Usually, end devices have a limited memory and CPU power, running on batteries, so connecting them with servers requires a light protocol. Enter MQTT, which was invented in 1999 by Andy Stanford-Clark from IBM and Arlen Nipper from Arcom, a SCADA protocol design initially for battery operated devices to supervise oil pipelines. Later in 2010, IBM released it as a royalty-free protocol. In 2014, OASIS announced that the MQTT v.3.1.1 had become an OASIS standard and a lot of MQTT clients were developed for all programming languages.

The characteristics of MQTT are listed as follows:

• Data agnostic: MQTT can transport all kind of data, from sensor data to images or over the air updates
• Lightweight and bandwidth efficient: Smallest frame is only 2 bytes long
• Provide QoS: Three Quality of Service (QoS) levels
• Runs on top of the TCP/IP stack
• Simple...

Eclipse Mosquitto^TM is an open source MQTT broker that implements the MQTT v3.1 and MQTT v.3.1.1 standards and provides a lightweight method to transport messages, allowing publish and subscription for low power sensors, mobile devices, embedded computers, and micro controllers.

You can install Mosquitto on a Raspberry Pi or on AWS instance or on a VirtualBox Linux instance directly from your Linux repository distribution; or you can get the source code and compile it yourself if you want support from websockets.

Installing from your Linux distribution repository:

1. First upgrade to the latest version:

    sudo apt update && sudo apt upgrade

2. Then install mosquito:

    sudo apt install mosquitto  

You should see the following screen:

3. After installing the Mosquitto broker verify that the broker is started and install...

To use the ESP8266 as a client for sending data to a broker you will need a library that offers MQTT support. For this you can use the PubSubClient library, which can be installed like other libraries; see Chapter 1, Getting Started with the ESP8266:

1. Go to Sketch | Include Library | Manage Libraries..., as follows:

2. And search for the PubSubClient library and click Install as in the following screenshot:

Start a new sketch via File | New in the Arduino IDE and paste in the following code. Include the ESP8266WiFi library and the PubSubClient one:

#include <ESP8266WiFi.h> 
#include <PubSubClient.h> 

Update these with values suitable for your network. Use ifconfig to get the IP address of the server where the Mosquitto broker is installed. If your server has an FQDN name such as myiotserver.com and is registered into the DNS, then instead of the IP address in mqtt_server you can use the FQDN name:

const char* wifi_network = "YOUR_WIFI_SSID"; 
const char* wifi_pass = "YOUR_WIFI_PASSWORD"; 
const char* mqtt_serv_address = "192.168.1.116"; 
const int mqtt_port_number = 1883; 

Instantiate a WiFiClient and pass it to the PubSubClient:

WiFiClient espClient; 
PubSubClient client(espClient); 
long lastMsg...

Now let's publish a message using mosquitto_pub and receive it in the ESP8266.

For this the ESP8266 needs to subscribe to the same topic on which mosquitto_pub will publish the message. Let's call the topic outdoor/light and it will publish on 0 or 1 values. If the ESP8266 receives the value as 1, it will turn on a LED connected to GPIO 12 and if it will receives a 0, it will turn off that LED:

#include <ESP8266WiFi.h> 
#include <PubSubClient.h> 

Update these with values suitable for your network:

 
const char* wifi_network= "YOUR_WIFI_SSID"; 
const char* password = "YOUR_WIFI_PASSWORD"; 
const char* mqtt_serv_address = "YOUR_MQTT_SERVER_IP"; 
const int mqtt_port_number = 1883; 
#define OUTDOOR_LIGHT  12 
 
WiFiClient  espClient; 
PubsubClient client(espClient); 
long lastMsg...