Websockets vs. Server-Sent events/EventSource

javascript

real-time-communication

server-sent-events

websockets

byAlex Kataev·Mar 10, 2025

Prefer WebSockets for full-duplex, two-way interaction:

let socket = new WebSocket('ws://yourserver/path');
socket.onmessage = (e) => console.log('Message from beyond:', e.data); // aliens saying hello 👽
socket.send('Hello aliens!');

Inclined towards Server-Sent Events (SSE) for unidirectional data transmission from server to a client:

let eventSource = new EventSource('http://yourserver/path');
eventSource.onmessage = (e) => console.log('Incoming transmission:', e.data); // space station broadcasts 📡

WebSockets: bidirectional chats, high-stakes gaming. SSE: client-only data such as news feeds, server-sided updates.

Choosing the right one

Data: Text vs. Binary

Opt for WebSockets if your application relies heavily on binary and non-textual data. SSE works well when you are dealing mostly with text-based UTF-8 data.

Firewall friendliness

SSE might be your savior if you are haunted by firewall restrictions, providing you peace by using standard HTTP protocol. WebSockets may have issues with certain firewalls as it upgrades the HTTP handshake and could trigger packet inspection.

Automatic reconnections

SSE has you covered if handling reconnection is a pain point, as it comes with automatic reconnections. For WebSockets, you'll have to roll up your sleeves and write some code to handle reconnections.

Simplicity in setup

If time is ticking, you might want to consider SSE over WebSockets due to its simplicity in backend setup and its handy HTTP protocol.

Usage contexts

For high-frequency, two-way data exchanges like in multiplayer games, choose WebSockets for the real-time requirements. If you need to push server-side event notifications to the client, use SSE for the job.

A closer look at implementation

WebSockets: unlimited connections with overhead

WebSockets allow more connections without a browser-imposed limit, unlike SSE's typical 6. However, each WebSocket client requires a dedicated socket, which could demand a bit more from our server.

Libraries and Polyfills

socket.io is your lifesaver for working with WebSockets, taking care of cross-browser compatibility and real-time communication. For SSE, libraries like EventSource.js provide polyfills for non-supporting browsers, making life easier.

Think about performance

Be careful about server-load and performance-considerations. WebSockets might outshine in case of ultra-high traffic, but SSE can be better in simpler scenarios due to efficiency and easier setup.

Practical tips and guidance

Sudden disconnections?

Handle connection disruptions properly with WebSockets and SSE. Make sure you are listening for the error event in both cases- it's basically like subscribing to the "Oops, something went wrong!" newsletter.

Going big? Consider load balancing

When your application scales up, for WebSockets, consider incorporating a message broker or pub/sub service. For SSE, having a load balancer with sticky sessions should do the job.

Don't forget security

Regardless of technology, a golden rule is to validate incoming data to avoid SQL injection attacks. Use wss:// with WebSockets and https:// with SSE for secure, encrypted connections.

explain-codes / Javascript / Websockets vs. Server-Sent events/EventSource

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