Server-Sent Events vs WebSockets
Learn how Server-Sent Events provide one-way real-time streaming, how they compare to WebSockets and long polling, and when to choose each technology.
Server-Sent Events vs WebSockets is a free WebSockets & Real-Time Systems with Spring lesson on CoddyKit — lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the WebSockets & Real-Time Systems with Spring learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Another Real-Time Option
WebSockets are not the only option. Server-Sent Events (SSE) give a simpler, one-way streaming channel from server to client over plain HTTP.
What Are Server-Sent Events?
SSE pushes a continuous stream of text events to the browser over one long-lived HTTP connection, received via the EventSource API. It is one-way only.
The SSE Wire Format
SSE uses a simple, line-based wire format over the text/event-stream content type — just data: and optional event: fields.
HTTP/1.1 200 OK
Content-Type: text/event-stream
data: First update
event: price
data: {"symbol":"ACME","price":42}
Receiving Events in the Browser
The client subscribes with EventSource, and the browser automatically reconnects if the stream drops — no manual retry logic needed.
const source = new EventSource('/stream');
source.onmessage = function (e) {
console.log('message:', e.data);
};
source.addEventListener('price', function (e) {
console.log('price event:', e.data);
});Built-In Reconnection
SSE has reconnection built in: the server sets the delay with a retry: field, and the client sends Last-Event-ID to resume where it left off.
SSE vs WebSockets
SSE vs WebSockets: SSE is one-way over plain HTTP with auto-reconnect and text only; WebSockets are full-duplex, use their own protocol, and support binary.
SSE vs Long Polling
Versus long polling, SSE keeps one connection open and streams many events, avoiding the cost of reopening a connection for every message.
When to Use SSE
Reach for SSE for one-way feeds: live notifications, activity streams, stock tickers, dashboards, job progress, and live scores.
When to Use WebSockets
Choose WebSockets when the client sends frequently too: chat, multiplayer, collaborative editing. If you only need server-to-client, SSE is simpler.
Limitations of SSE
SSE has limits: text only, no binary; browsers cap connections per domain (eased by HTTP/2); and older environments may lack support.
A Decision Helper
A simple rule of thumb decides most cases — need client-to-server or binary? Pick WebSocket. Otherwise, SSE.
def choose_transport(needs_client_to_server, needs_binary):
if needs_client_to_server or needs_binary:
return 'WebSocket'
return 'Server-Sent Events'
print(choose_transport(False, False)) # notifications
print(choose_transport(True, False)) # chatQuick Check
One feed, one direction, one connection — how well do you know SSE?
Recap
You learned Server-Sent Events: simple one-way streaming over HTTP with auto-reconnect, how they compare to WebSockets and long polling, and when to use each.
Frequently asked questions
Is the “Server-Sent Events vs WebSockets” lesson free?
Yes — the full text of “Server-Sent Events vs WebSockets” is free to read here on the web, and the WebSockets & Real-Time Systems with Spring course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the WebSockets & Real-Time Systems with Spring course, upgrade to CoddyKit PRO.
What will I learn in “Server-Sent Events vs WebSockets”?
Learn how Server-Sent Events provide one-way real-time streaming, how they compare to WebSockets and long polling, and when to choose each technology. You practise WebSockets & Real-Time Systems with Spring with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start WebSockets & Real-Time Systems with Spring?
No prior experience is required. WebSockets & Real-Time Systems with Spring on CoddyKit is structured for beginners through advanced learners; this is — lesson 4 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Server-Sent Events vs WebSockets” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this WebSockets & Real-Time Systems with Spring lesson?
Yes. Every WebSockets & Real-Time Systems with Spring lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Understanding Real-Time Communication
- WebSockets vs. HTTP Polling
- WebSocket Protocol Fundamentals
- Server-Sent Events vs WebSockets