sse
Server-Sent Events for real-time server-to-client streaming. Express, Fastify, FastAPI, Spring WebFlux SSE implementations. Event streams, reconnection, and EventSource API. USE WHEN: user mentions "SSE", "Server-Sent Events", "EventSource", "event stream", "text/event-stream", "live feed", "streaming updates" DO NOT USE FOR: bidirectional communication - use `socket-io`; WebRTC - use `webrtc`; LLM streaming - use AI SDK skills
What this skill does
# Server-Sent Events (SSE)
## Express Server
```typescript
app.get('/api/events', (req, res) => {
res.writeHead(200, {
'Content-Type': 'text/event-stream',
'Cache-Control': 'no-cache',
'Connection': 'keep-alive',
});
// Send initial data
res.write(`data: ${JSON.stringify({ type: 'connected' })}\n\n`);
// Named events
function sendEvent(event: string, data: unknown) {
res.write(`event: ${event}\ndata: ${JSON.stringify(data)}\n\n`);
}
// Keep-alive
const keepAlive = setInterval(() => res.write(': keep-alive\n\n'), 30000);
// Listen for updates
const handler = (data: unknown) => sendEvent('update', data);
eventEmitter.on('update', handler);
req.on('close', () => {
clearInterval(keepAlive);
eventEmitter.off('update', handler);
});
});
```
## FastAPI (Python)
```python
from fastapi import FastAPI
from fastapi.responses import StreamingResponse
import asyncio, json
app = FastAPI()
@app.get("/api/events")
async def events():
async def stream():
while True:
data = await get_next_update()
yield f"event: update\ndata: {json.dumps(data)}\n\n"
await asyncio.sleep(0.1)
return StreamingResponse(stream(), media_type="text/event-stream")
```
## Spring WebFlux
```java
@GetMapping(path = "/api/events", produces = MediaType.TEXT_EVENT_STREAM_VALUE)
public Flux<ServerSentEvent<String>> events() {
return Flux.interval(Duration.ofSeconds(1))
.map(seq -> ServerSentEvent.<String>builder()
.id(String.valueOf(seq))
.event("update")
.data("{\"count\":" + seq + "}")
.build());
}
```
## Browser Client (EventSource)
```typescript
const source = new EventSource('/api/events');
source.onopen = () => console.log('Connected');
// Default "message" event
source.onmessage = (e) => console.log(JSON.parse(e.data));
// Named events
source.addEventListener('update', (e) => {
const data = JSON.parse(e.data);
updateUI(data);
});
source.onerror = (e) => {
if (source.readyState === EventSource.CLOSED) {
console.log('Connection closed');
}
// Browser auto-reconnects
};
// Cleanup
source.close();
```
### With Auth Headers (fetch-based)
```typescript
async function* sseStream(url: string, token: string) {
const response = await fetch(url, {
headers: { Authorization: `Bearer ${token}` },
});
const reader = response.body!.getReader();
const decoder = new TextDecoder();
let buffer = '';
while (true) {
const { done, value } = await reader.read();
if (done) break;
buffer += decoder.decode(value, { stream: true });
const lines = buffer.split('\n\n');
buffer = lines.pop()!;
for (const block of lines) {
const dataLine = block.split('\n').find((l) => l.startsWith('data: '));
if (dataLine) yield JSON.parse(dataLine.slice(6));
}
}
}
```
## Anti-Patterns
| Anti-Pattern | Fix |
|--------------|-----|
| No keep-alive pings | Send comment (`: keep-alive`) every 30s |
| Missing cleanup on disconnect | Listen for `req.on('close')` and clean up |
| No event IDs for reconnection | Send `id:` field, use `Last-Event-ID` header |
| EventSource doesn't support auth headers | Use fetch-based SSE client for auth |
| No backpressure | Check `res.writableEnded` before writing |
## Production Checklist
- [ ] Keep-alive pings configured
- [ ] Cleanup on client disconnect
- [ ] Event IDs for reconnection support
- [ ] Connection limits per user
- [ ] Reverse proxy timeout configured (nginx: `proxy_read_timeout`)
- [ ] Load balancer sticky sessions or pub/sub for multi-server
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