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Understanding Eventlet Hubs
A hub forms the basis of Eventlet's event loop, which dispatches I/O events and schedules greenthreads. It is the existence of the hub that promotes coroutines (which can be tricky to program with) into greenthreads (which are easy).
Eventlet has multiple hub implementations, and when you start using it, it tries to select the best hub implementation for your system. The hubs that it supports are (in order of preference):
- epolls
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Requires Python 2.6 or the python-epoll package, and Linux. This is the fastest pure-Python hub.
- poll
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On platforms that support it
- selects
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Lowest-common-denominator, available everywhere.
- pyevent
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This is a libevent-based backend and is thus the fastest. It's disabled by default, because it does not support native threads, but you can enable it yourself if your use case doesn't require them. (You have to install pyevent, too.)
If the selected hub is not ideal for the application, another can be
selected. You can make the selection either with the environment
variable EVENTLET_HUB <env_vars>, or with use_hub.
eventlet.hubs.use_hub(hub=None)
Use this to control which hub Eventlet selects. Call it with the name of the desired hub module. Make sure to do this before the application starts doing any I/O! Calling use_hub completely eliminates the old hub, and any file descriptors or timers that it had been managing will be forgotten. Put the call as one of the first lines in the main module.:
""" This is the main module """
from eventlet import hubs
hubs.use_hub("pyevent")Hubs are implemented as thread-local class instances. eventlet.hubs.use_hub only
operates on the current thread. When using multiple threads that each
need their own hub, call eventlet.hubs.use_hub at the beginning of each thread
function that needs a specific hub. In practice, it may not be necessary
to specify a hub in each thread; it works to use one special hub for the
main thread, and let other threads use the default hub; this hybrid hub
configuration will work fine.
It is also possible to use a third-party hub module in place of one
of the built-in ones. Simply pass the module itself to eventlet.hubs.use_hub. The
task of writing such a hub is a little beyond the scope of this
document, it's probably a good idea to simply inspect the code of the
existing hubs to see how they work.:
from eventlet import hubs
from mypackage import myhub
hubs.use_hub(myhub)Supplying None as the argument to eventlet.hubs.use_hub causes it to select the default
hub.
How the Hubs Work
The hub has a main greenlet, MAINLOOP. When one of the running
coroutines needs to do some I/O, it registers a listener with the hub
(so that the hub knows when to wake it up again), and then switches to
MAINLOOP (via get_hub().switch()). If there are other
coroutines that are ready to run, MAINLOOP switches to them, and when
they complete or need to do more I/O, they switch back to the MAINLOOP.
In this manner, MAINLOOP ensures that every coroutine gets scheduled
when it has some work to do.
MAINLOOP is launched only when the first I/O operation happens, and it is not the same greenlet that __main__ is running in. This lazy launching is why it's not necessary to explicitly call a dispatch() method like other frameworks, which in turn means that code can start using Eventlet without needing to be substantially restructured.
More Hub-Related Functions
eventlet.hubs.get_hub
eventlet.hubs.get_default_hub
eventlet.hubs.trampoline