Server Sent Events for Starlette and FastAPI

Implements the Server-Sent Events specification.

Background: https://sysid.github.io/server-sent-events/

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:
              i += 1
              yield dict(data=i)
              await asyncio.sleep(0.2)
        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())

Thread Safety with SQLAlchemy Sessions and Similar Objects

The streaming portion of this package is accomplished via anyio TaskGroups. Care needs to be taken to avoid passing objects that are not thread-safe to generators you use to yield streaming data.

For example, if you are using SQLAlchemy, you should not use/pass an AsyncSession object to your generator:

# ❌ This can result in "The garbage collector is trying to clean up non-checked-in connection..." errors
async def bad_route():
    async with AsyncSession() as session:
        async def generator():
            async for row in session.execute(select(User)):
                yield dict(data=row)

    return EventSourceResponse(generator)

Instead, ensure you create sessions within the generator itself

# ✅ This is safe
async def good_route():
    async def generator():
        async with AsyncSession() as session:
            async for row in session.execute(select(User)):
                yield dict(data=row)

    return EventSourceResponse(generator)

Special use cases

Customize Ping

By default, the server sends a ping every 15 seconds. You can customize this by:

1. setting the ping parameter
2. by changing the ping event to a comment event so that it is not visible to the client

@router.get("")
async def handle():
    generator = numbers(1, 100)
    return EventSourceResponse(
        generator,
        headers={"Server": "nini"},
        ping=5,
        ping_message_factory=lambda: ServerSentEvent(**{"comment": "You can't see\r\nthis ping"}),
    )

SSE Send Timeout

To avoid 'hanging' connections in case HTTP connection from a certain client was kept open, but the client stopped reading from the connection you can specifiy a send timeout (see #89).

EventSourceResponse(..., send_timeout=5)  # terminate hanging send call after 5s

Fan out Proxies

Fan out proxies usually rely on response being cacheable. To support that, you can set the value of Cache-Control. For example:

return EventSourceResponse(
        generator(), headers={"Cache-Control": "public, max-age=29"}
    )

Error Handling

See example: examples/error_handling.py

Sending Responses without Async Generators

Async generators can expose tricky error and cleanup behavior especially when they are interrupted.

Background: Cleanup in async generators.

Example no_async_generators.py shows an alternative implementation that does not rely on async generators but instead uses memory channels (examples/no_async_generators.py).

Using pytest to test SSE Endpoints

When testing more than a single SSE endpoint via pytest, one may encounter the following error: RuntimeError: <asyncio.locks.Event object at 0xxxx [unset]> is bound to a different event loop.

A workaround to fix this error is to use the following fixture on all tests that use SSE:

@pytest.fixture
def reset_sse_starlette_appstatus_event():
    """
    Fixture that resets the appstatus event in the sse_starlette app.

    Should be used on any test that uses sse_starlette to stream events.
    """
    # See https://github.com/sysid/sse-starlette/issues/59
    from sse_starlette.sse import AppStatus

    AppStatus.should_exit_event = None

For details, see Issue#59.

Development, Contributing

1. install pdm: pip install pdm
2. install dependencies using pipenv: pdm install -d.
3. To run tests:

Makefile

• make sure your virtualenv is active
• check Makefile for available commands and development support, e.g. run the unit tests:

make test
make tox

For integration testing you can use the provided examples in tests and examples.

If you are using Postman, please see: #47 (comment)