[Web-SIG] Entry points and import maps (was Re: Scarecrow deployment config

[Phillip J. Eby]

At 03:02 AM 7/25/2005 -0400, Chris McDonough wrote:
>Actually, let me give this a shot.
>
>We package up an egg called helloworld.egg.  It happens to contain
>something that can be used as a WSGI component.  Let's say it's a WSGI
>application that always returns 'Hello World'.  And let's say it also
>contains middleware that lowercases anything that passes through before
>it's returned.
>
>The implementations of these components could be as follows:
>
>class HelloWorld:
>     def __init__(self, app, **kw):
>         pass # nothing to configure
>
>     def __call__(self, environ, start_response):
>         start_response('200 OK', [])
>         return ['Hello World']

I'm thinking that an application like this wouldn't take an 'app' 
constuctor parameter, and if it takes no configuration parameters it 
doesn't need **kw, but good so far.


>class Lowercaser:
>     def __init__(self, app, **kw):
>         self.app = app
>         # nothing else to configure
>
>     def __call__(self, environ, start_response):
>         for chunk in self.app(environ, start_response):
>             yield chunk.lower()

Again, no need for **kw if it doesn't take any configuration, but okay.


>An import map would ship inside of the egg-info dir:
>
>[wsgi.app_factories]
>helloworld = helloworld:HelloWorld
>lowercaser = helloworld:Lowercaser

I'm thinking it would be more like:

     [wsgi.middleware]
     lowercaser = helloworld:Lowercaser

     [wsgi.apps]
     helloworld = helloworld:HelloWorld

and you'd specify it in the setup script as something like this:

     setup(
         #...
         entry_points = {
             'wsgi.apps': ['helloworld = helloworld:HelloWorld']
             'wsgi.middleware': ['lowercaser = helloworld:Lowercaser']
         }
     )

(And the CVS version of setuptools already supports this.)



>So we install the egg and this does nothing except allow it to be used
>from within Python.
>
>But when we create a "deployment descriptor" like so in a text editor:
>
>[helloworld from helloworld]
>
>[lowercaser from helloworld]

Opposite order, though; the lowercaser comes first because it's the 
middleware; the application would always come last, because they're listed 
in the order in which they receive data, just like a pipes-and-filters 
command line.


>... and run some "starter" script that parses that as a pipeline,

... possibly using a #! line if you're using CGI or FastCGI with Apache or 
some other non-Python webserver.


>creates the two instances, wires them together, and we get a running
>pipeline?
>
>Am I on track?

Definitely.