Understanding aiohttp.TCPConnector pooling & connection limits

Question

I am experimenting with the limit and limit_per_host parameters to aiohttp.connector.TCPConnector.

In the script below, I pass connector = aiohttp.connector.TCPConnector(limit=25, limit_per_host=5) to aiohttp.ClientSession, then open 2 requests to docs.aiohttp.org and 3 to github.com.

The result of session.request is an instance of aiohttp.ClientResponse, and in this example I intentionally do not call .close() on it, either via .close() or __aexit__. I would assume this would leave the connection pool open and decrease the available connections to that (host, ssl, port) triple by -1.

The table below represents the ._available_connections() after each request. Why does the number hang at 4 even after completing the 2nd request to docs.aiohttp.org? Both of these connections are presumably still open and haven't accessed ._content yet or been closed. Shouldn't the available connections decrease by 1?

After Request Num.        To                    _available_connections
1                         docs.aiohttp.org      4
2                         docs.aiohttp.org      4   <--- Why?
3                         github.com            4
4                         github.com            3
5                         github.com            2

Furthermore, why does ._acquired_per_host only ever contain 1 key? I guess I may be understanding the methods of TCPConnector; what explains the behavior above?

Full script:

import aiohttp


async def main():
    connector = aiohttp.connector.TCPConnector(limit=25, limit_per_host=5)

    print("Connector arguments:")
    print("_limit:", connector._limit)
    print("_limit_per_host:", connector._limit_per_host)
    print("-" * 70, end="\n\n")

    async with aiohttp.client.ClientSession(
        connector=connector,
        headers={"User-Agent": "Mozilla/5.0 (Windows NT 6.3; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2225.0 Safari/537.36"},
        raise_for_status=True
    ) as session:

        # Make 2 connections to docs.aiohttp.org and 
        #      3 connections to github.com
        #
        # Note that these instances intentionally do not use
        # .close(), either explicitly or via __aexit__
        # in an async with block

        r1 = await session.request(
            "GET",
            "https://docs.aiohttp.org/en/stable/client_reference.html#connectors"
        )
        print_connector_attrs("r1", session)

        r2 = await session.request(
            "GET",
            "https://docs.aiohttp.org/en/stable/index.html"
        )
        print_connector_attrs("r2", session)

        r3 = await session.request(
            "GET",
            "https://github.com/aio-libs/aiohttp/blob/master/aiohttp/client.py"
        )
        print_connector_attrs("r3", session)

        r4 = await session.request(
            "GET",
            "https://github.com/python/cpython/blob/3.7/Lib/typing.py"
        )
        print_connector_attrs("r4", session)

        r5 = await session.request(
            "GET",
            "https://github.com/aio-libs/aiohttp"
        )
        print_connector_attrs("r5", session)


def print_connector_attrs(name: str, session: aiohttp.client.ClientSession):
    print("Connection attributes for", name, end="\n\n")
    conn = session._connector
    print("_conns:", conn._conns, end="\n\n")
    print("_acquired:", conn._acquired, end="\n\n")
    print("_acquired_per_host:", conn._acquired_per_host, end="\n\n")
    print("_available_connections:")
    for k in conn._acquired_per_host:
        print("\t", k, conn._available_connections(k))
    print("-" * 70, end="\n\n")


if __name__ == "__main__":
    import asyncio
    loop = asyncio.get_event_loop()
    loop.run_until_complete(main())

The output is pasted at https://pastebin.com/rvfzMTe3. I've put it there rather than here because the lines are long and not very wrap-able.

Answer 1

To address your main question “Why does the number hang at 4 even after completing the 2nd request to docs.aiohttp.org?”, the connection count will be decreased when aiohttp.connector.BaseConnector._release() is called and this would be called if you were to use the async with on session.request() or explicitly call .close() or after you read the response content with .read(). Or as it is in the case of docs.aiohttp.org requests when the server send an EOF (this happens when the server does not wait for you to stream the response content but send it all in response to the first request). And this is what is happening here. You can see for yourself when you put a breakpoint in the aiohttp.connector.BaseConnector._release() and examine the stack, you will see that aiohttp.http_parser.DeflateBuffer.feed_eof() is called. And that aiohttp.streams.StreamReade._eof_callbacks contains aiohttp.client_reqrep.ClientResponse._response_eof() that is calling self._connection.release()

As for your second question “why does ._acquired_per_host only ever contain 1 key”, well it is odd. But I can not see nothing about it in the doc. It is a private attribute so we should not mess with it. It is probably just badly named private attribute.