Add one clarification to docs on dependencies in websocket routes

[bhumkong]

First Check

• I added a very descriptive title here.
• I used the GitHub search to find a similar question and didn't find it.
• I searched the FastAPI documentation, with the integrated search.
• I already searched in Google "How to X in FastAPI" and didn't find any information.
• I already read and followed all the tutorial in the docs and didn't find an answer.
• I already checked if it is not related to FastAPI but to Pydantic.
• I already checked if it is not related to FastAPI but to Swagger UI.
• I already checked if it is not related to FastAPI but to ReDoc.

Commit to Help

• I commit to help with one of those options 👆

Example Code

async def get_async_db_session() -> AsyncSession:
    async with async_session_maker() as connection:
        yield connection

async def authenticate_user(
    db_session: AsyncSession = Depends(get_async_db_session),
    user_token: str = Header(),
) -> User:
    ...  # auth logic

@router.websocket("/ws")
async def websocket_endpoint(
    websocket: WebSocket,
    auth_user: User = Depends(authenticate_user),
):
    ...  # ws handling logic

Description

When you have a websocket endpoint which uses a dependency that holds some resource for the time of execution, this can lead to problems. Maybe this dependency itself does not acquire a resource, but it depends on another dependency which acquires a resource - so it's easy not to notice.

I propose to add a clarification to this page that you must not generally use dependencies with yield in a websocket endpoint.

I faced this problem myself:
Added websocket endpoint to my REST application. In this websocket endpoint I reused a dependency to authenticate users. This dependency requires DB session dependency. So, DB connection is acquired when client connects to ws and it's held until client disconnects from ws. But it's actually only needed in the beginning. I did not notice it, and because of this my app started having huge delays for both HTTP and websocket endpoints when a certain number of connected clients is reached. It was hard to debug.

Operating System

Linux

Operating System Details

No response

FastAPI Version

0.110.0

Pydantic Version

2.6.2

Python Version

3.11.7

Additional Context

I can make a PR, but first want to ensure this is a good idea.