starting

Dockerfile

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+FROM perl-mqtt
+COPY update-berths.pl /
+ENTRYPOINT "/update-berths.pl"

update-berths.pl

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+#!/usr/bin/env perl
+
+use strict;
+use warnings;
+
+use Net::MQTT::Simple;
+use JSON;
+use Data::Dumper;
+
+sub assert($;$) {
+    my ($required, $message) = @_;
+    unless ($required) {
+        my $msg = "Assertion failed";
+        $msg .= " ($message)" if $message;
+        die $msg;
+    }
+}
+
+sub _debug($) {
+    my $msg = shift;
+    return unless $ENV{DEBUG};
+    printf("%s [%s] %s\n", time, "DEBUG", $msg);
+}
+
+sub _error($) {
+    my $msg = shift;
+    printf("%s [%s] %s\n", time, "ERROR", $msg);
+}
+
+sub handle($$) {
+    my ($topic, $json) = @_;
+    my $data;
+
+    eval {
+        $data = from_json($json)
+    };
+
+    if ($data) {
+        _debug( Dumper($data) );
+    } else {
+        _error("Got invalid JSON data from $topic: $json");
+    }
+}
+
+### Check required environement variables are present
+foreach (
+    qw(
+        MQTT_SERVER
+        MQTT_MONITOR
+    )
+) {
+    assert($ENV{$_}, "`$_' not available");
+}
+
+## Connect to MQTT
+_debug("Connecting to mqtt://$ENV{MQTT_SERVER}/$ENV{MQTT_MONITOR}");
+my $mqtt = Net::MQTT::Simple->new($ENV{MQTT_SERVER}) or die "Failed to connect to mqtt://$ENV{MQTT_SERVER}";
+
+$mqtt->subscribe(
+    "$ENV{MQTT_MONITOR}" => sub {
+        my ($topic, $message) = @_;
+        handle($topic, $message);
+    },
+);
+
+$mqtt->run();