If you expect high throughput, subscription with a multi-level wildcard alone is an anti-pattern (see the best practices below). If you specify only the multi-level wildcard as a topic ( #), you receive all messages that are sent to the MQTT broker. When a client subscribes to a topic with a multi-level wildcard, it receives all messages of a topic that begins with the pattern before the wildcard character, no matter how long or deep the topic is. For the broker to determine which topics match, the multi-level wildcard must be placed as the last character in the topic and preceded by a forward slash. The hash symbol represents the multi-level wild card in the topic. The multi-level wildcard covers many topic levels. For example a subscription to myhome/groundfloor/+/temperature can produce the following results: The plus symbol represents a single-level wildcard in a topic.Īny topic matches a topic with single-level wildcard if it contains an arbitrary string instead of the wildcard. Single Level: +Īs the name suggests, a single-level wildcard replaces one topic level. There are two different kinds of wildcards: single-level and multi-level. A wildcard can only be used to subscribe to topics, not to publish a message. When a client subscribes to a topic, it can subscribe to the exact topic of a published message or it can use wildcards to subscribe to multiple topics simultaneously. Additionally, the forward slash alone is a valid topic. For example, myhome/temperature and MyHome/Temperature are two different topics. Note that each topic must contain at least 1 character and that the topic string permits empty spaces. ![]() USA/California/San Francisco/Silicon Valleyĥff4a2ce-e485-40f4-826c-b1a5d81be9b6/status Myhome/groundfloor/livingroom/temperature The broker accepts each valid topic without any prior initialization. The client does not need to create the desired topic before they publish or subscribe to it. In comparison to a message queue, MQTT topics are very lightweight. Each topic level is separated by a forward slash (topic level separator). ![]() The topic consists of one or more topic levels. ![]() In MQTT, the word topic refers to an UTF-8 string that the broker uses to filter messages for each connected client. 72 pages of MQTT education and learning for beginners and experts alike.
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