We are actually working on a project like this with Tygs (
https://github.com/Tygs/tygs, which will use crossbar.io), and right now it's taking ages just to get the app life cycle right.
Indeed, we want to make it very easy to use, especially a clean/discoverable API, simple debugging, clear errors, etc. Which is the stuff async frameworks are not good at, and an incredible added value.
It's a lot more work we initially thought. When you are doing async, you now think, not with a sequence of actions ordered in time, but with events. So for your framework to be usable, you must provide hooks to:
- do something at init
- register a component
- do something when a component is registered
- do something once it's ready
- do something when there is an error
- do something when it shuts down
With minimum boiler plate, and maximum clear error handling when the user try to do it at the wrong time or in the wrong way.
But, we learned while coding the project that, with asyncio, exceptions in coroutine don't break the event look (except KeyboardInterrupt which is a weird hybrid) while exceptions from the loop do break it.
Plus you have to make a nice setup, which auto starts the event loop with good default so that people don't have to think about it for simple apps. But it must be overridable, and handle the case where your framework is embeded inside an already started event loop, and make it easy to do so.
It's one of the strong point of gevent: you don't have to think about it. With asycio/twisted, you have the benefit a explicit process switch and parallelism, but you need to pay the price with verbosity and complexity. We try to create a balance, and it's turns out to be harder than expected.
Then you have to make clear error reporting, especially iron the implementation mixing Task, Futures, coroutine functions and coroutines. Provide helpers so that common scheduling is done easily...
And you haven't even talked about HTTP yet. This is just proper asyncio management. This is why nobody made a killer framework yet : it's a looooooot of work, it's hard, and it's very easy to get it wrong. Doing like but async doesn't cut it.