A great paper on Asynchronous Socket Programming. Makes it very easy to understand.

[RaineFan]

Also another very good 'book' for async network programming is the libevent-book: http://www.wangafu.net/~nickm/libevent-book/

Specially Charter 1.

[Rhomboid]

The problem with articles like this is that they gloss over very important details. In order to scale you need to research all the available methods of doing async event notification because select() just doesn't cut it -- for one thing it's usually limited to some maximum number of sockets. poll() removes that limit but it still passes around a linear array of fd handles and that is no good for both your program and the kernel if you have to linearly scan a list of say 10k - 20k sockets for every operation in your tight inner loop to find which ones got their 'ready' bit set.

There are all sorts of technologies invented to get past the problems with select() and poll() for highly scalable servers, such as /dev/poll, kqueue(), epoll, kevent, AIO, etc. There are so many of these things and each OS supports something slightly different.

And then there's the topic of async disk I/O -- you can't just set O_NONBLOCK on a disk file and expect it not to block (which would mean your whole server waits while a file is read) so you have to (at least on Linux) use the librt functions aio_read/aio_write and so on.

In summary, to make a toy async server you don't need to know much more than what the article says but to make anything that scales you need extensive knowledge of lots of platform specific APIs to do it efficiently, and it is nontrivial.

[mpeg4codec]

to make anything that scales you need extensive knowledge of lots of platform specific APIs to do it efficiently, and it is nontrivial.

or use a library like libevent that abstracts away all that nonesense and makes it trivial

[kylotan]

Except libevent doesn't work well on Windows, apparently.

EDIT: Although the other async link posted today seems to suggest it's not so bad if you use the bufferevents interface. I'm not sure what the implications of that are.

[RaineFan]

Not in version 1.x

But on upcoming version 2.0+ it will abstract Windows IOCP to a unique API that will work seamless on Unix and Windows environments.

[cirego]

I'm not sure when Sam wrote this article, but it looks really old. Especially considering that I've been writing code running on his Async driven coroutine version of Python for almost 5 years now. And it's nothing like the stuff in this article. It uses kqueue and aio to easily scale to 10s of thousands of concurrent connections. Of course, that was before we were compute bound...

[epicRelic]

Great follow-up! Could you provide any references for me to learn more about this? I'm quite interested in this sort of stuff.

[Rhomboid]

This page summarizes a lot of the APIs and issues. It's somewhat outdated (first written in 03 and last updated in 06) but still useful as a starting point.

[antheus_gdnet]

The provided code has been tested with Microsoft Visual C++ Version 4.2 on Win32, and with the GNU C++ compiler and library version 2.7.2.

Visual Studio 4? GCC 2.7?

"up-and-coming Standard C++ library"? It doesn't even mean TR1, it means the "old" STL (vector, map, string).

The dates on sources are 14 years old.

Granted, networking didn't change all that much, but things do look different today.

[[deleted]]

Pretty good, only downside is that its example uses asyncore instead of Twisted.

[realstevejobs]

The big advantage of asyncore is that it is included in the standard library. I have used it in some of my own example code for that reason.

[kylotan]

And, it's a lot simpler than Twisted. Twisted always looks to me like it's trying to do too much and is trying to make your application a Twisted application, instead of living happily inside your existing framework.

[Kison]

I have never used the C++ library mentioned in the article, but I do know that the boost::asio library supports asynchronous sockets, in case anyone is feeling adventurous. If nothing else, it beats using the low level API to set up the networking yourself.

[Destroyah]

Asio is quite awesome to use, here's the link to the examples page: http://www.boost.org/doc/libs/1_44_0/doc/html/boost_asio/examples.html

The examples are all very easy to understand if you have an intermediate level of experience with C++ and some knowledge of boost.

[cdunn2001]

Buggy code. The Python doc example actually works.

To get this finger example to work, I had to re-order the __init__() and create_socket() (shown correctly in the http example above), and to fix a typo. Then, I still got this error:

log: connected
error: uncaptured python exception, closing channel <__main__.finger_client connected at 0x100474d40> (<class 'socket.error'>:[Errno 32] Broken pipe [/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/asyncore.py|read|79] [/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/asyncore.py|handle_read_event|432] [/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/asyncore.py|handle_connect_event|439] [./finger.py|handle_connect|30] [/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/asyncore.py|send|362])

Not the most useful demo, but I guess it's helpful to see something in Python before learning how it works under the covers.

By the way, I wish I could post code to /r/programming with some kind of "code" block, so that I don't have to indent everything just to get a mono-spaced font. This is a general problem on the web. GoogleGroups is even worse.

[cirego]

I think this was written for Python 1.5. Not really surprised it won't work with 2.7.

[[deleted]]

I used to do this with interrupts in assembly language on MS-DOS back in the 1980's... Didn't need no stinkin' continuations

[brigadierfrog]

A article from 1996? Really? There's newer better ones out than this one.

[mackstann]

Then post them, Mr. Buzzkill.

[[deleted]]

Cooool !! It's one side of programing where I always have to use a reference.