MQTT is a very light weight protocol used commonly in sensor communications. MQTT stands for MQ Telemetry Transport. Unlike other messaging protocols, this is very simple, light weight and requires only very low bandwidth. Because of these features, it is widely used in sensor networks. Now with the emerge of Internet of Things, MQTT became very popular. Since it is very light, it consumes very less power and is ideal for low power applications. MQTT messages are delivered asynchronously through a publish-subscribe mechanism. The message packets are specially crafted in such a way to reduce the data size.
The basic communication model in an MQTT system is shown below.
Common terminologies in MQTT
Publisher – The one who publishes the messages. Usually the sensors emits the messages. For example in a temparature monitoring scenario, the temparature sensor emits the temparature frequently and the values are displayed in the mobile application. Here the sensor is publisher and the mobile application is the subscriber.
Subscriber – Subscriber is the one who subscribes the messages.
MQTT Client – A client can be a subscriber or publisher. A single client can act as a publisher as well as subscriber. MQTT client libraries are available in almost all programming languages. The complete list is available in MQTT wiki.
MQTT Broker/Server -All the clients are connected to the broker. This is the heart of the MQTT system. Depending upon the implementation, a broker can handle thousands of concurrently connected MQTT clients. The broker needs to be configured in such a way that it is accessible to all the clients. A client can be configured within any NAT network, but the broker needs to directly accessible from all the clients. There are several broker variants available. Some of them are Mosquitto, HiveMQ, Mosca, emqttd etc. Based on your choice and your application needs, you can choose one among these.
Topic – This is a UTF-8 string used by the broker to filter the messages for each connected client. There can be N number of topics. Topic is case sensitive.
MosquittoMQTT is one of the widely used MQTT servers. I have used Mosquitto in many projects. This can be easily installed in Linux and Windows.
By default MQTT uses TCP port 1883 for communication and for SSL enabled communication, it uses port 8883. These are configurable values and it can be changed.
Security
The messages can be secured with username and password. Communication with SSL is also possible. Apart from these, there is no advanced security mechanism present in MQTT. But we can have our own custom message encryption/decryption logic.
There are a lot to explain about MQTT. Instead of writing more, I feel it is better to demonstrate the working of a real implementation. I will be publishing a post on the real implementation of MQTT soon.
