Stop 14 · Real-time CommunicationTopics · 3/6

Server-Sent Events

In short

Live cricket scores, stock tickers and AI answers streaming word by word only need the server to talk. Server-Sent Events do exactly that over plain HTTP, with almost no setup.

A simpler one-way protocol (server-to-client only) for pushing live updates over a single long-lived HTTP connection — a lighter-weight alternative to WebSockets when you don't need bidirectional communication.

The blob listens excitedly to a transistor radio with a cricket bat and ball beside its chair.

Think of it like this

Cricket commentary on the radio

You tune the transistor to the match and just listen. The commentator talks whenever something happens; you don't call the station to ask for each ball. If the signal fades, you retune. SSE is a one-way commentary channel from your server.

Watch it happen

One long response, many events

Browser

Server

GET /score

1/5new EventSource("/score") sends a normal GET with Accept: text/event-stream.

The big ideas

What you need to know

Server to client only

The client can't send on this stream. For the rare message upwards, use a normal POST.

Just HTTP

Works through most proxies and load balancers, uses normal cookies for auth, and needs no special library on the server.

Auto-reconnect built in

The browser's EventSource retries for you. Send an id: with each event so it can resume from the right spot.

Text only

SSE carries UTF-8 text. Send JSON strings; for binary data like audio, WebSockets fit better.

See it in code

Walk through the code

1app.get("/score", (req, res) => {2  res.set({3    "Content-Type": "text/event-stream",4    "Cache-Control": "no-cache",5    Connection: "keep-alive",6  });7  res.flushHeaders();8 9  const timer = setInterval(() => {10    res.write(`data: ${JSON.stringify({ runs: getRuns() })}\n\n`);11  }, 2000);12 13  req.on("close", () => clearInterval(timer));14});15 16// Browser: new EventSource("/score").onmessage = (e) => console.log(e.data);

1/4These headers tell the browser it's an event stream and tell caches not to store it.

If you sit behind Nginx, turn off response buffering for this route (proxy_buffering off), or events arrive in clumps instead of live.

Save yourself the pain

Common mistakes

Oops

Forgetting the blank line after each data: line.

Do this instead

End every event with \n\n. Without it, the browser keeps waiting for the event to finish.

Oops

Reaching for WebSockets just to stream an AI reply.

Do this instead

Streaming one way is exactly what SSE is for. Many AI chat APIs stream their tokens this way.

Your mission

A live score ticker

~20 min

Quick check

Your dashboard only needs the server to push new order counts. Which fits best with the least effort?

Go deeper

1Build an SSE endpoint that streams live updates to a browser using `EventSource`.
2Compare SSE's simplicity to WebSockets for a use case like live notifications.
3Learn SSE automatically reconnects on the client side, unlike raw WebSockets.