> this is already going to be a heavy web request with multiple web service calls
Sure, but it can now be a less heavy web request! ¯\_(ツ)_/¯
> a request typically has a single transaction going out to the database
The fact of the matter is, as applications develop, become richer, and grow larger, it becomes less and less uncommon to have more than one query per page. Especially in the context of larger organizations, it's very common to have everything wrapped behind a service call with an entire armada of infrastructure hidden behind it, and having to make many service calls to put together one web API result or page.
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sigh Slight tangent. Look at where we are now and how we came here.
Back in the non-ajax days we used to do them all on the server side, then render the whole page all in one go. This would have come in handy back then! Imagine doing 5x 50ms queries asynchronously, dropping a 250ms response delay down to 50ms! But this stuff was hard back then, and we mostly left it alone.
This is also along the times when we figured out that since we can have pages that take a long time to load and block the interpreter, perhaps it's not such a great idea to serve many requests with a single interpreter, so people started using stuff like nginx to run multiple python interpreters in parallel (not even getting into threads here), which was easier to reason about since each python process is a separate universe that can block entirely, but overall we can still serve a new request with a new interpreter, so for the most part things are good.
Then the twisted people thought that this was silly, and why should we block in the first place, and they decided that the way to fix this was to change the way we program entirely, and re-create or wrap an entire ecosystem of software. It sort of worked, except there wasn't a good twisted package for your thing. But all in all it worked.
Then the greenlets (or one of its other 20 names) people came and wanted to instead use fine-grained implicit concurrency, and said "no no, we can get something with nicer abstraction packaging while mostly not changing the code we have", and that was even nicer, except when something didn't get monkey patched correctly for some reason. We got stuff like gunicorn, which was impressive.
Then as we moved more stuff to the client to create more responsive (in the original meaning of the word) applications, so we pushed the burden of requesting and fetching data to the browser side, which means that as a page loads, it might call REST APIs one by one (hopefully asynchronously!), each of which might make a single (finer-grained) database or service call behind the scenes.
So how different is this now from the gunicorn model? In the latter, you get fine threads of control, each working asynchronously to fetch their own thing, which gets put together in the server side, and then sent back to the client. In the former, you get similarly fine threads of control, but the fine threads perhaps live in their own universes, and it doesn't get all put back together until it travels over the internet to the browser.
So it's a little bit different, but overall what's happening is similar. It feels like we just keep moving concerns and procedures up and down the stack.
Surely there's reasons for all this. Times and technologies change, and we find ways to adapt. I like the "async" stuff because it makes things explicit. It's the middle-ground result of the culmination of our learnings that hiding async behavior makes libraries hard to design and can result in frustrating and unpredictable behavior, whilst changing the entire programming model isn't great either. So we get asyncio. I'm mostly happy with this result. Admittedly this article isn't doing any of this justice.
