MQTT Protocol

MQTT is a lightweight, machine-to-machine connectivity publish/subscribe protocol. Because of its small code footprint, and its ability to support wireless networks with varying levels of latency, it is becoming extremely popular for IoT and other real-world applications.

Toby abstracts the process of creating an MQTT connection between your devices. It uses MQTT 3.1.

MQTT Terminology

• MQTT Message
  • The data carried by the MQTT protocol across the network for the application.
  • Messages have an associated Quality of Service and a Topic Name.
• Client
  • A program or device that uses MQTT. A Client always establishes the Network Connection to the Server and supports the following methods.
  • Publish Messages that other Clients might be interested in.
  • Subscribe to request Messages that it is interested in receiving.
  • Unsubscribe to remove a request for Messages.
  • Disconnect from the Server.
• Server
  • A program or device that acts as an intermediary between Clients which publish Messages and Clients which have made Subscriptions.
  • Accepts Network Connections from Clients.
  • Accepts Messages published by Clients.
  • Processes Subscribe and Unsubscribe requests from Clients.
  • Forwards Messages that match Client Subscriptions.
• Subscription
  • A Subscription comprises a Topic Filter and a maximum QoS.
  • A Subscription is associated with a single Session. A Session can contain more than one Subscription.
  • Each Subscription within a session has a different Topic Filter.
• Topic Name
  • The label attached to a Message which is matched against the Subscriptions known to the Server. The Server sends a copy of the Message to each Client that has a matching Subscription.
• Topic Filter
  • An expression contained in a Subscription, to indicate an interest in one or more topics.
  • A Topic Filter can include wildcard characters.
• Session
  • A stateful interaction between a Client and a Server. Some Sessions last only as long as the Network Connection ("non-persistent"), others can span multiple consecutive Network Connections between a Client and a Server ("persistent").