Pub/Sub

According to the MQTT protocol, entities involved in sending and receiving messages can be categorized as Publishers and Subscribers. The basic model is depicted below:

• Publishing Messages: Clients can publish messages to the server through a connection. Messages are published based on topics, which describe the content of the communication and are hierarchical in nature ( e.g., home/bedroom/temperature).

  When publishing a message, clients need to specify a topic, and the message payload contains the actual content of the communication. Additionally, the publisher must specify the Quality of Service (QoS) for the message. BifroMQ supports all MQTT QoS levels: QoS0, QoS1, and QoS2. Their meanings are as follows:

  QoS	Description
  QoS0	At most once delivery
  QoS1	At least once delivery
  QoS2	Exactly once delivery

• Subscribing to Topics: In subscription, the subscribed topics are called topicFilter. The server receives subscription requests from clients and records these subscriptions. Generally, subscriptions can be either non-wildcard subscriptions or wildcard subscriptions.

  • For non-wildcard subscriptions: Clients subscribe to a specific topic without using wildcards. For example, if a client subscribes to home/bedroom/temperature, it will only match messages with the topic home/bedroom/temperature.

  • For wildcard subscriptions: The subscription topic includes wildcard characters (+ or #). The + wildcard matches a single level in the topic hierarchy, while the # wildcard matches multiple levels. Below are examples of how to use wildcards for subscriptions:

    Using the + wildcard:

    • Subscribing to sensor/+/temperature matches topics like sensor/bedroom/temperature, sensor/kitchen/temperature, etc.
    • Subscribing to home/+/light/+ matches topics like home/livingroom/light/on, home/bedroom/light/off, etc.

    Using the # wildcard:

    • Subscribing to home/bedroom/# matches topics like home/bedroom/light/on, home/bedroom/temperature, and their subtopics.
    • Subscribing to home/+/# matches topics like home/bedroom/light/on, home/kitchen/temperature, and their subtopics.
    • Subscribing to # matches all topics.

    Note that wildcard subscriptions can result in a large number of topics being subscribed to, so performance and resource consumption should be carefully considered when using wildcard subscriptions.

    Similar to publishing messages, when subscribing to topics, the client needs to specify the QoS for that topic. BifroMQ, according to the MQTT protocol, will take the minimum of the QoS of the published message and the subscribed topic when forwarding messages, i.e., Min(PublishQoS, SubscribeQoS). This means that even if the subscriber has subscribed to a topic with QoS2, it might still receive messages with QoS0.