[Web-SIG] Re: Just lost another one to Rails

[Ian Bicking]

Shannon -jj Behrens wrote:
> At the risk of offending you guys with my continual blabbering, I do
> like the "tools not policy" approach shared by the FreeBSD and Mason
> worlds.  

This is exactly what I see WSGI accomplishing.  It's too lame of an 
interface to be anyone's policy (in a good way, of course).  It's always 
been possible to use external libraries, but it's difficult to use 
libraries that participate in the request/response cycle, and WSGI makes 
that possible.

This can be used in both mundane and interesting ways.  I think sessions 
are a bit mundane, but they work well, allowing the session "library" to 
modify both request and response, providing an isolated and well-scoped 
way to add session services to a WSGI application.  This doesn't 
preclude such a library (middleware) relying on other libraries for the 
bulk of their work (e.g., persistence); indeed, I think it's best to 
keep middleware as small as possible.

A more interesting example of this kind of reuse is URL parsing. 
Typically URL parsing is one of those intimate details of a framework 
that can't be factored out.  But paste.urlparser.URLParser is fairly 
successful, I think, at implementing a specific algorithm without 
greatly effecting other parts of the system; it doesn't make any 
assumptions about the applications it delegates to, or about the 
environment it is run in.  It's not the only URL parser you'd want -- 
but that's a big part of what makes it interesting; it separates one 
kind of policy (URL resolution) from another (how resources are 
expressed).

I think other people have worried that this sort of thing is all fun and 
games, but it doesn't give us a concise and unified framework.  But by 
firmly keeping policy either decoupled (urlparser) or entirely removed 
(session middleware) it makes it possible to create a framework that is 
the amalgam of these pieces of code without sacrificing internal 
consistency.

-- 
Ian Bicking  /  ianb at colorstudy.com  /  http://blog.ianbicking.org