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Jan Klopper, 2019-11-04 15:50

PageMaker is the Controller of the MVC approach in µWeb. After a request is received by the web server (either Standalone or Apache) and wrapped inside a Request object, it is routed here to be answered.

In the PageMaker, there might be database lookups done through the data abstraction layer (model) and likely output is sent back making use of the TemplateParser.

A very minimal PageMaker

In the simplest form, a PageMaker for a project subclasses from µWeb's default PageMaker class and provides its own methods to handle requests. The full source for this would look something like this:

#!/usr/bin/python
"""PageMaker demonstration module""" 

# uWeb framework
import uweb

class Minimalist(uweb.PageMaker):
  def Index(self):
    return 'Welcome to our website, it is still very much under construction.'

  def Catchall(self, path):
    return 'The requested page %r does not exist yet' % path

DebuggingPageMaker

Before we do anything else, during development you are strongly advised to use µWeb's DebuggingPageMaker. This has a lot of additional features for when something goes wrong on the server side. When the regular PageMaker runs into a server side error, it returns a very plain HTTP 500 response:

INTERNAL SERVER ERROR (HTTP 500) DURING PROCESSING OF '/'

Where '/' is the path requested by the client. When running DebuggingPageMaker there is a significantly more helpful (for the developer at least) page whenever an internal server error is encountered. It will show a full stack trace, the local variables on each stack level (typically at the point of calling another function), which helps to arrive to the point of failure more quickly.

To use, just subclass your PageMaker from DebuggingPageMaker:

class Minimalist(uweb.DebuggingPageMaker)

Example Internal Server Error response as image or in the µWeb demo project

In all cases, an internal server error will cause a full stacktrace to be logged in the log file database.

Templateparser

The µWeb TemplateParser is available on the standard PageMaker instance. When using PageMaker, an instantiated TemplateParser instance is available through the parser member of PageMaker. Basic usage looks like this:

import uweb
import time

class TemplateDemo(uweb.PageMaker):
  def VersionPage(self):
    return self.parser.Parse(
      'version.utp', year=time.strftime('%Y'), version=uweb.__version__)

The example template for the above file could look something like this:

<!DOCTYPE html>
<html>
  <head>
    <title>µWeb version info</title>
  </head>
  <body>
    <p>µWeb version [version] - Copyright 2010-[year] Underdark</p>
  </body>
</html>

And would result in the following output:

<!DOCTYPE html>
<html>
  <head>
    <title>µWeb version info</title>
  </head>
  <body>
    <p>µWeb version 0.12 - Copyright 2010-2012 Underdark</p>
  </body>
</html>

Full documentation, with plenty of example template uses can be found on the TemplateParser wiki-entry.

Template directory configuration

By default, template are loaded from the 'templates' directory that is expected to be on the same path as the pagemaker module. If your pagemaker is located on /var/www/uweb_project/project.py, then templates will be automatically loaded from /var/www/uweb_project/templates/.

To change the default template loading path, define a new path in the class variable TEMPLATE_DIR. This should be a relative path (and defaults to 'templates').

Serving static content

Your website most likely has a few static files that need to be served up. If you have a large website you would run many of these from a separate domain (to reduce the amount of overhead from a heavy web server and complex processes), but often there are at least some files that need to be served up from the local disk.

µWeb has built-in facilities to serve static files, which prevent filesystem traversal by those lesser-behaved browsers. A browser requesting http://example.com/../secret_configuration.txt should not get the keys to your database server. The static handler has a base (configurable) directory from which all static content is served. For a client it is impossible to 'browse' up from that directory, preventing these leaks of information.

The MIME-Type for the content will be determined using the mimetypes module (available by default in Python), based on the file's extension.

Static handler demonstration

In your router configuration, add a route the directs to the static handler:

ROUTES = (
    ...
    ('/images/(.*)', 'Static'),
    ...
    )
This will cause the following behaviour:

Static directory configuration

As can be seen in the previous example, the content is served from the 'static' directory (this is not dependent on the selected route. This path is relative to the absolute path of the PageMaker itself. If the PageMaker module exists on '/var/www/project/controller.py' then the default static directory is '/var/www/project/static/'.

