From Gevent to CherryPy

I’ve been working on a project for the last few months on GitHub called Commissaire along with some other smart folks. Without getting to deep into what the software is supposed to do, just know it’s a REST service which needs to handle some asynchronous tasks. When prototyping the web service I started utilizing gevent for it’s WSGI server and coroutines but, as it turns out, it didn’t end up being the best fit. This is not a post about gevent sucking because it doesn’t suck. gevent is pretty awesome but it’s not for every use case.

The Problem

One of the asynchronous tasks we do in Commissaire utilizes Ansible. We use the Ansible python API to handle part of host bootstrapping of a new host. Under the covers Ansible uses the multiprocessing module when executing it’s work. Specifically, this occurs when the TaskQueueManager starts its run. Under normal circumstances this is no problem but when gevent is in use it’s monkey patching ends up causing some problems. As noted in the post using monkey.patch_all(thread=False, socket=False) can be a solution. What this ends up doing is patching everything except thread and socket. But even this wasn’t enough for us to get past problems we were facing between multiprocessing, gevent, and Ansible. The closest patch we found was to also disable os, subprocess and a few other things making most of gevents great features unavailable. At this point it seemed pretty obvious gevent was not going to be a good fit.

Looking Elsewhere

There are no lack of options when looking for a Python web application server. Here are the requirements that I figured we would need:

Requirements

  • Importable as a library
  • Supports WSGI
  • Supports TLS
  • Active user base
  • Active development
  • Does not require a reverse proxy
  • Does not require greenlets
  • Supports Python 2 and 3

Based on the name of this post you already know we chose CherryPy. It hit all the requirements and came with a few added benefits. The plugin system which allows for calls to be published over an internal bus let’s us decouple our data saving internals (though couples us with CherryPy as it is doing the abstraction). The server is also already available in many Linux distributions at new enough versions. That’s a big boon hoping to have software easily installed via traditional means.

The runner up was Waitress. Unlike CherryPy which assumes you are developing within the CherryPy web framework, Waitress assumes WSGI. Unfortunately, Waitress requires a reverse proxy for TLS. If it had first class support for TLS we would have probably have picked it.

Going back to a more traditional threading server is definitely not as sexy as utilizing greenlets/coroutines but it has provided a consistent result when paired with a multiprocessing worker process and that is what matters.

Porting Time

Porting to a different library can be an annoying task and can feel like busy work. Porting can be even worse when you liked the library in use in the first place as I did (and still do!) with gevent.

Initial porting of main functionality from gevent to CherryPy took roughly four hours. After that, porting it took about another 6 hours to iron out some rough edges followed by updating unit tests. Really, the unit testing updates ended up being more work, in terms of time, than the actual functionality. A lot of that was our fault in how we use mock, but I digress. That’s really not much time!

So What

So far I’m happy with the results. The application functionality works as expected, the request/response speeds are more than acceptable, and CherryPy as a server has been fun to work with. Assuming no crazy corner cases don’t crop up I don’t see use moving off CherryPy anytime soon.

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