The topic is represented by a string and it has a hierarchical structure where each layer (subtopic) is separated by “/” character. For example, imagine that a company wants to monitor some physical data inside its buildings with a lot of sensors (temperature, humidity, …) for each floor and room; the topic structure for publishing and receiving sensors data could be the following :

building1/floor1/room1/temperature

building1/floor1/room1/humidity

building2/floor3/room4/temperature

The hierarchy starts from the building to a specific room through the floor and at the end we have the specific sensor.

Figure 3.4 : Topic hierarchical structure

A publisher can publish only to an absolute topic but a subscriber can subscribe to an absolute topic or can use wildcards so that it can receives messages published on more topics. There are two types of wildcards:

• Single-level wildcard “+” that represents any subtopic to a specific level of the hierarchy
• Multi-level wildcard “#” that appears only at the end of the string and it represents one or more levels of the hierarchy

If we consider the above examples, we could be interested in all temperature values for all rooms on floor1 in the building1. In this case, we can use the wildcard “+” to collapse the room level inside our topic hierarchy in the following way :

building1/floor1/+/temperature

In the same manner, if we are interested in all data sensors (temperature, humidity, …) in room3 on floor2 in the building2, we can use the wildcard ”#” to collapse the last level of the topic hierarchy in the following way :

building2/floor2/room3/#