µWeb

h1. The Request Router

When a request is sent to µWeb, the first thing that needs to be made is the decision where and how to handle it. The request router is the place where this happens. The router employed in µWeb is based on regular expressions, and delegates the request to the handler associated with the first matching expression.

Before we explain everything there is know about routes and handlers, an example of how a router would look. The example given is a very stripped down version of the handler from the @uweb_info@ example project:

<pre><code class="python">

import uweb

from uweb.uweb_info import pages

PAGE_CLASS = pages.PageMaker

ROUTES = (

    ('/static/(.*)', 'Static'),

    ('/(broken.*)', 'FourOhFour'),

    ('/haltandcatchfire', 'MakeFail'),

    ('/json', 'Json'),

    ('/text', 'Text'),

    ('/redirect/(.*)', 'Redirect'),

    ('/OpenIDLogin', '_OpenIdInitiate'),

    ('/OpenIDValidate', '_OpenIdValidate'),

    ('/ULF-Challenge', '_ULF_Challenge'),

    ('/ULF-Login', '_ULF_Verify'),

    ('/(.*)', 'Index'))

uweb.ServerSetup()

</code></pre>

In the example above, the following things happen:

* Firstly, we import the @uweb@ package, since we need that to configure the webserver from

* Secondly, we import the module where the class with our handling methods is defined (the @uweb.uweb_info.pages@ module).

* @PAGE_CLASS@ is the global that holds a reference to the [[PageMaker]] subclass for your project, from where requests are handled.

* @ROUTES@ is the global that defines the various requests that the router understands, and directs them to the (named) method of the @PAGE_CLASS@.

* Finally, @uweb.ServerSetup()@ starts the webserver for those cases where it runs [[StandAlone]] (and does some initial setting up for when it runs using [[Apache]])

When a request arrives, it is checked against all the defined routes. Each of the routes is a 2-tuple, with a regular expression and a method name (as string). The request URL is matched against the regular expression, and if it succeeds, the method name will be resolved on the @PAGE_CLASS@. This method will then be executed, and the router will _not_ continue searching for a next match.

To illustrate the previous with an example request "@/haltandcatchfire@":

# The PAGE_CLASS is instantiated into a live [[PageMaker]]

# The first route tuple is inspected

#* Its regex is @'/static/(.*)'@

#* The request does *not* match the regex

# The second route tuple is inspected

#* Its regex is @'/(broken.*)'@

#* The request does *not* match the regex

# The third route tuple is inspected

#* Its regex is @'/haltandcatchfire'@

#* The request *matches* the regex

#* The associated handler method name is @'MakeFail'@

#* The method @MakeFail@ is resolved on our @PageMaker@ instance, resulting in @pages.PageMaker.MakeFail@

#* This method is executed and its results will be sent to the client

# Router has ended after the third inspected route

h2. Arguments from the request string

Oftentimes, there are parts of the request string that are needed further on in the process. While it's possible to extract these in the PageMaker methods, this is inconvenient, and the router has the means to do this. As can be seen in the router example above, some of the regular expressions have capture groups defined. These capture groups are provided to the PageMaker method as positional arguments. For example:

Requesting "@/user/elmer/edit/27@" on the following router:

<pre><code class="python">

PAGE_CLASS = blog.BlogPages

ROUTES = (

    ('/user/(\w*)/edit/(\d*)', 'EditArticle'),

    ('/', 'Index'))

</code></pre>

Will end up calling the method @blog.BlogPages.EditArticle@ with the arguments @('Elmer', '27')@. Note that all arguments are provided as strings, type-conversion is left to the developer.

h2. Typical regexes for request matching

How to use various other url formats:

* To match one or more letters, numbers, dashes and spaces, use this: @'([\w\- ]+)'@

* Match optional trailing slashes (so that requests don't end up 404'ing because of an added slash) add: @'/?'@

* To match optional an optional page number: @'article/([\w\- ]+)/?(\d+)?'@

** This matches @/article/cookies_are_delicious@

** As well as @/article/cookies_are_delicious/2@

** And also @/article/cookies_are_delicious/@ – to allow for trailing slashes that are almost certain to happen