aiohttp: Should a ClientSession be re-used or created newly foreach request?

I'm using Python 3.11 with aiohttp as a client to POST data to a server. In the end I'll be posting 3 million single requests to the same endpoint, just with different request bodies.

The server alone is quite fast.

class MyClient:
    async def create(single_request_body: dict)->bool:
        """"returns true iff the request was successful"""
        async with aiohttp.ClientSession() as session:
            async with session.post(
                    "https://my-server.org/endpoint", data=single_request_body
                ) as response:
                     return response.status == 201

Now I'm processing my 3million POSTs like this:

all_request_bodys: list[dict] = 3_000_000*[{...}]
my_client = MyClient()
all_post_tasks = [my_client.create(x) for x in all_request_bodies]
await asyncio.gather(*all_post_tasks)

And it's way too slow.

I'm wondering if this is the right way to do it and I was wondering if:

Would it speed up my application, if the ClientSession was an instance variable of the client and I'd re-use the same session instead of creating new sessions for each POST?

In my first tests it didn't seem so, but maybe I'm using it wrong?

Yes your code definitely lead to performance issues. try this one:

class MyClient:
    def __init__(self):
        self.session = aiohttp.ClientSession()

    async def create(self, single_request_body: dict) -> bool:
        """"returns True if the request was successful"""
        async with self.session.post("https://my-server.org/endpoint", data=single_request_body) as response:
            return response.status == 201

async def main():
    all_request_bodies = [{}] * 3_000_000  # Example list of request bodies
    my_client = MyClient()
    all_post_tasks = [my_client.create(x) for x in all_request_bodies]
    await asyncio.gather(*all_post_tasks)

# Run the event loop
asyncio.run(main())

Creating a new session for each request implies a new TCP connection, including all the associated overhead. You create a session in order to reuse already established connections. With that said,

• you can further improve the performance by increasing the number of the concurrent open connections (by default 100). Note that this is specific to the server

 connector = aiohttp.TCPConnector(limit=1000)  # Adjust as needed
 self.session = aiohttp.ClientSession(connector=connector)

• in aiohttp, sessions must be created inside a coroutine. You can pass it as an argument to the create function or instantiate it after creating MyClient

class MyClient:
    def __init__(self):
        self.session = None

    async def create_session(self):
        self.session = aiohttp.ClientSession()

    async def create(self, single_request_body: dict) -> bool:
        if self.session is None:
            raise ValueError("create the session before calling")

        async with self.session.post(
            "https://my-server.org/endpoint", data=single_request_body
        ) as response:
            return response.status == 201