Long Polling vs WebSockets vs SSE

Standard HTTP is a request-response protocol. The client asks, the server answers. The server cannot proactively push data to the client. But many modern applications (chat apps, live sports scores, stock tickers) need the server to send data to the client the instant it becomes available.

1. Short Polling (The Naive Approach)

The client repeatedly sends HTTP requests to the server at regular intervals (e.g., every 2 seconds) asking "Any new data?". The server responds immediately with data or an empty response.

• Pros: Simple to implement.
• Cons: Extremely wasteful. 99% of responses are empty "no new data" responses. Creates massive unnecessary server load and network traffic.

2. Long Polling

An improvement over short polling. The client sends a request to the server. Instead of responding immediately, the server holds the connection open until new data is available (or a timeout occurs). Once the server responds, the client immediately sends a new request, and the cycle repeats.

• Pros: Near-real-time updates. No wasted empty responses.
• Cons: Each "held" request consumes a server thread/connection. Scaling to millions of concurrent users requires enormous server resources. Message ordering can be tricky.
• Used by: Early implementations of Facebook Messenger.

3. WebSockets

WebSockets provide a full-duplex, persistent connection between the client and server. After an initial HTTP handshake (upgrade request), the connection is "upgraded" to a WebSocket connection. Both the client and server can send messages to each other at any time over this single, long-lived TCP connection.

• Pros: True real-time, bidirectional communication. Extremely low overhead (no HTTP headers on every message). Ideal for high-frequency data exchange.
• Cons: More complex to implement (requires WebSocket server infrastructure). Stateful connections make horizontal scaling harder (need sticky sessions or a pub/sub layer like Redis).
• Used by: Slack, Discord, online multiplayer games, collaborative editing (Google Docs), live trading platforms.

4. Server-Sent Events (SSE)

SSE provides a unidirectional, persistent connection from the server to the client. The client opens a connection, and the server pushes events through it whenever new data is available. The client cannot send data back over this connection.

• Pros: Simpler than WebSockets (uses standard HTTP). Built-in browser support via the EventSource API. Automatic reconnection.
• Cons: Unidirectional only (server to client). Limited to text data (no binary). Subject to browser connection limits.
• Used by: Live news feeds, social media timelines, notification streams.

5. Comparison