MQTT is a publish/subscribe messaging protocol designed for lightweight M2M communications.
This was originally developed by IBM and now it is a standard.
MQTT is a client server protocol where every sensor is a client and it connects to a server, known as broker, over TCP.
MQTT is message oriented, every message is a discrete chunk of data, opaque to the broker.
Every message is published to an address, known as topic. Clients may subscribe to multiple topics. Every client subscribed to a topic receives every message published to the topic.
Below is the concept exemplified with 3 clients and a broker.
Client B, C subscribed for light on event. When A publishes light ON, Broker server sends that as Notify to B & C.
MQTT supports 3 application level QOS. Fire and Forget, devlivered at least once, delivered exactly once.
MQTT clients can register a custom “lat will and testament” message to be sent by the broker when they disconnect. These messages can be
used to signal to subscribers when a device disconnects.
Persistence : MQTT has support for persistent messages stored on the Broker. When client publish, it can request broker to store the message when a client subscribe to a topic, any persisted message will be sent to the client. Unlike message queues, MQTT brokers do not allow persisted messages to be backed up inside the server.
Security MQTT brokers may require username and password authentication from the client. To ensure privacy, TCP connection can be encrypted using SSL/TLS.
MQTT-SN. Even though MQTT is designed to be light weight, it has two drawbacks for very constrained devices.
1. Every MQTT client must support TCP and will typically hold a connection open to the broker a all the times. For some environments where packet loss is high or computing resources are scarce, this is a problem.
2. MQTT topic names are often long strings which make them impractical for 802.15.4
Both of these shortcomings are addressed for MQTT-SN protocol. which defines UDP mapping of MQTT and adds broker support for indexing topic names.
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