in Plackstic.

Mojolicious also has nice WebSocket-related offerings. See Mojolicious::Plugin::MountPSGI or (hi Joel!)


Yanick Champoux <>


This software is copyright (c) 2021, 2019, 2017 by Yanick Champoux.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.