MQTT Topics and Wildcards

An MQTT topic is a UTF-8 string with levels separated by forward slashes that labels each published message; subscribers select messages with exact topics or with the single-level (+) and multi-level (#) wildcards.

Topics are created implicitly when a client publishes to them, with no need to register them on the broker. Each level narrows the scope, as in site/area/line/machine/measurement. A subscription to site/area/+/machine1/temperature matches that measurement on any line, while site/area/# matches everything below the area. Wildcards can be used only in subscriptions, not when publishing, and topics beginning with $ are reserved for broker-specific use, such as $SYS statistics topics.

A well-designed topic hierarchy is central to IIoT and unified namespace architectures, where it often mirrors the physical or ISA-95 equipment hierarchy so that users and applications can find data intuitively. Topics are case-sensitive, so consistent naming conventions, character sets and case rules avoid duplicate topics that differ only in spelling. The Sparkplug specification defines its own fixed topic structure for industrial interoperability.

Very deep or very granular hierarchies can increase broker load and complicate access control, while overly flat ones make subscriptions coarse. Retained messages give new subscribers the latest value on a topic, which suits state data. Access control lists usually grant permissions by topic pattern, so the hierarchy should also be designed with security boundaries in mind.

Key points

Topics are hierarchical strings with levels separated by forward slashes
The + wildcard matches one level; # matches all remaining levels
Wildcards are allowed in subscriptions only
Topics are case-sensitive and created implicitly on first publish
Hierarchy design shapes discoverability and access control

Where AiVibe comes in

AiAmbA IoT is AiVibe's edge protocol layer: it discovers devices and normalises industrial signals, with supported protocols including Siemens S7, Mitsubishi MC, Delta Modbus, OPC UA and MQTT, and driver availability confirmed per installation. Writes to machines are deny-by-default.

Explore AiVibe’s work in Industrial IoT & Edge →

Related terms

Terms that refer to MQTT Topics and Wildcards

MQTT Last Will and Testament (LWT)Industrial IoT & Edge