Redis Caching & Messaging (Pub/Sub, Streams) · 课时

设计实时聊天

实现一个基础的实时聊天应用，展示如何使用发布/订阅传递消息

设计实时聊天 是 CoddyKit 上的免费 Redis Caching & Messaging (Pub/Sub, Streams) 课时。这是第 2 节课，共 4 节。你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。这是 Redis Caching & Messaging (Pub/Sub, Streams) 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Redis Caching & Messaging (Pub/Sub, Streams) 课程共包含 4 节课。

本课时的部分内容尚未翻译，以英文显示。

Real-time Chat with Pub/Sub

Imagine building a chat application where messages appear instantly! This is where Redis Pub/Sub shines. It's perfect for real-time communication without constant 'checking for new messages'.

In this lesson, you'll learn how to design a basic chat system using Redis Publish/Subscribe, turning chat rooms into channels and messages into published events.

Chat Rooms as Redis Channels

The core idea for a chat application with Pub/Sub is simple: each chat room corresponds to a unique Redis Channel.

When a user wants to join a chat room (e.g., 'general' or 'support'), their client application will subscribe to the corresponding Redis channel.
When a user sends a message in that room, their client will publish the message to that specific channel.

Subscribing to Join a Room

To 'join' a chat room, a client needs to start listening for messages on its channel. This is done by subscribing. Once subscribed, the client will receive all messages published to that channel.

Below is a simplified Java example using a Redis client library (like Jedis) to subscribe to a channel named chat:general.

import redis.clients.jedis.Jedis;
import redis.clients.jedis.JedisPubSub;

public class ChatSubscriber {
  public static void main(String[] args) {
    try (Jedis jedis = new Jedis("localhost")) {
      System.out.println("Subscribing to chat:general...");
      jedis.subscribe(new JedisPubSub() {
        @Override
        public void onMessage(String channel, String message) {
          System.out.println("Received on " + channel + ": " + message);
        }
      }, "chat:general");
    } catch (Exception e) {
      System.err.println("Error: " + e.getMessage());
    }
  }
}

Publishing to Send Messages

When a user types a message and hits 'send', their client application will publish that message to the channel corresponding to the current chat room. All clients currently subscribed to that channel will immediately receive the message.

This Java example shows how a client would publish a message to the chat:general channel.

import redis.clients.jedis.Jedis;

public class ChatPublisher {
  public static void main(String[] args) {
    try (Jedis jedis = new Jedis("localhost")) {
      String message = "Hello, everyone!";
      jedis.publish("chat:general", message);
      System.out.println("Published: '" + message + "'");
    } catch (Exception e) {
      System.err.println("Error: " + e.getMessage());
    }
  }
}

Demo: User A Listens

Let's see this in action! Run this program in one terminal. It simulates User A joining the chat:general room and waiting for messages. It will block and print any message it receives.

import redis.clients.jedis.Jedis;
import redis.clients.jedis.JedisPubSub;

public class UserA {
  public static void main(String[] args) {
    try (Jedis jedis = new Jedis("localhost")) {
      System.out.println("User A is listening on chat:general...");
      jedis.subscribe(new JedisPubSub() {
        @Override
        public void onMessage(String channel, String message) {
          System.out.println("User A received: " + message);
        }
      }, "chat:general");
    } catch (Exception e) {
      System.err.println("Error: " + e.getMessage());
    }
  }
}

Demo: User B Sends

Now, open a separate terminal and run this program. It simulates User B sending a message to the chat:general room. Watch User A's terminal – it should instantly receive User B's message!

import redis.clients.jedis.Jedis;

public class UserB {
  public static void main(String[] args) {
    try (Jedis jedis = new Jedis("localhost")) {
      String message = "Hey User A, are you there?";
      jedis.publish("chat:general", message);
      System.out.println("User B published: '" + message + "'");
    } catch (Exception e) {
      System.err.println("Error: " + e.getMessage());
    }
  }
}

Designing Your Chat Message

A simple string message isn't usually enough for a chat application. You'll want to include more context. A common approach is to serialize chat message data into a format like JSON before publishing it.

Consider including:

sender: The username or ID of who sent the message.
timestamp: When the message was sent (for ordering).
text: The actual content of the message.
roomId: (Optional) The channel/room ID, useful for client-side processing.

Handling Multiple Chat Rooms

To support many different chat rooms (e.g., 'general', 'private:alice-bob', 'developers'), you simply use different channel names.

Each room gets a unique channel name (e.g., chat:general, chat:support).
Clients subscribe only to the channels of the rooms they are active in.
When a user switches rooms, their client unsubscribes from the old channel and subscribes to the new one.

Pub/Sub's Non-Persistent Nature

It's crucial to remember that Redis Pub/Sub is a fire-and-forget system. Messages are delivered to all currently active subscribers and then disappear.

If a user is offline or not subscribed when a message is published, they will not receive that message when they come back online.
For persistent chat history or messages to offline users, you would need to combine Pub/Sub with other Redis data structures (like Lists or Streams) or a separate database.

Chat Design Quiz

You're building a chat app with Redis Pub/Sub. Users join rooms by subscribing to channels. If User A is subscribed to chat:tech and User B publishes a message to chat:random, what happens?

Recap: Real-time Chat Design

You've learned the basics of designing a real-time chat application using Redis Pub/Sub:

Each chat room maps to a unique Redis channel.
Users subscribe to a channel to join a room and receive messages.
Users publish messages to a channel to send them to all subscribers.
Messages should be structured (e.g., JSON) to include sender, timestamp, and content.
Remember that Pub/Sub is non-persistent; messages are gone once delivered.

This foundation allows for highly responsive chat experiences!

常见问题解答

「设计实时聊天」课时是免费的吗？

是的 — 「设计实时聊天」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 Redis Caching & Messaging (Pub/Sub, Streams) 课程的其余内容，请升级到 CoddyKit PRO。 Redis Caching & Messaging (Pub/Sub, Streams) 课程共包含 4 节课。

「设计实时聊天」这节课中我会学到什么？

实现一个基础的实时聊天应用，展示如何使用发布/订阅传递消息你通过在浏览器中直接运行的动手代码来练习 Redis Caching & Messaging (Pub/Sub, Streams)，全天候 AI 导师会在你学习这节课的过程中回答你的问题。

学习 Redis Caching & Messaging (Pub/Sub, Streams) 需要有经验吗？

无需任何先前经验。CoddyKit 上的 Redis Caching & Messaging (Pub/Sub, Streams) 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。这是第 2 节课，共 4 节。

「设计实时聊天」课时需要多长时间？

大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。

我能在这节 Redis Caching & Messaging (Pub/Sub, Streams) 课中编写并运行代码吗？

能。每节 Redis Caching & Messaging (Pub/Sub, Streams) 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。

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