Eventlet all to synchronous code to adopt an asynchronous behavior.

Many Openstack services needs both kind of synchronicity (synchronous, and asynchronous).

Introducing the Asyncio hub will allow to migrate toward an only asynchronous behavior. That's not enough for Openstack. We need to document a specific conception pattern to allow doing synchronous network calls through code migrated to asyncio (asynchronous based).

I already proposed a draft of this pattern into the community goal, but we need a more robust documentation.

https://review.opendev.org/c/openstack/governance/+/902585