MQTT

Connect to an MQTT broker and read values published to topics, with optional QoS and encryption.

MQTT reads values published to topics on an MQTT broker. You connect to the broker once and create a tag for each topic you want to read.

Connecting to a broker

Quality of Service and retained messages

• Quality of Service (QoS) — how reliably messages are delivered: 0 – at most once, 1 – at least once, or 2 – exactly once. Higher levels give greater assurance of receipt at the cost of more traffic.
• Retained — ensures the last message on a topic is kept by the broker and delivered to new subscribers as soon as they connect, so a tag has a value immediately rather than waiting for the next publish.
• Receipt acknowledgement — sends an acknowledgement on successful receipt; required for QoS 1 and QoS 2 to work correctly on the target device.

Data encryption

Optionally enable data encryption for payloads, choosing an algorithm (AES or DES) and supplying the key and initialization vector (their required sizes depend on the algorithm).

Health check

Optionally enable a health check that periodically verifies the connection, with an interval (default 60 seconds) and a timeout (default 10 seconds).

Reading a value: the topic

Each MQTT tag reads one topic, appended to the device’s base topic. THMI treats a value’s topic as a base and exchanges JSON messages on two sub-topics underneath it:

So a tag with base topic building-a and topic boiler/temp reads from building-a/boiler/temp/read. Set the data type to match the published value (see Data types). Writable lets users with permission publish a value on the …/write sub-topic (see Values).