Welcome! In this unit, we will delve into implementing Pub/Sub for notifications within our Redis-based backend system project. You've already learned how to manage user data, handle transactions, and use streams for event logging. Now, we'll add another powerful feature to our project: real-time notifications using Redis Pub/Sub (publish/subscribe). This will enable our system to send and receive messages instantaneously.

In this unit, we'll focus on creating a simple real-time notification system using Redis Pub/Sub. Specifically, we'll cover:

1. Publishing messages: how to send notifications.
2. Subscribing to channels: how to receive and handle notifications.

We'll implement a complete C++ program using Boost.Redis that demonstrates both publishing and subscribing. The example will:

• Create separate connections for subscribing and publishing, which is required for Pub/Sub.
• Subscribe to a channel and listen for messages asynchronously.
• Publish a JSON-formatted message to that channel.
• Process and display the received message.

Messages will use JSON format for structured data:

This first section sets up the headers, type aliases, and the function that listens for Pub/Sub messages.

This section prepares the program to receive messages from Redis.

It includes the libraries for:

• Redis communication with Boost.Redis
• asynchronous execution with Boost.Asio

This section creates the Redis connections, starts them, and subscribes to a channel.

This part sets up the runtime environment and the two Redis connections.

The program creates:

• sub_conn for SUBSCRIBE
• pub_conn for PUBLISH

This separation is required for Redis Pub/Sub, because a connection in subscriber mode is dedicated to Pub/Sub commands.

Both connections are started with async_run(), which lets them process Redis communication through the io_context.

Next, the code defines the channel name as and starts the receive loop on the subscriber connection.

This final section publishes a message after a short delay and then runs the event loop.

This section sends the actual notification.

A steady_timer delays the publish operation slightly so the subscription can be established first. When the timer fires, the program creates a PUBLISH request and builds a JSON message:

The JSON object is converted to a string with message.dump() and sent to Redis on the chat_room channel.

The response to PUBLISH is an integer representing how many subscribers received the message. The code prints that number, then cancels the publisher connection.

When you run this program, you should see output similar to:

This demonstrates a complete publish-subscribe cycle: the program subscribes to a channel, publishes a message to that channel, receives the message, and then exits cleanly.

Congratulations! You've just learned how to implement real-time notifications using Redis Pub/Sub. Let's recap the key concepts:

• Dual Connections: Redis Pub/Sub requires separate connections for subscribers and publishers, as subscriber connections can only execute Pub/Sub commands.
• Asynchronous Message Handling: Using Boost.Redis's async operations with callbacks to handle messages without blocking.
• RESP3 Push Messages: Understanding how Redis delivers published messages in the format ["push", "message", "<channel>", "<payload>"].
• Structured Data with JSON: Leveraging JSON payloads for flexible, extensible message formats.

This foundation enables you to build sophisticated real-time features like chat systems, live notifications, and event broadcasting in your backend applications.

Now it's time to put this into practice. In the upcoming exercises, you'll build upon this foundation to create a more interactive notification system that handles multiple messages and different types of notifications.

Happy coding!