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Private messaging - Part IV

This guide has four distinct parts:

  • Part I: initial implementation
  • Part II : persistent user ID
  • Part III : persistent messages
  • Part IV (current): scaling up

Here's where we were at the end of the 3rd part:

Chat

We will see now how we can scale to multiple Socket.IO servers, for high availability / load-balancing purposes.

Installation​

Let's checkout the branch for part IV:

git checkout examples/private-messaging-part-4

Here's what you should see in the current directory:

├── babel.config.js
├── package.json
├── public
│   ├── favicon.ico
│   ├── fonts
│   │   └── Lato-Regular.ttf
│   └── index.html
├── README.md
├── server
│   ├── cluster.js (created)
│   ├── docker-compose.yml (created)
│   ├── index.js (updated)
│   ├── messageStore.js (updated)
│   ├── package.json (updated)
│   └── sessionStore.js (updated)
└── src
    ├── App.vue
    ├── components
    │   ├── Chat.vue
    │   ├── MessagePanel.vue
    │   ├── SelectUsername.vue
    │   ├── StatusIcon.vue
    │   └── User.vue
    ├── main.js
    └── socket.js

The complete diff can be found here.

Updating the server​

For this last part, we need 3 additional dependencies on the server-side:

We also need a Redis instance. For your convenience, a docker-compose.yml file is provided:

cd server
docker-compose up -d

npm install
npm start

This will create 4 Node.js workers, each running the same index.js file.

On the client-side, no change is needed, we will focus on the server-side here.

How it works​

Creating multiple servers​

When creating multiple Socket.IO servers, there are two things to do:

  • you need to enable sticky-session (please see here for the complete explanation)
  • you need to replace the default in-memory adapter with the Redis adapter (or another compatible adapter)

In our example, the @socket.io/sticky module is used to ensure that requests from a given client are always routed to the same Socket.IO server. This is what is called "sticky-session":

Sticky session

Note: we could also have created several processes listening to different ports (or used multiple hosts), and add a reverse-proxy in front of them. Enabling sticky-session for common reverse-proxy solutions like NginX or HAProxy is covered in the documentation.

The cluster is created in the server/cluster.js file:

const cluster = require("cluster");
const http = require("http");
const { setupMaster } = require("@socket.io/sticky");

const WORKERS_COUNT = 4;

if (cluster.isMaster) {
  console.log(`Master ${process.pid} is running`);

  for (let i = 0; i < WORKERS_COUNT; i++) {
    cluster.fork();
  }

  cluster.on("exit", (worker) => {
    console.log(`Worker ${worker.process.pid} died`);
    cluster.fork();
  });

  const httpServer = http.createServer();
  setupMaster(httpServer, {
    loadBalancingMethod: "least-connection", // either "random", "round-robin" or "least-connection"
  });
  const PORT = process.env.PORT || 3000;

  httpServer.listen(PORT, () =>
    console.log(`server listening at http://localhost:${PORT}`)
  );
} else {
  console.log(`Worker ${process.pid} started`);
  require("./index");
}

In our existing server/index.js file, there is a single change: the HTTP server created by the worker process does not actually listen to a given port, the requests will be handled by the master process and then forwarded to the right worker.

Before:

httpServer.listen(PORT, () =>
  console.log(`server listening at http://localhost:${PORT}`)
);

After:

setupWorker(io);

The setupWorker method provided by the @socket.io/sticky will take care of the synchronization between the master and the worker.

Sessions & messages​

Now that sticky-session is enabled, we need to share sessions and messages across the Socket.IO servers.

We create a new SessionStore based on Redis. We will store each session in a Redis hash, with the HSET command:

class RedisSessionStore extends SessionStore {
  // ...
  saveSession(id, { userID, username, connected }) {
    this.redisClient
      .multi()
      .hset(`session:${id}`, "userID", userID, "username", username, "connected", connected)
      .expire(`session:${id}`, SESSION_TTL)
      .exec();
  }
  // ...
}

We also set an expiry to the key in order to clean up old sessions.

