Building Real-Time Features with WebSockets and Server-Sent Events
Our early real-time features all reached for WebSockets by default — a live collaborative editing feature, a notifications feed, a live order-status tracker, all built on the same socket connection. That worked, but it meant every one of those features inherited WebSocket's operational complexity: connection state management, reconnection logic, and load balancer configuration that supports sticky sessions or a shared pub/sub layer across server instances.
The order-status tracker was the feature that made us reconsider. It's genuinely one-directional — the server pushes status updates, the client never sends anything back over that channel — which is exactly the case Server-Sent Events was built for. Moving it from a WebSocket to SSE cut the client-side code roughly in half, since SSE reconnects automatically out of the box via the EventSource API without us writing custom reconnection logic, and it plays more naturally with standard HTTP infrastructure since it's just a long-lived HTTP response rather than a protocol upgrade.
The collaborative editing feature stayed on WebSockets, correctly, because it's genuinely bidirectional — each client both sends and receives cursor positions and document changes in real time. We used Socket.io on top of raw WebSockets specifically for its automatic fallback and room-based broadcasting, backed by a Redis adapter so that messages broadcast correctly across multiple server instances behind a load balancer rather than only reaching clients connected to the same server process.
A production incident pushed the SSE migration further: during a traffic spike, we hit our WebSocket server's connection limit, and every real-time feature on the platform degraded simultaneously because they all shared one connection pool. Splitting one-directional features onto SSE, which rides on standard HTTP and scales more like ordinary HTTP traffic, meant a spike in notification volume no longer competed with the collaborative editor for the same finite connection budget.
The rule we apply now: if data only flows server-to-client, SSE first, since it's simpler to operate and degrades more gracefully. WebSockets are reserved for genuinely bidirectional, low-latency cases like collaborative editing or multiplayer features, where the added complexity is buying you something SSE structurally can't provide.
