The broker has to make sure that subscribers only receive the messages they are interested in. It is possible to filter messages based on different criteria in a publish-subscribe pattern. We will focus on analyzing topic-based filtering, also known as subject-based filtering.
Consider that each message belongs to a topic. When a publisher requests the broker to publish a message, it must specify both the topic and the message. The broker receives the message and delivers it to all the subscribers that have subscribed to the topic to which the message belongs.
The broker doesn't need to check the payload for the message to deliver it to the corresponding subscribers; it just needs to check the topic for each message that has arrived and needs to be filtered before publishing it to the corresponding subscribers.
A subscriber can subscribe to more than one topic. In this case, the broker has to make sure that the subscriber receives the messages that belong to all the topics to which it has subscribed. It is easy to understand how things work with another simple diagram. The following diagram shows two future publishers that haven't published any messages yet, a broker and two subscribers connected to the broker:
A Raspberry Pi 3 board with an altitude sensor wired to it and an Intel Edison board with a temperature sensor wired to it will be two publishers. An iOS smartphone and an Android tablet are two subscribers that establish a connection to the broker.
The iOS smartphone indicates to the broker that it wants to subscribe to all the messages that belong to the sensor1/altitude topic. The Android tablet indicates to the broker that it wants to subscribe to all the messages that belong to any of the following two topics: sensor1/altitude and sensor42/temperature. Hence, the Android tablet is subscribed to two topics while the iOS smartphone is subscribed to just one topic.
The following diagram shows what happens after the two publishers connect and publish messages to different topics through the broker:
The Raspberry Pi 3 board publishes a message with 120 feet as the payload and sensor1/altitude as the topic. The board, that is, the publisher, sends the publish request to the broker. The broker distributes the message to the two clients that are subscribed to the sensor1/altitude topic: the iOS smartphone and the Android tablet.
The Intel Edison board publishes a message with 75 F as the payload and sensor42/temperature as the topic. The board, that is, the publisher, sends the publish request to the broker. The broker distributes the message to the only client that is subscribed to the sensor42/temperature topic: the Android Tablet. Thus, the Android tablet receives two messages.
