Motivation

A migration path off of eventlet for libraries/applications that exclusively rely on eventlet

Eventlet has different users with different use cases. This issue covers one specific subset of users: those who deliberately choose eventlet as their exclusive networking/event loop solution. A notable and relevant example is OpenStack.

My basic assumption is that the monkey-patching approach used by eventlet is fundamentally non-viable without massive engineering resources. For example, consider all the effort being put into patching RLock, when patching it actually introduces bugs (e.g. https://bugs.python.org/issue13697). A correct solution would involve maintaining a custom C RLock, and C Queue, and so on. And the more the Python stdlib changes, the harder eventlet maintenance becomes.

As such, I believe eventlet-only users are likely better off switching to event-driven frameworks like asyncio.

Access to asyncio libraries

Even if you disagree with the above, or you're a user who doesn't fit the above criteria, access to asyncio libraries in eventlet code is useful. Eventlet used to have Twisted support for this reason.

In general I don't think new features should be added to eventlet (see #835), but given that this aids with migration I think it's worth it.

I also believe it's easier to do this inside of eventlet rather than as an external project (see eventlet/aiohub#2).

The work involved

Prior art for gevent suggests this all should be possible. Each of these should be done as its own issue.

• An asyncio hub for eventlet #869
• Support multiple readers in the asyncio hub #874
• Asyncio hub support (or nice error message) for Python 3.9 and earlier #885
• Support awaiting GreenThread in an async def context. #887
• Support calling asyncio-using async def functions from inside greenlets #873
• Figure out how to deal with Locks and Queues etc when doing asyncio+eventlet integration
• GreenPool-specific support for turning a future into a GreenThread
• Document asyncio integration and how to use it for migration path off of eventlet #891
• Make sure asyncio hub + dnspython work correctly together #936
• Maybe make the asyncio hub the default, after enough releases and real-world usage demonstrates correctness

Questions

@temoto @tipabu what is your opinion on this approach of integrating asyncio and eventlet?