A (long) while ago I wrote a web-spider that I multithreaded to enable concurrent requests to occur at the same time. That was in my Python youth, in the days before I knew about the GIL and the associated woes it creates for multithreaded code (IE, most of the time stuff just ends up serialized!)…
I’d like to rework this code to make it more robust and perform better. There are basically two ways I could do this: I could use the new multiprocessing module in 2.6+ or I could go for a reactor / event-based model of some sort. I would rather do the later since it’s far simpler and less error-prone.
So the question relates to what framework would be best suited to my needs. The following is a list of the options I know about so far:
- Twisted: The granddaddy of Python reactor frameworks: seems complex and a bit bloated however. Steep learning curve for a small task.
- Eventlet: From the guys at lindenlab. Greenlet based framework that’s geared towards these kinds of tasks. I had a look at the code though and it’s not too pretty: non-pep8 compliant, scattered with prints (why do people do this in a framework!?), API seems a little inconsistent.
- PyEv: Immature, doesn’t seem to be anyone using it right now though it is based on libevent so it’s got a solid backend.
- asyncore: From the stdlib: über low-level, seems like a lot of legwork involved just to get something off the ground.
- tornado: Though this is a server oriented product designed to server dynamic websites it does feature an async HTTP client and a simple ioloop. Looks like it could get the job done but not what it was intended for.
[edit: doesn’t run on Windows unfortunately, which counts it out for me – its a requirement for me to support this lame platform]
Is there anything I have missed at all? Surely there must be a library out there that fits the sweet-spot of a simplified async networking library!
[edit: big thanks to intgr for his pointer to this page. If you scroll to the bottom you will see there is a really nice list of projects that aim to tackle this task in one way or another. It seems actually that things have indeed moved on since the inception of Twisted: people now seem to favour a co-routine based solution rather than a traditional reactor / callback oriented one. The benefits of this approach are clearer more direct code: I’ve certainly found in the past, especially when working with boost.asio in C++ that callback based code can lead to designs that can be hard-to-follow and are relatively obscure to the untrained eye. Using co-routines allows you to write code that looks a little more synchronous at least. I guess now my task is to work out which one of these many libraries I like the look of and give it a go! Glad I asked now…]
[edit: perhaps of interest to anyone who followed or stumbled on this this question or cares about this topic in any sense: I found a really great writeup of the current state of the available tools for this job]
I liked the concurrence Python module which relies on either Stackless Python microthreads or Greenlets for light-weight threading. All blocking network I/O is transparently made asynchronous through a single
libevent loop, so it should be nearly as efficient as an real asynchronous server.
I suppose it’s similar to Eventlet in this way.
The downside is that its API is quite different from Python’s
threading modules; you need to rewrite a fair bit of your application (or write a compatibility shim layer)
Edit: It seems that there’s also cogen, which is similar, but uses Python 2.5’s enhanced generators for its coroutines, instead of Greenlets. This makes it more portable than concurrence and other alternatives. Network I/O is done directly with epoll/kqueue/iocp.
Twisted is complex, you’re right about that. Twisted is not bloated.
If you take a look here: http://twistedmatrix.com/trac/browser/trunk/twisted you’ll find an organized, comprehensive, and very well tested suite of many protocols of the internet, as well as helper code to write and deploy very sophisticated network applications. I wouldn’t confuse bloat with comprehensiveness.
It’s well known that the Twisted documentation isn’t the most user-friendly from first glance, and I believe this turns away an unfortunate number of people. But Twisted is amazing (IMHO) if you put in the time. I did and it proved to be worth it, and I’d recommend to others to try the same.
API-wise it follows the same conventions as the standard library (in particular, threading and multiprocessing modules) where it makes sense. So you have familiar things like Queue and Event to work with.
It only supports libevent (update: libev since 1.0) as reactor implementation but takes full advantage of it, featuring a fast WSGI server based on libevent-http and resolving DNS queries through libevent-dns as opposed to using a thread pool like most other libraries do. (update: since 1.0 c-ares is used to make async DNS queries; threadpool is also an option.)
Like eventlet, it makes the callbacks and Deferreds unnecessary by using greenlets.
A really interesting comparison of such frameworks was compiled by Nicholas Piël on his blog: it’s well worth a read!
None of these solutions will avoid that fact that the GIL prevents CPU parallelism – they are just better ways of getting IO parallelism that you already have with threads. If you think you can do better IO, by all means pursue one of these, but if your bottleneck is in processing the results nothing here will help except for the multiprocessing module.
I wouldn’t go as far as to call Twisted bloated, but it is difficult to wrap your head around. I avoided really settling in an learn for quite a while as I always wanted something a little easier for ‘small tasks’.
However, now that I have worked with it some more I have to say having all the batteries included is VERY nice.
All the other async libraries I’ve worked with end being way less mature than they even appear. Twisted’s event loop is solid.
I’m not quite sure how to solve the steep Twisted learning curve. It might help if someone would fork it and clean a few things up, like removing all the backwards compatability cruft and the dead projects. But that’s the nature of mature software I guess.
I’ve started to use twisted for some things. The beauty of it almost is because it’s “bloated.” There are connectors for just about any of the main protocols out there. You can have a jabber bot that will take commands and post to an irc server, email them to someone, run a command, read from an NNTP server, and monitor a web page for changes. The bad news is it can do all of that and can make things overly complex for simple tasks like the OP explained. The advantage of python though is you only include what you need. So while the download may be 20mb, you may only include 2mb of libraries (which is still a lot). My biggest complaint with twisted is although they include examples, anything beyond a basic tcp server you’re on your own.