Fetching the session is quite straightforward, with the HMGET command:

const mapSession = ([userID, username, connected]) =>
  userID ? { userID, username, connected: connected === "true" } : undefined;

class RedisSessionStore extends SessionStore {
  // ...
  findSession(id) {
    return this.redisClient
      .hmget(`session:${id}`, "userID", "username", "connected")
      .then(mapSession);
  }
  // ...
}

Fetching all sessions is a bit more complex:

class RedisSessionStore extends SessionStore {
  // ...
  async findAllSessions() {
    // first, we fetch all the keys with the SCAN command
    const keys = new Set();
    let nextIndex = 0;
    do {
      const [nextIndexAsStr, results] = await this.redisClient.scan(
        nextIndex,
        "MATCH",
        "session:*",
        "COUNT",
        "100"
      );
      nextIndex = parseInt(nextIndexAsStr, 10);
      results.forEach((s) => keys.add(s));
    } while (nextIndex !== 0);

    // and then we retrieve the session details with multiple HMGET commands
    const commands = [];
    keys.forEach((key) => {
      commands.push(["hmget", key, "userID", "username", "connected"]);
    });
    return this.redisClient
      .multi(commands)
      .exec()
      .then((results) => {
        return results
          .map(([err, session]) => (err ? undefined : mapSession(session)))
          .filter((v) => !!v);
      });
  }
}

Similarly, we create a new MessageStore based on Redis. We will store all the messages linked to a given user in a Redis list, with the RPUSH command:

class RedisMessageStore extends MessageStore {
  // ...
  saveMessage(message) {
    const value = JSON.stringify(message);
    this.redisClient
      .multi()
      .rpush(`messages:${message.from}`, value)
      .rpush(`messages:${message.to}`, value)
      .expire(`messages:${message.from}`, CONVERSATION_TTL)
      .expire(`messages:${message.to}`, CONVERSATION_TTL)
      .exec();
  }
  // ...
}

Retrieving the messages is done with the LRANGE command:

class RedisMessageStore extends MessageStore {
  // ...
  findMessagesForUser(userID) {
    return this.redisClient
      .lrange(`messages:${userID}`, 0, -1)
      .then((results) => {
        return results.map((result) => JSON.parse(result));
      });
  }
}

Forwarding messages​

There is one last modification that is needed: we need to make sure that messages actually reach the recipient, even if this recipient is not connected on the same Socket.IO server:

Broadcasting with the Redis adapter

This is the duty of the Redis adapter, which relies on the Redis pub/sub mechanism to broadcast messages between the Socket.IO servers and eventually reach all clients.

const httpServer = require("http").createServer();
const Redis = require("ioredis");
const redisClient = new Redis();
const io = require("socket.io")(httpServer, {
  cors: {
    origin: "http://localhost:8080",
  },
  adapter: require("socket.io-redis")({
    pubClient: redisClient,
    subClient: redisClient.duplicate(),
  }),
});

And that's it! If you have a Redis CLI on your machine, you can check the messages that are sent on the wire:

$ redis-cli
127.0.0.1:6379> PSUBSCRIBE socket.io*
Reading messages... (press Ctrl-C to quit)
1) "psubscribe"
2) "socket.io*"
3) (integer) 1
1) "pmessage"
2) "socket.io*"
3) "socket.io#/#"
4) "\x93\xa6XFD3OF\x83..."

Documentation:

Note: with the Redis adapter, the allSockets() method which is used in the "disconnect" handler automatically returns the Socket IDs across all Socket.IO servers, so there is nothing to update.

Review​

OK, so let's sum it up: we have created a fully functional chat (yes, once again!), robust, ready to scale horizontally, which allowed us to introduce some useful Socket.IO features:

Thanks for reading!