Real-time communication: WebSockets vs Server-Sent Events

Real-time communication is a standard expectation in modern web applications. Two primary technologies deliver it: WebSockets and Server-Sent Events (SSE). While both enable real-time data flow from server to client, they have fundamental differences that make each suitable for different scenarios.

This guide compares both approaches and helps you choose.

WebSockets

WebSockets provide full-duplex communication — both client and server can send messages at any time. The connection starts as an HTTP request that upgrades to the WebSocket protocol (ws:// or wss://).

Best for:

• Two-way communication (chat, collaborative editing, multiplayer games)
• Interactive applications where both sides send frequent messages
• Low-latency requirements

Characteristics:

• Full-duplex: both client and server send and receive
• Protocol: custom WebSocket protocol (not HTTP)
• Browser support: excellent (all modern browsers)
• Libraries: Socket.io, ws (Node.js), SignalR (.NET)

Limitations:

• No automatic reconnection (must implement manually)
• No built-in event types (messages are opaque blobs)
• Complex to scale horizontally (stateful connections)
• Firewalls and proxies sometimes block WebSocket connections

Server-Sent Events (SSE)

SSE is a standard HTTP mechanism where the server sends events to the client over a single persistent HTTP connection. The client subscribes to messages using the EventSource API.

const source = new EventSource("/api/events");
source.addEventListener("message", (event) => {
  console.log("Received:", event.data);
});

Best for:

• One-way data streams (notifications, live feeds, status updates)
• Server-to-client updates
• Applications already using HTTP infrastructure

Characteristics:

• Simplex: server pushes, client receives (client can send via separate HTTP requests)
• Protocol: standard HTTP (works through all firewalls and proxies)
• Browser support: good (Firefox, Chrome, Safari), not in some older browsers
• Automatic reconnection built into the EventSource API
• Built-in event types (named events with event: field)

Limitations:

• One-way only — server cannot receive messages on the same connection
• Limited to 6 concurrent connections per domain (browser limitation)
• No binary message support (text-only by default)
• Less mature ecosystem than WebSockets

Performance comparison

| Aspect | WebSockets | SSE | |--------|------------|-----| | Latency | Minimal (persistent connection) | Minimal (persistent connection) | | Overhead | Low (after upgrade) | Low (standard HTTP) | | Concurrency per domain | Unlimited (separate connection) | ~6 connections (browser limit) | | Message format | Binary or text | Text only | | Scaling | Complex (stateful) | Simpler (stateless design) |

When to use each

Use WebSockets when you need bidirectional communication: chat applications, collaborative editing tools, multiplayer games, real-time financial trading platforms.

Use SSE when you need server-to-client updates only: live notifications, social media feeds, real-time dashboards, progress bars for long-running jobs, push-based content updates.

The hybrid approach

Some applications use both. A chat application uses WebSockets for message exchange but falls back to SSE for notifications when the user is less active. Trading platforms use WebSockets for market data and SSE for account notifications.

---

Real-time communication is not one-size-fits-all. The protocol choice should match your data flow pattern — bidirectional for interactive apps, server-push for notification-style updates.

Implementing real-time features in your web application? At Vynta select and implement the right real-time communication protocol for your specific use case.