I recently read that around 100 species become extinct every year. So why not mosquitoes? I am merciless on the issue, because I happen to be allergic to their stings. I can't sleep, with just one of these pests buzzing around my bedroom. My weapon of choice – always at hand to decimate the population of my enemy – is a 30-year-old school atlas beside the bed. But it is still a Sisyphean task.

And the bloodstains on the wallpaper also impact on the kind of atmosphere that I would like have in my bedroom, which explains why I repaper the walls nearly every year. I also failed to secure a majority vote in the family council for my suggestions of adopting mosquito-eating spiders or using a Linux and LiDAR (light detection and ranging) based mosquito laser. War is always miserable.

You will thus understand that my response to a piece of software named after these flying bloodsuckers was somewhat skeptical. However, this much I will reveal in advance, Mosquitto [1] is allowed to stay. The tool was given its name, because the developer wanted to have the letters MQTT in there somewhere, as the acronym for message queue telemetry transport. And telemetry is the right keyword for describing Mosquitto's task.

Mosquitto operates a messaging hub; in MQTT speak, this is known as the message broker. Any client wanting to communicate sends its message to the broker (publisher), and all clients that have a subscription with the broker (subscribers) then receive the message. The broker is simply named mosquitto and runs as a daemon; the clients that publish and subscribe go by the names of mosquitto_pub and mosquitto_sub.

First, I start the broker. It normally runs in the background, but I can also stop the daemon and start it in the foreground on the console. Because Mosquitto runs on port 1883, I don't even need root privileges to do this:

mosquitto -v

The -v parameter sets a higher verbosity level, so that I can send to the broker at work. Then I use the mosquitto_sub client to subscribe to a topic. The topic is freely selectable and should ideally describe the type of data that you can expect in a meaningful way. As an example, I can subscribe to the topic date:

mosquitto_sub -h 127.0.0.1 -i subscriber -t date

The -i parameter describes the client's identity and is freely definable. I used subscriber here; in a practical application, however, it would make more sense to use the client's hostname.

Mosquitto News

Time to publish something. On another console, I typed the following:

mosquitto_pub -h 127.0.0.1 -i publisher -t date -m "30.02.2017 ;-)"

See Figure 1. The text passed in with -m <text> immediately appears on the console where mosquitto_sub is waiting for data.

Figure 1: In the upper console, a client is sending a message to another client – the console at the bottom receives it immediately.

Impressively simple, don't you think? In daily use, topics are often hierarchically organized; users separate the hierarchy levels with a slash, as in: readings/humidity/outdoor. You can secure communication with TLS and also stipulate that the server and clients identify themselves with certificates. I must admit: Mosquitto is useful. But my atlas is still staying where it is!