websockets-sse

WebSockets and SSE Patterns

Real-time communication patterns for FastAPI: WebSockets for bidirectional, SSE for server-push.

When to Activate

• Building real-time features (live updates, streaming LLM output, notifications)
• Choosing between WebSockets and SSE for a use case
• Implementing connection management (connect/disconnect, broadcast)
• Scaling real-time across multiple workers with Redis
• Handling backpressure or slow consumers disconnecting other clients
• Writing tests for SSE or WebSocket endpoints
• Implementing Node.js server-side WebSocket handling
• Debugging connection drops, backpressure, or missed events
• Implementing heartbeats and client-side reconnection

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WebSocket vs SSE — When to Use Which

	WebSocket	SSE
Direction	Bidirectional	Server → client only
Protocol	ws:// / wss://	Plain HTTP
Browser reconnect	Manual	Automatic (EventSource)
Proxy / CDN support	Needs config	Works everywhere
Use case	Chat, collaborative editing, games	Notifications, live feeds, LLM streaming
Load balancer sticky sessions	Required (or Redis)	Required (or Redis)

Rule of thumb: use SSE unless the client needs to send frequent messages (more than form submit). LLM token streaming → SSE. Chat → WebSocket.

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SSE with FastAPI

Basic SSE endpoint

import asyncio
from fastapi import APIRouter
from fastapi.responses import StreamingResponse

router = APIRouter()

async def event_generator(task_id: str):
    """Yield SSE-formatted strings."""
    while True:
        data = await get_next_update(task_id)   # your async data source
        if data is None:
            break
        yield f"data: {data}\n\n"   # SSE format: "data: ...\n\n"
        await asyncio.sleep(0)       # yield control to event loop

@router.get("/tasks/{task_id}/stream")
async def stream_task(task_id: str):
    return StreamingResponse(
        event_generator(task_id),
        media_type="text/event-stream",
        headers={
            "Cache-Control": "no-cache",
            "X-Accel-Buffering": "no",   # disable nginx buffering
        },
    )