TL;DR

Some playing with MQTT, Net::MQTT, and Net::MQTT::Simple.

I recently discovered Software Tools for Hobby-Scale Projects and there’s the interesting presence of Free MQTT servers.

Now I don’t even know what’s the exact use case for MQTT, but I think it’s fun to see that Perl gets me covered anyway with Net::MQTT.

So, after installing it, we can start using the provided example programs and they work!

In one terminal we can set to listen for incoming messages, i.e. act as a subscriber on one or more topic. We’ll subscribe to two topics, this and and-that:

local/bin/net-mqtt-sub -host test.mosquitto.org this and-that

Then, in another terminal, we can connect as a publisher, e.g. on topic this:

local/bin/net-mqtt-pub -host test.mosquitto.org this

Now whatever we write in the latter one will appear in the former. We can also use a third terminal (or replace the program in the second one) to connect, as a publisher, to the other topic:

local/bin/net-mqtt-pub -host test.mosquitto.org and-that

The subscriber is able to get the topic, as shown by the sample output:

this hello from this
this how are you doing?
and-that hello from the other one
and-that I hope everything is fine!

The code is not super-user-friendly, the documentation is a bit blunt and it seems that we’re supposed to do the Net part all by ourselves. So, I guess, it could have been named Data::MQTT or something like this.

For something more ready-to-use, Net::MQTT::Simple seems more friendly. Stealing from the SYNOPSIS, implementing a publisher is a no-brainer, easily done from the command line:

perl -MNet::MQTT::Simple=test.mosquitto.org \
     -nle 'retain "topic/here" => $_'

or programmatically:

use Net::MQTT::Simple "test.mosquitto.org";
 
publish "topic/here" => "Message here";
retain  "topic/here" => "Retained message here";

The subscriber is marginally more complicated, but it provides the flexibility of connecting to multiple topics at the same time much like we have in Net::MQTT:

use Net::MQTT::Simple;
 
my $mqtt = Net::MQTT::Simple->new("mosquitto.example.org");
$mqtt->run(
    "sensors/+/temperature" => sub {
        my ($topic, $message) = @_;
        die "The building's on fire" if $message > 150;
    },
    "#" => sub {
        my ($topic, $message) = @_;
        print "[$topic] $message\n";
    },
);

I guess this is it!