Make your topics as short and concise as possible. Keep the MQTT topic short and conciseĮach topic is included in every message in which it is used. UTF-8 has many different white space types, such uncommon characters should be avoided. As with leading forward slashes, just because something is allowed, doesn’t mean it should be used. ![]() When things are not going the way they should, spaces make it much harder to read and debug topics. Never use spaces in a topicĪ space is the natural enemy of every programmer. The zero does not provide any benefit and often leads to confusion. However, the leading forward slash introduces an unnecessary topic level with a zero character at the front. For example, /myhome/groundfloor/livingroom. Use the comments to start a conversation, Let us know your best practices or if you disagree with one of ours! MQTT Best practices Never use a leading forward slashĪ leading forward slash is permitted in MQTT. We have collected the best practices that we have learned from working extensively with MQTT in various projects and are always open to suggestions or a discussion about these practices. When you use wildcards in real-world applications, there are some challenges you should be aware of. As you can see, MQTT topics are dynamic and provide great flexibility. These are the basics of MQTT message topics. $SYS/broker/uptime Summary of MQTT Topics One suggestion for $SYS-topics is in the MQTT GitHub wiki. Commonly, $SYS/ is used for all the following information, but broker implementations varies. At the moment, there is no official standardization for such topics. Clients cannot publish messages to these topics. The $-symbol topics are reserved for internal statistics of the MQTT broker. These topics are not part of the subscription when you subscribe to the multi-level wildcard as a topic (#). However, there is one exception: Topics that start with a $ symbol have a different purpose. Generally, you can name your MQTT topics as you wish. 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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