From: Julian Fitzell <julian@...4.com>
Date: 2003-08-07T00:55:05+09:00
Subject: Re: Why does Ruby have callcc?

james_b wrote:
> Brian Candler wrote:
> 
>> On Wed, Aug 06, 2003 at 08:38:25PM +0900, Harry Ohlsen wrote:
>>
>>> I'd be interested to read about some real-world examples where people 
>>> have
>>> used callcc, not just to try it out, but because it was the "right" 
>>> way to
>>> do something (for efficiency, or code brevity or whatever reason).
>>
>>
>>
>> I think Seaside/Borges falls into this category.
>>
>> I haven't had a chance to use it in anger, but I think the idea is that
>> continuations can represent CGI session state; you code can be written 
>> in a
>> linear fashion (e.g. display page, fetch response, display next page, 
>> fetch
>> next response etc) without any explicit session maintenance. 
>> Furthermore, by
>> retaining continuation objects which represent earlier pages, it will 
>> work
>> even if the user clicks 'back' in their browser and re-enters data in a
>> previous form.
>>
>> I'd have a number of concerns in a practical application - I'd want old
>> continuations to expire and be garbage-collected so that they don't cause
>> infinite memory growth - but otherwise it sounds like a great way to 
>> write
>> web applications.
> 
> 
> As I recall from RubyConf 2002, the use of continuations in Borges was 
> elegant but did not scale.  Tracking all that previous state gets 
> expensive.
> 
> Perhaps see Avi Bryant's presentation slides:
> http://www.zenspider.com/Languages/Ruby/RubyConf2002/index.html
> 
> James

Well now, it's not that it doesn't scale - you just have to be careful 
how you let it scale.  There are several useful deployed applications 
out there using Seaside.  One in particular that Avi and I wrote has 
been running for a year and a half with almost no maintenance on our 
part.  Now you're right that you probably wouldn't want to use it for a 
site that received millions of hits in a day, but you have a quite a bit 
of control over what state gets stored for backtracking and how many 
previous page views you want to keep around for a session, etc.  You can 
definitely reasonably have a session kept under a couple of megs, which 
could give you hundreds of concurrent users on a box with a Gig of ram.

Now I'm trying to remember the state of Borges (and the limitations of 
the basic part he ported back to ruby for that talk) and I can't 
really... so it may be that what he demoed didn't scale for some reason. 
  So it's possible you're remembering correctly ;)

Julian