WebSockets vs Server-Sent Events

Overview

WebSockets and Server-Sent Events (SSE) both push data to clients in real time, but they differ in direction and complexity. WebSockets open a persistent, full-duplex channel where client and server exchange messages freely. SSE keeps a single HTTP response open and streams a one-way sequence of events from server to client.

Key Differences

Directionality is the defining distinction. WebSockets are bidirectional: either side can send at any time, which suits interactive applications. SSE is unidirectional, server to client only; if the client needs to send data, it makes ordinary HTTP requests alongside the stream.

SSE is simpler and rides native HTTP. It needs no protocol upgrade, works through standard proxies and HTTP/2, and the browser's EventSource API handles parsing and, crucially, automatic reconnection with a Last-Event-ID header so the server can resume from where it left off. WebSockets upgrade to a separate ws:// connection, can require special handling at proxies and firewalls, and leave reconnection logic to the developer.

Data capabilities differ: WebSockets carry both text and binary frames, while SSE transmits UTF-8 text only. For binary payloads or very high message rates in both directions, WebSockets are the stronger fit.

When to Choose WebSockets

Choose WebSockets when the client must send frequently and with low latency: chat applications, multiplayer games, collaborative editors, trading interfaces, and any scenario needing genuine two-way communication or binary streaming.

When to Choose Server-Sent Events

Choose SSE when the server mainly pushes updates and the client mostly listens: live dashboards, notifications, activity feeds, build or job progress, and streaming LLM token output to a web UI. SSE's automatic reconnection and HTTP friendliness make it markedly simpler to deploy and operate.

Verdict

WebSockets and SSE are not strict competitors; they suit different communication shapes. Reach for SSE first when updates flow one way—it is simpler, resilient, and infrastructure-friendly. Reach for WebSockets when you need real bidirectional, low-latency, or binary exchange. Some applications combine both, using SSE for broadcasts and WebSockets for interactive sessions.