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Detecting Offline

The server already knows if Bob is online — it just has to check the Redis registry

The connection registry was built to route messages to the right WebSocket server. It doubles as an online/offline detector for free.

The Redis connection registry

Every time a user establishes a WebSocket connection, the WS server writes an entry to Redis:

SET ws:bob → ws_server_3
SET ws:alice → ws_server_1

Every time a user disconnects, the entry is deleted.

This means at any point in time, the Redis registry is a live map of who is online. If ws:bob exists in Redis — Bob is online. If it doesn't — Bob is offline.

How the server uses this for delivery

When Alice's message arrives and needs to be delivered to Bob:

Step 1 → GET ws:bob from Redis

Step 2a → Entry found: ws_server_3
           → Bob is online
           → Forward message to ws_server_3
           → ws_server_3 pushes to Bob's WebSocket
           → Done

Step 2b → No entry found
           → Bob is offline
           → Do not attempt WebSocket delivery
           → Trigger offline delivery flow (covered in next files)

No separate online/offline status service needed. The registry check that was already needed for routing doubles as the online check. One Redis lookup, two purposes.

What "offline" actually means here

Offline means one of three things:

1. Bob closed WhatsApp              → WebSocket closed, registry entry deleted
2. Bob's internet dropped           → WebSocket timed out, registry entry deleted
3. Bob's phone is off entirely      → Same as above from the server's perspective

From the server's perspective, all three look identical — no registry entry. The offline delivery mechanism handles all three the same way.