Question 1

What are Server-Sent Events (SSE)?

Accepted Answer

Server-Sent Events is a browser API (EventSource) that allows a server to push updates to the client over a single, long-lived HTTP connection. Unlike WebSockets, SSE is one-directional (server → client only), uses plain HTTP, and automatically reconnects. It's ideal for live feeds, notifications, progress updates, and streaming AI responses.

Question 2

What is the difference between SSE, WebSocket, and Long Polling?

Accepted Answer

SSE (Server-Sent Events) is one-way server→client over HTTP/1.1, with automatic reconnect and built-in event IDs — simple to implement on any HTTP server. WebSockets are full-duplex bidirectional, require an upgrade handshake, and suit real-time chat or games. Long Polling repeatedly fires HTTP requests and waits — it works everywhere but is inefficient and adds latency. For read-only streams (dashboards, notifications, AI responses), SSE is the simplest and most efficient choice.

Question 3

How do I test SSE endpoints in development?

Accepted Answer

You can test SSE endpoints using this browser-based tester (note CORS restrictions apply), curl (curl -N http://localhost:3000/events), or browser DevTools → Network → filter by 'EventStream'. For local dev, the endpoint must serve text/event-stream with CORS headers if testing from a different origin. Use the mock demo in this tool to see the SSE format without needing a real server.

Question 4

What is the EventSource API?

Accepted Answer

EventSource is the browser API for consuming SSE streams. Usage: const es = new EventSource('/events'); es.onmessage = e => console.log(e.data); es.addEventListener('myEvent', e => ...); es.onerror = e => .... The browser automatically reconnects on disconnect using the Last-Event-ID header. EventSource only supports GET requests and cannot send custom request headers directly — use a fetch-based polyfill or backend auth proxy if you need headers or POST.

Question 5

How do I add custom event types in SSE?

Accepted Answer

In SSE, each message block can have an event: field to specify a named event type. Example server output: event: userJoined
data: {"user": "Alice"}

. On the client, listen with: es.addEventListener('userJoined', e => console.log(JSON.parse(e.data))). Messages without an event: field are dispatched as the default 'message' event and caught by es.onmessage.

SSE Server-Sent Events Tester

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