[Web-SIG] [ANN] twsgi: asynchronous WSGI implementation for Twisted Web

[Manlio Perillo]

Gustavo Narea ha scritto:
> Hello,
> 
> Maybe I'm missing something obvious, but if the gateway doesn't support
> applications that return write() callables, then it's not WSGI.
> 
> A callable that raises an exception does not even count. It's obvious
> that they must not raise exceptions -- Then what's the point of
> providing the callable?
> 

Nothing is obvious in an official specification ;-).

The reason I choose to not completely remove the write callable is
because it will raise a nice error message if someone even try to use my
implementation to execute a WSGI application that requires the write
callable.

Moreover some middlewares or applications may assume the write callable
exists and the value returned by start_response is not None, even if it
is never used.


> That said, I *think* it might be OK to disable support for the write()
> callable *optionally* on a per application basis. For example, the
> gateway could look at the "requires_write" attribute of the application
> callable, if any:
> """
> def wsgi_app(environ, start_response):
>     # ... process the request and return a response....
> 
> wsgi_app.requires_write = False
> """
> 
> That way, applications which don't use the write() callable can let your
> gateway know and thus it won't pass one on.
> 

The problem is that applications that requires the write callable, are
not aware of this extension.

This is really a no problem, IMHO.
If you try to execute an application, and you get a NotImplementedError
extension, then you *know* that write callable is required.

Then, you just configure the WSGI gateway to use the required adapter.
See http://bitbucket.org/mperillo/txwsgi/src/tip/doc/examples/demo_write.py
for a pratical example using txwsgi.

With ngx_http_wsgi_module, you just have to add a
    wsgi_middleware  txwsgi.greenlet write_adapter;
directive in Nginx configuration file.

> We could even standardize this (at wsgi.org) so that any WSGI middleware
> which wraps an application can expose the "requires_write" attribute of
> the wrapped application... As long as such a middleware doesn't use
> write() either.
> 
> On the other hand, I would avoid using "middleware" in this context for
> something specific to your implementation as people will believe it's a
> proper WSGI middleware. 

Yes.
I now use the term "adapter".


Regards   Manlio