greenlet

If you're familiar with greenlet, the first function is greenlet() and the second two functions do the job of greenlet.switch().

The request object is a threadlocal (or, if greenlet is installed, a greenlet-local).

Threads could be mean green threads (as in Python greenlet co-routines), it could be Go's channels, it could be Erlang's processes.

BTW Python has something like it based on the greenlet library (eventlet and gevent libraries are based on).

When the os-threads of disk-io-scheduler (or network-io-scheduler) finish the io, they queue the greenlet/task back into the cpu-scheduler.

> The request object is a threadlocal (or, if greenlet is installed, a greenlet-local).

Werkzeug uses the thread/greenlet id as key in its storage and cleans up when the request completes.

FYI, greenlet already includes stack switching code, so it would make more sense to use that.

That said, historically we mostly chose gevent as it seemed like the better interface to greenlet, which we really liked.

Check out Spawning [1], which uses eventlet [2], which uses greenlet [3].

To emulate that, you need an extra greenlet for each queue.

For example Python's eventlet and gevent are based on greenlet.

Some of these cooperatively multithreaded implementations have "green" varieties of all your standard functions; these are greenlet-aware (that is, aware of the cooperative threading & I/O loop that's happening) functions that do things like, for example, sleep.

Lua has a more general thing that is maybe comparable to Greenlet.

Greenlet-based approaches seemed easier to explain than other options, and sure enough we rarely hear complaints about development complexity, etc.

Greenlet is based underneath on large select/epoll/kqueue/poll loop.