I’m using server sent events to send real time updates to clients. I’ve thought about 2 ways on how I can implement this

1. subscribe to a redis channel and use nodejs event emitter. Whenever there is a new message received from channel, use event emitter listener to write message to response stream. The problem I have with this is that the event emitter is synchronous and will block the event loop if there are a larger number of connections.

2. whenever a new request is made to subscribe to events, I create a new redis client and subscribe the client to channel. So this approach leads to N number of redisClients where N = number of users connected to server. Now, I’m aware I’m limited by the max limit of connections on redis server. Not a great way but if this is the way to go, I’ll use redis clusters.

I’ve added some code I wrote for both the approaches

1. using event emitter

const emitter = new EventEmitter();

redisClient.subscribe(CHANNEL_NAME, (msg, channel) => {
  emitter.emit("event",msg);
});


function subscribe(req: Request, res: Response) {
    const handleEvent = (msg: string) => {
      res.write(formatSseData("message", msg));
    };

    emitter.addListener("event", handleEvent);

    req.on("close", () => {
      emitter.removeListener("event", handleEvent);
    });
  }

2. Setting up new redis client for every new user connected

async function subscribe(req: Request, res: Response) {
    try {
      const redisClient = createClient({ url: "redis://localhost:6379" });

      await redisClient.connect();

      redisClient.subscribe(CHANNEL_NAME, (msg, channel) => {
        res.write(formatSseData("message", msg));
      });

      req.on("close", () => {
        redisClient
          .unsubscribe(CHANNEL_NAME)
          .catch(err => {
            console.log("error while unsubscribing", err);
          });
      });
    } catch (error) {
      console.error("Failed to connect: %s", (error as Error).message);
    }
  }

Which approach would be better here when scaling the app?