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Re: Twisted or Eventlet?


Hi Michael,

++ On all points. Over the past few days while researching all this,
I went from being in the middle to the Eventlet side. Another couple
points to make:

* We are not actually using twisted for what it is powerful for -
  non-blocking I/O handling. The AMQP and Redis libs are still using
  blocking sockets and queues are being polled with a timer every 100ms
  with sync operations. There is still some work to be done to use
  it properly (replacing libs with twisted versions, restructure app).

* The places we are using twisted currently may not even need this,
  using a multi-process or multi-thread model may make more sense. See
  active thread discussing this right now.

* The application where we do need twisted/eventlet - nova-api - is
  currently still on tornado, so we can start using eventlet there and
  don't need to worry about converting twisted code already written
  (no wasted resources).

* We have eventlet experts and a large scale app already on hand
  with swift, so we can bug them with any questions. :)


On Wed, Aug 04, 2010 at 12:03:49PM -0400, Michael Gundlach wrote:
>    All,
>    I propose that we go ahead and decide to use Eventlet.  We've all had a
>    chance to talk, the decision isn't going to make itself, and it seems like
>    we're coalescing around Eventlet:
>      * In a long discussion about how to keep our code loosely coupled and
>        easy to maintain (see Architecture For Shared Components on the
>        openstack list), we concluded that WSGI would be a great solution, and
>        eday has prototyped an Eventlet-powered WSGI app talking AMQP to
>        Rabbit.  The Twisted WSGI approach apparently isn't stable or
>        standard, but WSGI works just as you'd expect in Eventlet.
>      * As hazmat points out in the "twisted v. eventlet" thread, you can't
>        use pdb with Twisted, while pdb works just as you'd hope in Eventlet.
>      * hazmat's experience during his lightning talk (see the same thread) is
>        a good datapoint verifying that people unfamiliar with either project
>        will probably prefer to work with Eventlet over Twisted: good for
>        recruiting.
>      * It's a weaker argument, but it's really hard to find any comparisons
>        between Twisted and Eventlet on the web that say Twisted is preferable
>        (has anyone found any?)
>    And as always, there's the argument that Eventlet code is easier to read
>    and understand, and therefore to maintain.
>    I'm going to be working on adding the RS API to Nova, and I'm going to
>    proceed in Eventlet.  If anyone feels strongly that we must use Twisted
>    instead, speak up; otherwise, shall we call this decision made?
>    trying-to-build-consensus-ly yrs,
>    Michael
>    On Tue, Aug 3, 2010 at 8:13 AM, Soren Hansen <soren.hansen@xxxxxxxxxxxxx>
>    wrote:
>      On Mon, Aug 02, 2010 at 05:21:23PM -0500, Eric Day wrote:
>      > I took a closer look at the inner workings of how Twisted is currently
>      > being used, and I think there is still a good chunk of work to be done
>      > to make things work properly. For example, both Redis and AMQP modules
>      > are not using the Twisted I/O event loop. Both are still issuing sync
>      > socket calls in their own libs which could block the entire
>      > application.
>      Yup, this definitely needs fixing.
>      > These things are very doable in Twisted, it will just require using
>      > the txRedis and txAMQP modules. It will also introduce more deferreds
>      > into the mix since we'll have non-blocking connections dependent on
>      > each other. From this perspective, Eventlet may be easier since we
>      > could still use the current libraries by patching
>      > (http://eventlet.net/doc/patching.html).
>      Yes, both python-redis and python-amqplib use Python's standard socket
>      module, so they can be patched.
>      Does anyone know how eventlet would deal with libvirt calls?
>      python-libvirt just calls into libvirt obviously doesn't use Python's
>      socket module.
>      --
>      Soren Hansen
>      Systems Architect
>      The Rackspace Cloud
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