This default path can be changed by setting the PUBLIC_DIR class variable of PageMaker. The path can be made absolute simply by providing one:

import uweb

class DifferentPublic(uweb.PageMaker):
  PUBLIC_DIR = '/var/www/project/public_http'

404 on static content

Whenever a request for static content (through Static) cannot be fulfilled, the method _StaticNotFound is called, with the requested relative path as the sole argument. The default response for which is a simple plain text:

  def _StaticNotFound(self, _path):
    message = 'This is not the path you\'re looking for. No such file %r' % (
      self.req.env['PATH_INFO'])
    return response.Response(message, content_type='text/plain', httpcode=404)

Override this page if you want to provide your user with a more informative or styled response.

Model admin

µWeb comes with a minimal admin interface based on the model

Add the Admin mixin class to PageMaker

This is as simple as adding the Admin mixin class as one of the ancestors of your PageMaker. It is generally advisable to place mixin classes before the base class):

# Package imports
from . import model

# uWeb imports
import uweb
from uweb.pagemaker import admin

class Pages(admin.AdminMixin, uweb.PageMaker):

After this, you will want to tell the admin code where to find your model, this happens inside your PageMaker class:

  ADMIN_MODEL = model

Router details

To setup a route to the admin pages, you will need to add a specific route to your router.

ROUTES = (
    ...
    # Admin routes
    ('/admin(/.*)?', '_Admin'),
    ...
)

After this, you can navigate to http://localhost:8082/admin to see the database's contents, manipulate records and view the documentation stored inside your model's methods.

Login and sessions

OpenID

To enable users of your website to log in using OpenID, there are only a few steps that need to be taken:

Add the OpenID mixin class to PageMaker

This is as simple as adding the OpenID mixin class as one of the ancestors of your PageMaker. It is generally advisable to place mixin classes before the base class):

import uweb
from uweb.pagemaker import login

class Pages(login.OpenIdMixin, uweb.PageMaker):

Set up routes to the OpenID validator

The following routes (or similar ones) should be added to the router:

ROUTES = (
    ...
    ('/OpenIDLogin/?(\w+)?', '_OpenIdInitiate')
    ('/OpenIDValidate', '_OpenIdValidate')
    ...
    )

The optional capture after 'OpenIDLogin' here is to provide the optional provider URL (instead of through the POST field 'openid_provider').

Add handlers to PageMaker for success, failure, etc:

  def OpenIdAuthCancel(self, message):
    return 'OpenID Authentication canceled by user: %s' % message

  def OpenIdAuthFailure(self, message):
    return 'Authentication failed: %s' % message

  def OpenIdAuthSuccess(self, auth_dict):
    # Authentication succeeded, the auth_dict contains the information we received from the provider
    #
    # Next: Retrieve user information from database, or create a new user
    #       Store the user's session (in the database, cookie, or both)
    session_id = base64.urlsafe_b64encode(os.urandom(30))
    self.req.AddCookie('OpenIDSession', session_id, max_age=86400)
    return 'OpenID Authentication successful!'

  def OpenIdProviderBadLink(self, message):
    return 'Bad OpenID Provider URL: %s' % message

  def OpenIdProviderError(self, message):
    return 'The OpenID provider did not respond as expected: %r' % message

Underdark Login Framework

Using the Underdark Login Framework requires steps comparable to using OpenID, but comes with a little more default setup. The system has two modes for logging in, one that is a straightforward plaintext form submit. This is slightly easier to implement, but when used without SSL it is highly vulnerable to man-in-the-middle (MITM) attacks. The second mode is a Javascript enabled mode where the password is hashed (using the SHA-1 algorithm), and to prevent replay attacks (from a MITM), a random 'challenge' is provided for each login attempt, which is also hashed with the result. This prevents a MITM from learning the password value, or using it to log in later (though the plaintext communication remains visible).

Add the ULF mixin class to PageMaker

import uweb
from uweb.pagemaker import login

class Pages(login.LoginMixin, uweb.PageMaker):

Set up routes to the OpenID validator

The following routes (or similar ones) should be added to the router:

ROUTES = (
    ...
    ('/ULF-Challenge', '_ULF_Challenge'),
    ('/ULF-Login', '_ULF_Verify'),
    ...
    )

Add handlers to PageMaker for success and failure:

  def _ULF_Failure(self, secure):
    reutrn 'ULF authentication failed (secure mode: %d)' % secure

  def _ULF_Success(self, secure):
    return 'ULF authentication successful! (secure mode: %d)' % secure

Persistent storage between requests

µWeb allows you to store objects in a process-persistent storage. This means that the storage will be properly persistent and available when µWeb is in standalone mode. When running on top of apache, this persistence is only as good as the apache process, which is typically a couple hundred to a few thousand requests.

