WSGI Framework Components and other thoughts on WSGI

In light of Phillip Eby's recent post concerning WSGI Middleware as harmful, I've had more than a few thoughts on the issue. None of them are all that new, but given the post I think its useful to get some of them out there.

First, I agree 100% with PJE's post. The issue it raises results in two lines of thought. Without a doubt these objects using the WSGI specification should not be called WSGI Middleware or WSGI Applications. This means that either: 1) People should stop using the WSGI spec for non WSGI application/middleware objects.

Or… 2) WSGI needs new terminology for this application of the specification, and should not be muddling up the WSGI middleware/application definitions and environ namespace with meta-framework API's.

To answer either of these possibilities it helps to evaluate why things are developing like this right now, and almost all of it comes down to one thing… tool developers are incredibly picky and opinionated.

To avoid further muddling up WSGI definitions, I'll be using the following term:

WSGI Framework Component (WFC) – A WSGI specification based component that acts possibly as either a WSGI application or WSGI middleware, or some mix of both. Example, a WFC that ensures users are logged in before accessing your WSGI application (thus acting as WSGI middleware), but will render its own form and go through its own login procedure should they need to login (thus acting like a WSGI application). This is referred to as a WFC because using WSGI is seen as a way to avoid binding it to a specific framework, while its clear that an application using it actually requires it to be there to operate (thus its not WSGI middleware).

Dealing with Disagreement the WSGI Way

WSGI makes it a lot easier to disagree, yet still harness the code and development efforts of those that disagreed with you. WFC's allow re-usable code that isn't utterly dependent on your framework of choice as long as its WSGI compatible. Thus the fact that many frameworks are WSGI compatible at various levels makes it very enticing to build re-usable components at the WSGI level instead of using framework-bound API's.

A thought that started cropping up, and hitting the Web-SIG mail list, and which I believe one target of Phillip Eby's post, regards putting ‘standard' keys into the WSGI environ for applications to utilize. This would to an extent allow you to swap WFC's that do similar things, but in different ways. Maybe you want to swap two resolving middleware, so you use the routing spec to determine how the URL was resolved then dispatch appropriately. You can now swap WFC's that do routing to an extent since there's a further specification in place.

There are other specifications underway, and lots of various WFC's being developed. If I'm using Pylons, and someone using CleverHarold or some other WSGI type application makes a WFC that does something cool, I can use it as is without having to agree with the design of their application of WSGI based framework.

Compare that to a CherryPy2 filter, CherryPy3 tool, or Django middleware. To use any of those, I need to use the whole framework. For CherryPy, this may not be the case in the future should it allow a CherryPy tool to act in the middle like middleware. Robert Brewer has said in the past he wants CherryPy to be the end-point and not continue dispatching elsewhere which would rule out its use as a library for a WFC. Thus, I'm labeling CherryPy as a framework in the context of WFC creation, while Paste and Yaro are libraries usable both in WSGI apps/middleware and in WFC's (Note that CherryPy3 is almost capable of being used as a WFC, except it can only dispatch to non-CP3 WSGI apps).

Going Overboard with WFC's and WSGI

The other aspect to these new WFC's that I think Phillip hit on the head, is that there's quite a few being pushed into this layer that really don't belong there. No one has put out a solid checklist to know when something should be in a library, a plugin API (possibly using setuptools entrypoints like the TG Template Plugin API), actual WSGI middleware, or a WFC. As a result, there are WFC's that do very little, and in some cases have no reason to be operating at the WSGI layer.

So, I'm going to propose some guidelines, a rough draft as I'm sure there'll be plenty of useful feedback, on when something should be considered for a WFC and when it should be a library. It's also useful to note that libraries can operate on things from WSGI, vs WFC's which get plugged into a framework/app as if it was WSGI middleware.

The guidelines for WFC's should roughly follow the same guidelines you'd want for any WSGI middleware. There's some conditions that make it more obvious than others on where some functionality belongs and of course there's always exceptions to the rules.

Signs some code would be a good candidate for a WFC (It's assumed that if you're thinking of making a WFC, you will be wrapping your actual ‘application' with it):

  • A set of operations needs to always occur before and after the application is called, and requires knowledge of the incoming and outgoing headers
  • Modifications are done to the HTTP headers and/or content being returned to the client (cookies, HTTP caching, content transformation)
  • The application may not be called at all (authentication, authorization, conditional dispatching)

Note that the first condition doesn't apply to functionality that merely requires something to setup. It's overkill using WSGI just to run a function at the start of every request — even if it needs environ — there's no reason you couldn't just put the function call in your app, call it every request, and put the function in its own module/package (thus easy to re-use).

A lot of the Paste functions operate like this, and many of them just take the environ as their call giving you a nice API without requiring a WFC (which Phillip Eby advocates as well):

request = paste.wsgiwrapper.WSGIRequest(environ)
print request.cookies, request.path_info

There's no reason a variety of WFC's I see on the WSGI middleware and utils list couldn't operate like this as well. Take wsgiakismet for example, which parses the form submission and screens it against Akismet. The example as a WFC actually looks more involved than I could see a library based version looking:

theoretical library version of wsgiakismet
from akismetverify import verify_akismet

def app(environ, start_response):
# Wordpress API Key and website name are required arguments
usersub = verify_akismet(key='3489012ab121', site='', environ)
start_response('200 OK', [('Content-type', 'text/plain')])
return ['Comment is %s' % usersub['comment'][0]]

Note that using it like this as a function that takes environ and the other 2 keys actually makes it easier to use than the original sample requiring you to import cgi and re-parse the form vars.

So some good ways to know you might be on the wrong track with a WFC:

  • Only a few things are being done on setup, and stuffed into environ
  • Some environ keys are manipulated
  • Your code never alters or does anything with the status codes, headers, or content
  • … or none of the conditions to know when it should be a WFC exist

I'm sure there's more criteria I've missed, and it'd be great to have a page possibly on regarding design decisions to hopefully avoid having anymore functionality pushed into the WSGI layer when there's no good reason for it.

Ben Bangert
Ben Bangert
Software Contriver

Code. Homebrew. Hike. Rollerblade.