In shortMQTT is a lightweight publish/subscribe messaging protocol that works through a broker; it was designed for constrained devices and unreliable networks.
In MQTT a client that produces data (the publisher) sends a message to a topic; clients subscribed to that topic receive it from the broker. Publisher and subscriber do not need to know each other, and this loose coupling makes it easy to distribute the same data to several consumers.
Topics are hierarchical names separated by “/”. Subscription filters use “+” for one level and “#” for any number of levels. Delivery assurance is chosen with three QoS levels (QoS 0, 1, 2); retain lets new subscribers receive the last known value immediately; Last Will defines the message the broker publishes when a client disconnects unexpectedly.
In industry MQTT is often mentioned together with a Unified Namespace and Sparkplug B; these are naming and message contracts layered on top of MQTT.
In OPC Router projects
In OPC Router the MQTT Client publishes to or subscribes from a broker; the Sparkplug plug-in configures MQTT-based industrial messaging. Topic naming and QoS selection are agreed in the project together with the data contract.
Frequently asked questions
What are the default MQTT ports?
Typically 1883 for unencrypted connections and 8883 for TLS-encrypted connections.
How should I name and configure MQTT topics?
Use the MQTT topic builder to generate topics and subscription filters and read the MQTT production guide.