Default users of the persistent storage

By default, the TemplateParser and the various database connectors are stored in the persistent storage. This has the benefit that pre-parsed templates will not need to be read from disk on subsequent requests. For databases the benefit is that connections need not be made on-the-fly, but can mostly be retrieved from the storage.

Storing persistent values

Storing persistent values is done with the Set method, as follows:

def _PostInit(self):
  if 'connection' not in self.persistent:
    self.persistent.Set('connection', self._MakeConnection())

In the example above, the database connection is only created, and added to the persistent storage, if it's not already present. This way expensive but reusable actions can be optimized by performing them only once (or once every few so many requests, if running on Apache).

Retrieving persistent values

Retrieving stored values works much like this, but uses the Get method:

def DatabaseAccess(self):
  with self.persistent.Get('connection') as cursor:
    cursor.Execute('INSERT INTO `message` SET `text` = "success!"')

This uses the connection we created (or still had) during _PostInit, and uses it to update the database.

In case a key has is not present in the persistent storage (because it wasn't set in the process' lifetime or because it was explicitly dropped), the Get method has an optional second argument, that is returned when the key is not present:

def FirstVisit(self):
  when = self.persistent.Get('first_visit_time', 'just now')
  return 'Your first visit was %s.' % when

This will return the stored date and time when there was a previously recorded visit, or the text just now if there was no previous time logged.

Finally, the persistent storage has a SetDefault method, that acts much like the similarly named dictionary method. It returns the value for the given key, but if it's not present, it will set the key to the provided value, and return it as well. With this, we can improve on our first-visit tracker, and in one call retrieve or store the first time someone visited:

def FirstVisit(self):
  when = self.persistent.SetDefault('first_visit_time', datetime.datetime.now())
  return 'Your first visit was %s.' % when

Deleting persistent values

If for any reason you need to delete a value from the persistent storage, this can be done using the Del method. The given key name is removed from the storage. N.B.: If the key was already removed from the storage (this can happen if the delete code runs more than once, or the key was not defined in the process' lifetime), no error is raised. It is assumed that removing the key is the only desired action.

def DeletePersistentKey(self, key):
  self.persistent.Del(key)

Logging:

You can use the bundled logging tool and vieuwer to log various messages and view them.
For example:

In a pagemaker function you can do:

uweb.logging.LogDebug('Some Debug message')
uweb.logging.LogDebug('Some Debug message', variables, a=b, c=d)

Both arguments and keyword based arguments will be logged.

Various log levels are present, each with their own method Name

  • LogDebug
  • LoginInfo
  • LogWarning
  • LogError
  • LogException
  • LogCritical

The logged errors, including those that the platform itself logs when using for example the 'debuggingpagemaker' are stored in you log folder inside a sqlite database.
A viewer for theses databases is included in he package and can be started by issuing:

uweb start logviewer

which will start a webserver on port 8001.

Log locations:

By default logs are stored in:
  • /var/log/underdark
  • ~/.underdark
    The choice of this depends on the write rights the user running the application has.

Using the config file for settings

uweb can use a configfile to store settings. Using these user-configurable settings in our application will allow for greater flexibility.

The location of the configfile is given in the router by setting it as such:

CONFIG = '../myconfig.conf'

n.b. The config is usually stored outside any web-accessible paths.

From there on you can store settings in a tiered key-value style, you can make chapters which in turn hold keys and values:

[general]
name = Uweb project

[somethingelse]
primaryip = 127.0.0.4
secundaryip = 127.0.0.4

You can access these values by their chapter and key using the following python code inside a pagemaker method:

name = self.options['general']['name']
ip = self.options['somethingelse']['primaryip']

How to set code on alle pagemaker calls.

If you want to set some environment variables on all your pagemaker calls, for example set http headers, you can use the _PostInit call.


  def _PostInit(self, *args, **kwargs):
    """Sets up a few basic security headers.""" 
    self.req.AddHeader('X-XSS-Protection', '1; mode=block')
    self.req.AddHeader('Referrer-Policy', 'same-origin')
    self.req.AddHeader('Content-Security-Policy', 'SAMEORIGIN')
    self.req.AddHeader('X-Frame-Options', 'SAMEORIGIN')
    super(PageMaker, self)._PostInit(*args, **kwargs)