Friday, June 26, 2009

NWSGI 2.0: Configuration Details

For the most part, NWSGI can be used without any configuration. All you have to do is create a .wsgi file, throw it in a directory, let IIS know what’s going on, and you’re good to go! Of course, if it was always that easy, configuration wouldn’t exist. To really work with NWSGI, you need to understand its configuration. It helps if you’re already familiar with web.config files; if not, check this out (but hurry back!).

First Things First

NWSGI uses .NET’s built-in configuration system (System.Configuration), so the first thing that needs to be done is to let the .NET Framework know about the custom NWSGI configuration section:

    <section name="wsgi" type="NWSGI.WsgiSection" />

This must be the first entry in web.config, directly under the <configuration> element.

The <wsgi> element

All of the configuration options for NWSGI are under the <wsgi> element. The first ones we will look at are specified as attributes of the <wsgi> element: enableExtensions, adaptiveCompilation and frames.

Enable extensions to NWSGI. Relying on these will make your application nonportable to other WSGI implementations. Required to use ASP.NET's built-in Session support, for example. Choices: true, false.
Control how IronPython compiles code. Choices: Adaptive, Compiled.
Control sys._getframe. Enabling sys._getframe causes a performance hit. Choices: Off, On, Full.


<wsgi adaptiveCompilation="false" frames="Full" />

Adding Python Search Paths

If your application needs to access Python modules that are not in the default NWSGI search paths (~/, ~/Bin/ and ~/Bin/Lib/), you can use the <pythonPaths> element to add them. The <pythonPaths> element contains a set of <path> elements that are added (in order) to sys.path before the application is run. The <path> element has one required attribute – path – that contains the full path to a folder on disk containing the modules you want to use.


        <path path="C:\django" />

Adding WSGI Environment Variables

Most applications will require some configuration of their own; these options are passed in the environ parameter of the WSGI application. You can add entries to this list by using the <wsgiEnviron> element, which contains a set of <variable> elements. Each <variable> element has two required attributes, name and value.


        <variable name="trac.env_path" value="C:\trac\projects" />

Advanced Script Usage

There are cases where you may need to configure how NWSGI handles a particular script file. In these cases you need a virtual script mapping; NWSGI will always check script mappings before looking for a file on disk. Not surprisingly, these are stored in the <scriptMappings> element, which contains a set of <scriptMapping> elements. There are three parts to a script mapping:

The name of the virtual script; overrides any scripts on disk.
The path to the actual file to execute. This can be a full path (i.e. C:\myapp\myapp.wsgi) or an app-relative path (i.e. ~/scripts/myapp.wsgi).
If physicalPath is set, the name of the callable object in that file; if it is not set, an entry point for the application.

Entry points are used to specify a particular function in a Python module on sys.path (see the <pythonPaths> element above for how to control sys.path). For example, for Django the entry point would be "django.core.handlers.wsgi.WSGIHandler()". This mimics the standard Python syntax for creating an object; it is this object that is used as the application. If, instead, the application provides a function (like trac), the entry point would be "trac.web.main.dispatch_request".


    <scriptMapping scriptName="myapp.wsgi" physicalPath="C:\myapp\myapp.wsgi" callable="myapp" />

Wildcard Settings

When IIS is configured in wildcard mode, all requests for an application are passed to NWSGI, instead of only requests for .wsgi files. You can enable this by setting path="*" in the handler/httpHandler definition. Because NWSGI normally expects there to be a .wsgi file in the URL, but in wildcard mode there isn't one, the <wildcard> element is required to tell NWSGI what script to execute. The available settings – physicalPath and callable – have the same meaning as the same settings for script mappings (above). If the <wildcard> element is present, the <scriptMappings> element is ignored.


<wildcard physicalPath="C:\HelloWorld\hello.wsgi" callable="hello" />


NWSGI is extremely configurable; in many cases, you don't even need a .wsgi file. That said, the configuration can be complex, so please ask if you run into any problems. A later post will go into more detail on the mechanics of callables.