Server-to-Client Data Push: Comparing Polling, WebSocket, and SSE with Implementation Guide

Implementation Scenarios for Server-to-Client Data Push

In many web applications, the server needs to actively push data to the client, such as real‑time dashboards, unread messages, or chat.

Common Solutions

Polling

WebSocket

SSE

Polling Overview

Polling repeatedly sends HTTP requests from the client, creating unnecessary overhead, consuming connections, and providing a pseudo‑push experience.

Drawbacks include repeated handshakes, constant client processing, limited concurrent connections, and latency.

WebSocket Overview

WebSocket provides full‑duplex communication, allowing bidirectional messages, but requires support for the ws/wss protocol and is heavier.

SSE Overview

Server‑Sent Events (SSE) is a unidirectional, lightweight HTTP‑based protocol for server‑to‑client pushes, with automatic reconnection and low client overhead, but not supported by IE.

Differences Between WebSocket and SSE

WebSocket is full‑duplex and requires a separate protocol; SSE is one‑way, built on HTTP, lighter, and easier to deploy.

SSE API

Creating a connection: var source = new EventSource(url);

Connection State

source.readyState returns 0 (CONNECTING), 1 (OPEN), or 2 (CLOSED).

Events

open – triggered when the connection opens

message – triggered on incoming data

error – triggered on communication errors

Data Format

Content-Type: text/event-stream
Cache-Control: no-cache
Connection: keep-alive

Demo

Front‑end HTML/JS demo and back‑end Node.js Express server that streams JSON messages every second.

Conclusion

SSE is lighter than WebSocket and works over HTTP.

Use SSE for one‑way push scenarios (dashboards, notifications).

Use WebSocket for bidirectional communication (chat).

Polling should be avoided except for compatibility edge cases.