What is the publish/subscribe model

Publish/Subscribe (Pub/Sub, aka Publish/Subscribe) is an architectural design pattern for asynchronous communication between different components or services in a distributed system. Although Publish/Subscribe is based on earlier design patterns such as message queues and event brokers, it is more flexible and scalable. The key is that Publish/Subscribe allows messages to be passed between different components of a system without these components knowing each other's identities (i.e. they are decoupled).

The emergence of the Publish/Subscribe model stems from the need to expand the scale of information systems. Before the Internet era, and even in the early days of the Internet, systems were mostly expanded in a static way. However, with the expansion of the Internet and the popularity of web-based applications, as well as the widespread use of mobile devices and IoT devices, systems need to expand dynamically.

The decoupled nature of the Publish/Subscribe model makes it ideal for managing dynamically scalable system architectures. Publish/Subscribe makes it possible to manage expansion without adding program logic burdens to system components.

Pub/Sub Architecture

Pub/Sub provides a framework for exchanging messages between publishers (components that create and send messages) and subscribers (components that receive and consume messages). Note that publishers do not send messages to specific subscribers in a point-to-point manner. Instead, an intermediary - Pub/Sub message broker is used, which groups messages into entities called channels (or topics).

How the Publish/Subscribe model works:

Subscribers first choose to subscribe to a specific topic so that they can receive all messages under that topic.

Publishers then send messages to these selected topics, which contain the information or instructions to be delivered.

The message broker, as the core intermediary, receives messages from publishers and stores them in the corresponding topics. At the same time, the broker is also responsible for distributing these messages to all subscribers who have subscribed to the topic.

To understand the operation of the Pub/Sub system more specifically, we can consider the following real-life scenario:

Imagine that you have installed a series of IoT devices in your home, which are responsible for monitoring smoke concentration and controlling the supply of electricity and gas. To ensure smooth and reliable communication between these devices, you can use an MQTT broker as the central hub for messaging.

In this scenario, once the smoke detection device (as a publisher) detects smoke and determines that it is an alarm state, it immediately sends a message containing alarm information to the MQTT broker. Your smartphone (as one of the subscribers) has subscribed to the relevant smoke alarm topic, so it will immediately receive the message and notify you that there is smoke in your home.

It is worth noting that the flexibility of the Pub/Sub model is also reflected in the ability of subscribers to become publishers. For example, after receiving the smoke alarm, your smartphone can act as a publisher and send a message to another specific topic to shut down the power and gas supply. At this time, the IoT device responsible for controlling the power and gas supply (as a subscriber) will receive this instruction and perform the shutdown operation accordingly to ensure the safety of the family.

In this way, even if only one message is sent, it can trigger two independent IoT devices to respond at the same time, thereby achieving efficient cross-device communication and collaboration.

What are the advantages of the Pub/Sub model?

The Pub/Sub pattern brings many benefits to the table, including but not limited to:

Loose coupling between components, making your system more modular and flexible.

High scalability (in theory, Pub/Sub allows any number of publishers to communicate with any number of subscribers).

Language and protocol agnostic, which makes integrating Pub/Sub into your technology stack quick and easy.

Asynchronous, event-driven communication, ideal for real-time, low-latency applications.

When should you use the Pub/Sub pattern?

The loose coupling, asynchronous nature, and inherent scalability of Pub/Sub make it a great solution for distributed systems with high and fluctuating numbers of publishers and subscribers. You can use Pub/Sub for many different purposes, such as:

Sending event notifications.

Distributed caching.

Distributed logging.

Working with multiple data sources.

Broadcasting updates (one-to-many messaging).

Building responsive, low-latency end-user experiences, such as real-time chat and multiplayer game collaboration features.