MQTT Essentials - A Lightweight IoT Protocol

Understanding wildcards

When we analyzed the subscription operation, we learn that an MQTT client can subscribe to one or more topic filters. In case we specify a topic name as a topic filter, we will only subscribe to a single topic. We can take advantage of the following two wildcards to create topic filters that subscribe to all the topics that match the filter:

• Plus sign (+): It is a single level wildcard that matches any name for a specific topic level. We can use this wildcard instead of specifying a name for any topic level in the topic filter.
• Hash (#): It is a multi level wildcard that we can use only at the end of the topic filter, as the last level and matches any topic whose first levels are the same as the topic levels specified at ...