Horizontal Scaling

Stateless HTTP scales by adding servers; WebSocket does not

An HTTP API can scale by adding identical servers behind a round-robin LB; each request lives entirely within its handler. WebSocket connections are persistent — adding a server adds capacity for new connections, but does nothing to bridge messages between connections on different servers. To get cross-server fan-out, you need a message bus.

Redis Pub/Sub is the standard answer

Track 4 introduced this. In production, every WebSocket server publishes outbound messages to Redis and subscribes to receive them; local fan-out happens on each server. Add servers freely; capacity scales linearly until Redis itself becomes the bottleneck (which happens at very large scale — at that point, look at Redis Cluster or Kafka).

Socket.IO ships an adapter

If you are using Socket.IO, scaling is one line: client_manager=socketio.AsyncRedisManager(...). Every sio.emit() automatically bridges across all servers. You write zero pub/sub code.

Code

Topology·text

  +----------+   +----------+   +----------+
  | Server 1 |   | Server 2 |   | Server 3 |
  | Users A,B|   | Users C,D|   | Users E,F|
  +----+-----+   +----+-----+   +----+-----+
       |              |              |
       +-------+------+-------+------+
               |    Redis Pub/Sub
               |  +-----------+
               +->| redis     |
                  +-----------+

  User A sends "hi" to room R
  Server 1 publishes "ws:room:R" -> Redis
  Redis fans out to all three servers
  Each server broadcasts locally to room R members
  -> A,B,C,D,E,F all receive "hi" within ms

Socket.IO Redis adapter·python

import socketio

sio = socketio.AsyncServer(
    async_mode='asgi',
    client_manager=socketio.AsyncRedisManager('redis://redis:6379'),
)

# That's the entire change. sio.emit() now spans all servers.

Stateless HTTP scales by adding servers; WebSocket does not

Redis Pub/Sub is the standard answer

Socket.IO ships an adapter

Code

External links

Exercise

Progress

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