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Many IoT applications begin with a small number of connected devices and then grow over time to a substantial number of devices. Therefore, it is important for organizations to understand how their IoT infrastructure will scale to handle an increase in the number of connected devices and the corresponding message throughput. 

So, it is vital to ensure that your IoT application can scale to handle any anticipated growth in usage. That’s where MQTT comes in.

What is MQTT ?

MQTT is the dominant standard used in IoT communications. It allows assets/sensors to publish data, for example, a weather sensor can publish the current temperature, wind metrics, etc. MQTT also defines how consumers can receive that data. In environments using Low Power Wide Area Network (LPWAN) solutions, sensor data is sent over wireless where it is received by one or more central base stations. These data are small as individual packets but massive when aggregated together, is then sent to analytics and visualization tools whether in the cloud or on-premises. Residing on top of the TCP/IP network stack, MQTT is a lightweight publish/subscribe messaging protocol designed for low-bandwidth, small size, low power usage, minimized data packets, high latency, unreliable networks. MQTT’s features make it an excellent option for sending high volumes of sensor messages to analytics platforms and cloud solutions.

How does MQTT work ?

The core of the MQTT protocol are clients and servers that send many-to-many communications between multiple clients using the following:
  • Topics provide a way of categorizing the types of message that may be sent. As one example, if a sensor measures temperature, the topic might be defined as “TEMP” and the sensor sends messages labeled “TEMP.”
  • Publishers include the sensors that are configured to send out messages containing data. In the “TEMP” example, the sensor would be considered the publisher.
  • In addition to transmitting data, IoT devices can be configured as Subscribers that receive data related to pre-defined topics. Devices can subscribe to multiple topics.
  • The broker is the server at the center of it all, transmitting published messages to servers or clients that have subscribed to specific topics.

Comparison between MQTT and HTTP protocols

Features MQTT HTTP
Architecture It has publish/subscribe architecture. Here devices can publish any topics and can also subscribe for any topics for any updates. It has request/response means Client/Server architecture.
Upper layer protocol It runs over TCP. It runs over TCP and UDP.
Message size Small Large
Message format Binary with 2Byte header ASCII format.
Data security Yes, It uses SSL/TLS for security No, hence HTTPS is used to provide data security
Complexity Simple Client more complex (ASCII parser)
Encryption It encrypts payload data are not encrypted before transmission

Publish/Subscribe Model

MQTT clients are split into two groups: A sender (referred to as a publisher in MQTT) and a consumer that receives the data (an MQTT subscriber). The publisher and the subscriber do not know anything about each other, and, in fact, are never in direct contact with each other. A third component i.e. (MQTT broker) responsible for receiving all messages, filtering the messages, determining who is subscribed to each message, and sending the message to these subscribed clients.
Example:
Let’s say there is a device that has a BME280 sensor. Certainly, it wants to send his readings to the broker. On the other side, a phone/desktop application wants to receive this temperature value. Therefore, 2 things will happen:
  • The device defines the topic it wants to publish on, ex: “temp” & “humidity”. Then, it publishes the message “temperature value” & “humidity value”.
  • The phone/desktop application subscribes to the topic “temp” & “humidity”. Then, it receives the message that the device has published, which is the temperature value or humidity value.
The broker role here is to take the message “temperature value” & “humidity value” and deliver it to phone/desktop application

NodeMCU — MQTT Basic Example

This example will show the basic MQTT protocol usage on the NodeMCU board.We use the MQTTBox as the MQTT client here, and we will use the NodeMCU to complete following  operations:
  • Publish “hello world” to the topic “outTopic” every two seconds.
  • Subscribe to the topic “inTopic”, print out any received messages.
  • It assumes the received payloads are strings not binaries.
  • If the subscribed message is “1”, light the onboards LED.Turn off the onboard LED if the message to subscribe is “0”.
It will reconnect to the server if the connection is lost using a blocking reconnect function. See the ‘mqtt_reconnect_nonblocking’ example for how to achieve the same result without blocking the main loop.
Preparation:
  • NodeMCU x1
  • USB cable x1
  • Arduino IDE(Version latest)
Library Installation
We need to install MQTT endpoint library(PubSubClient) to communicate with MQTT broker,  please download the library from following link: https://github.com/knolleary/pubsubclient Unzip the above downloaded file, move the unzipped folder to Arduino IDE library folder. Open the Arduino IED,you can find the “pubsubclient” on the “Examples” column:
MQTT Client Installation:
We used the MQTTBox as the MQTT client. (You can also use another client ) You will see this page after installation completed: As you can see below, I have created one: Go to Create MQTT Client & fill the details, according to your need, then click on SAVE.
Upload sketch
For uploading, go with the below path: Open Arduino IDE–>File–>Example–>pubsubclient–>mqtt esp8266 Edit the code to fit your own WiFi and MQTT settings as following operations:
MQTT  Server  Address Setting
You can use your own MQTT broker URL or IP address to set above mqtt_server value. You can also use some famous free MQTT server to test the project. const char* mqtt_server = “Your MQTT broker URL”;
Topic setting:
  • Make sure your MQTT client publish topic is same as your Arduino sketch subscribe topicinTopic here.
  • Make sure your MQTT client subscribe topic is same as your Arduino sketch publish topicoutTopic here.
As you can see on the Sketch side:
Program Running Result:
Once the upload done, if the wifi hotspot name and password setting is ok, and MQTT broker is connected, open the Serial Monitor,you will see following result: You can see the publish message “hello world” on the serial monitor. Then, open the MQTT client and publish payload “1” to the topic, this NodeMCU will recevie these messages by subscribing to the “inTopic”,and the LED will be light. Publish payload “0” to this topic, the NodeMCU LED will be turned off.
Video:
NodeMCU — MQTT Basic

Online Free MQTT Brokers for Testing

You can also do some online testing on some Brokers. However, not to publish anything sensitive. Topics are accessible by anyone.
Broker Server Ports Websocket
Mosquitto  iot.eclipse.org 1883 / 8883 N/A
HiveMQ broker.hivemq.com 1883 8000
Mosquitto test.mosquitto.org 1883 / 8883 / 8884 8080 / 8081
HiveMQ broker.mqttdashboard.com 1883 N/A
mosca  test.mosca.io 1883 80

Conclusion

MQTT provides a lots of functions for the Internet of Things. It can help providing a great performance and create new area for messaging and can handle billion of things connected through the internet. It is a very light weight protocol that can work with every types of devices and work using a minimum bandwidth. Now-a-days facebook.com is using MQTT protocol for their messenger which working great in our messaging in social network.
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