![]() 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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