On the web there are a ton of instructions and how to’s on how to do this, but I found it easiest to just issue two commands on Raspian Jessie:
sudo apt-get update
sudo apt-get install mosquitto mosquitto-clients python-mosquitto
then to test if it was all working, I opened 2 ssh terminals to my raspberry with:
ssh firstname.lastname@example.org (the latter being the ip nr from my raspberry)
and then in one of those terminals I entered:
mosquitto_sub -d -t hello/world
this made the terminal I will call Terminal 1 subscribe to the topic “hello/world”
in the other ssh terminal window (Terminal 2) I entered
mosquitto_pub -d -t hello/world -m "Greetings from Terminal2"
and immediately I got that message published in Terminal 1
(actually you don’t need the ‘-d’ option. That is just for diagnostics)
So I knew my local Mosquitto broker was working, at least within one machine, the raspberry on which it was installed. Time to check if it also worked on other machines. To do that I opened a third terminal on my desktop that didnt ssh to the raspberry but just gave a command line to the machine I was working on (a regular Linux desktop).
Before I could have that machine subscribe to an MQTT topic, I first had to install the MQTT client with:
sudo apt-get install mosquitto-clients
when that was done I entered:
mosquitto_sub -h 192.168.1.102 -t hello/world
Now when I published a message on terminal2 (thats the raspberry) it got published on terminal1 (that same raspberry) and on terminal3 (a regular desktop), so now I knew everything was hunky dory.
However, having to use terminal command lines to check your MQTT broker is not the most convenient. I can advise everybody who works with MQTT to install MQTT-spy. That is a jar file that basically is a publisher and a client that keeps track of MQTT messages on your network.
It comes pre-configured with 3 public brokers but adding your local broker is very simple:
After starting MQTT-spy go to ‘Connections-New Connection” Fill out a name for your connection and under the ‘connectivity tab’ add (in my case): 192.168.1.102:1883 with 1883 being the standard MQTT port.
Then go to the ‘other’tab and check ‘auto subscribe when opened’.
Now in the main window add a subscription tab called ‘hello/world’ and republish the message from Terminal2: it should appear in the receive window of mqtt-spy. Vice versa, if you publish a message from within mqtt spy, it will appear on terminal 1 also