Server-Sent Events vs. WebSocket – Definition and Differences
Compare one-way SSE with bidirectional WebSockets on direction, reconnection, infrastructure compatibility, and which to choose.
Both push data from server to client without polling. SSE does only that; WebSockets carry traffic in both directions. The choice usually follows from whether the client needs to send anything.
1. Key Differences
| Dimension | Server-sent events | WebSocket |
|---|---|---|
| Direction | Server to client | Both |
| Protocol | Ordinary HTTP response | Upgraded connection, own framing |
| Data | Text only | Text or binary |
| Reconnection | Automatic in browsers, with resume | Implement it yourself |
| Event identifiers | Built in, resumable | Application-defined |
| Proxy compatibility | High — it is HTTP | Occasionally problematic |
| Complexity | Low | Higher |
2. Reconnection Is the Understated Difference
A browser reconnects an SSE stream automatically and sends the last event identifier it received, so the server can resume. That is a meaningful amount of correctness handed over for free.
WebSocket clients must detect the drop, back off, reconnect, re-authenticate, and decide whether to resume or resynchronise. All of it is straightforward and all of it is code that must exist.
3. Infrastructure Compatibility
SSE is an HTTP response that stays open, so it passes through proxies, load balancers, and content delivery networks that understand HTTP.
WebSockets require an upgrade that intermediaries must support and preserve. Support is now broad, and occasional environments still interfere.
4. When One-Way Suffices
- Notifications and alerts.
- Live metrics and dashboards.
- Progress on a long-running job.
- Streaming generated text token by token.
- Price, status, and inventory updates.
If the client only listens, SSE is simpler in every dimension that matters.
5. When Bidirectional Is Required
- Chat and collaborative editing.
- Interactive control protocols, including browser automation.
- Multiplayer state synchronisation.
- Anything sending binary in both directions.
6. The Common Mistake
Choosing a WebSocket for a one-way feed, then writing reconnection and resumption logic that SSE would have provided.
The reverse mistake — using SSE where the client must send frequently — leads to a separate request channel alongside the stream, which is workable and inelegant.
7. Practical Notes
Under HTTP/1.1, browsers limit connections per host, which constrains how many SSE streams a page can hold; HTTP/2 multiplexing removes that. Both approaches need keepalives so intermediaries do not reap idle connections, and both are stateful, so scaling requires either sticky routing or shared state.
8. Summary
Server-sent events push text from server to client over ordinary HTTP with automatic reconnection and resumption. WebSockets carry both directions with binary support, at the cost of implementing reconnection.
Choose by direction: one-way feeds should be SSE, and genuine two-way interaction should be a WebSocket.