How to reuse aiohttp ClientSession pool?

The docs say to reuse the ClientSession:

Don’t create a session per request. Most likely you need a session per application which performs all requests altogether.

A session contains a connection pool inside, connection reusage and keep-alives (both are on by default) may speed up total performance.1

But there doesn't seem to be any explanation in the docs about how to do this? There is one example that's maybe relevant, but it does not show how to reuse the pool elsewhere: http://aiohttp.readthedocs.io/en/stable/client.html#keep-alive-connection-pooling-and-cookie-sharing

Would something like this be the correct way to do it?

@app.listener('before_server_start')
async def before_server_start(app, loop):
    app.pg_pool = await asyncpg.create_pool(**DB_CONFIG, loop=loop, max_size=100)
    app.http_session_pool = aiohttp.ClientSession()


@app.listener('after_server_stop')
async def after_server_stop(app, loop):
    app.http_session_pool.close()
    app.pg_pool.close()


@app.post("/api/register")
async def register(request):
    # json validation
    async with app.pg_pool.acquire() as pg:
        await pg.execute()  # create unactivated user in db
        async with app.http_session_pool as session:
            # TODO send activation email using SES API
            async with session.post('http://httpbin.org/post', data=b'data') as resp:
                print(resp.status)
                print(await resp.text())
        return HTTPResponse(status=204)

Solution

There're few things I think can be improved:

Instance of ClientSession is one session object. This on session contains pool of connections, but it's not "session_pool" itself. I would suggest rename http_session_pool to http_session or may be client_session.

Session's close() method is a corountine. Your should await it:

await app.client_session.close()

Or even better (IMHO), instead of thinking about how to properly open/close session use standard async context manager with awaiting of __aenter__ / __aexit__:

@app.listener('before_server_start')
async def before_server_start(app, loop):
    # ...
    app.client_session = await aiohttp.ClientSession().__aenter__()


@app.listener('after_server_stop')
async def after_server_stop(app, loop):
    await app.client_session.__aexit__(None, None, None)
    # ...

Pay attention to this info:

However, if the event loop is stopped before the underlying connection is closed, an ResourceWarning: unclosed transport warning is emitted (when warnings are enabled).

To avoid this situation, a small delay must be added before closing the event loop to allow any open underlying connections to close.

I'm not sure it's mandatory in your case but there's nothing bad in adding await asyncio.sleep(0) inside after_server_stop as documentation advices:

@app.listener('after_server_stop')
async def after_server_stop(app, loop):
    # ...
    await asyncio.sleep(0)  # http://aiohttp.readthedocs.io/en/stable/client.html#graceful-shutdown

Upd:

Class that implements __aenter__ / __aexit__ can be used as async context manager (can be used in async with statement). It allows to do some actions before executing internal block and after it. This is very similar to regular context managers, but asyncio related. Same as regular context manager async one can be used directly (without async with) manually awaiting __aenter__ / __aexit__.

Why do I think it's better to create/free session using __aenter__ / __aexit__ manually instead of using close(), for example? Because we shouldn't worry what actually happens inside __aenter__ / __aexit__. Imagine in future versions of aiohttp creating of session will be changed with the need to await open() for example. If you'll use __aenter__ / __aexit__ you wouldn't need to somehow change your code.