mqtt — An MQTT client for IoT services
Message Queue Telemetry Transport (MQTT) is a popular and widely used messaging protocol for IoT communication. It is an application layer protocol that operates over the standard TCP/IP stack. Standard transport-layer security can be applied. MQTT is a client-to-broker protocol. Connections are long-lived and bi-directional, or duplex, meaning the the client can send messages to the broker and vice-versa. It has a built-in quality of service mechanism that ensures that messages are delivered exactly once, at most once, or at least once, and this can be executed across lost connections. It provides a last will and testament mechanism for when the device goes offline in an uncontrolled manner (e.g. power removal). It is suitable for a constrained, embedded platform such as DL-7450.
MQTT is supported by many IoT cloud providers such as Azure Iot Hub, and is also a popular choice for bespoke IoT cloud backends. It is the mechanism we use for communicating with our development backend, and the delivery mechanism for Python code for the Workshop. We have validated and demonstrated that it supports Azure Iot Hub applications, using the Device twin and application message mechanisms. We have shown that in-field firmware updates can be managed through both Azure IoT hub and our backend development server.
More information about the MQTT protocol can be found at https://mqtt.org/
Constants
The following quality-of-service constants for the MQTT protocol are defined
in the mqtt module. The QOS level is used by the client when
publishing messages and by the server when sending messages on subscribed
topics. The optional will message also has an associated QOS.
- mqtt.QOS0
This is service-level at most once, or fire and forget. The sender waits for no acknowledgements of receipt and makes no attempt to resend the message.
- mqtt.QOS1
This is service-level at least once. The sender expects an acknowledgement of receipt and will retry sending after a time defined in the connect options. The message may eventually be received multiple times.
- mqtt.QOS2
This is service-level exactly once. A four-step handshake is used to ensure that the message is received exactly once.