HA: MQTT + Node Red

Message Queue Telemetry Transport is a lightweight publish / subscribe message transport.  For example Sensor hardware publishes values which the MQTT Broker passes on to all subscribers.
Alternatively a client can publish an instruction (topic) which wll be passed on by the MQTT Broker to all subscribers for action.
There are lots of MQTT resources/tutorials, for example at mqtt.org

Home Assistant has an integration for an MQTT Broker which can be utilised with a wide variety of devices. Typically, for me, the client will be an ESP8266 or ESP32.  For testing I use a NodeMCU ESP8266 and the Arduino IDE.

HA has an integration for the Mosquitto MQTT Broker which is widely used and very easy to install on HA. I used a good video by EverythingSmartHome. Once the install has completed the broker provides you with a screen where you can experiment, sending packets and listening to Topics.  This will be very useful going forward to watch messages flowing through the system.

I followed a good tutorial from emqx.com to setup a NodeMCU ESP8266 as an MQTT client.  It uses the ESP8266WiFi library and PubSubClient which is allows messages to be published / received.

Our inital sketch simply sets up wifi, sets up MQTT client and publishes a "hello" message which is promptly displayed on the Broker Topic Monitor.
The sketch is very straightforward so I have included it below.  

We dont have sensors attached on functions to carry out yet but you can see how easy they will be to add.

Node Red is a HA add-on / integration which allows you to configure how devices interact using flow diagrams.  I set up a simple test using my ESP8266 keypad and MQTT.  With the help of the Hook Up Guy I was quickly able to use Node Red to instruct HA to say a phrase when button 7 or 8 are pressed.  This isn't a trivial exercise and it is impressive that Node Red plus the wizardry linking it to HA makes it so easy.