justindujardin/sse-starlette

Server Sent Events for Starlette and FastAPI

Background: https://sysid.github.io/sse/

Installation:

pip install sse-starlette

Usage:

import asyncio
import uvicorn
from starlette.applications import Starlette
from starlette.routing import Route
from sse_starlette.sse import EventSourceResponse

async def numbers(minimum, maximum):
    for i in range(minimum, maximum + 1):
        await asyncio.sleep(0.9)
        yield dict(data=i)

async def sse(request):
    generator = numbers(1, 5)
    return EventSourceResponse(generator)

routes = [
    Route("/", endpoint=sse)
]

app = Starlette(debug=True, routes=routes)

if __name__ == "__main__":
    uvicorn.run(app, host="0.0.0.0", port=8000, log_level='info')

Output:

Caveat: SSE streaming does not work in combination with GZipMiddleware.

Be aware that for proper server shutdown your application must stop all running tasks (generators). Otherwise you might experience the following warnings at shutdown: Waiting for background tasks to complete. (CTRL+C to force quit).

Client disconnects need to be handled in your Request handler (see example.py):

async def endless(req: Request):
    async def event_publisher():
        i = 0
        try:
          while True:
              disconnected = await req.is_disconnected()
              if disconnected:
                  _log.info(f"Disconnecting client {req.client}")
                  break
              i += 1
              yield dict(data=i)
              await asyncio.sleep(0.2)
          _log.info(f"Disconnected from client {req.client}")
        except asyncio.CancelledError as e:
          _log.info(f"Disconnected from client (via refresh/close) {req.client}")
          # Do any other cleanup, if any
          raise e
    return EventSourceResponse(event_publisher())

Run the tests:

make test

Changelog

0.6.1 (2020-10-24)

• updated example with proper error handling

0.6.0 (2020-10-24)

• In case uvicorn is used: monkeypatch uvicorn signal-handler, in order to gracefully shutdown long-running handlers/generators.