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On Wed, Jan 28, 2015, Vishvananda Ishaya <vishvananda at gmail.com> wrote:
> On Jan 28, 2015, at 4:03 PM, Doug Hellmann <doug at doughellmann.com> wrote:
> > I hope someone who was around at the time will chime in with more detail
> > about why green threads were deemed better than regular threads, and I
> > look forward to seeing your analysis of a change. There is already a
> > thread-based executor in oslo.messaging, which *should* be usable in the
> > applications when you remove eventlet.
> 
> Threading was never really considered. The initial version tried to get a
> working api server up as quickly as possible and it used tonado. This was
> quickly replaced with twisted since tornado was really new at the time and
> had bugs. We then switched to eventlet when swift joined the party so we
> didn’t have multiple concurrency stacks.
> 
> By the time someone came up with the idea of using different concurrency
> models for the api server and the backend services, we were already pretty
> far down the greenthread path.

Not sure if it helps more than this explanation, but there was a
blueprint and accompanying wiki page that explains the move from twisted
to eventlet:

https://blueprints.launchpad.net/nova/+spec/unified-service-architecture

https://wiki.openstack.org/wiki/UnifiedServiceArchitecture

JE