Krang Element System

This document describes the Krang Element System at a technical level. The information given here is neither exhaustive nor authoritative but should provide a good introduction. For more detail, see the documentation in the modules referenced in this text.

Element Library

The element library is a directory in CVS containing all existing element sets available for use with Krang. The intent is to support the maximum possible degree of reusability in Krang elements.

The element library will be developed and versioned separately from Krang. One element set can use elements from another set, either by directly referencing element classes in foreign sets or by declaring another set as a parent set.

For example, the root directory of the element library might contain:


Each Krang instance will declare the element sets they wish to use with the ElementSet configuration option in krang.conf. For example, a site using the Flex element set would declare:

  ElementSet Flex

Element Set

An element set contains element classes that power the elements for an instance. Elements provide the building blocks for stories.

Each element set contains a configuration file, set.conf, which contains a list of top-level element classes in the set. Each element set can be configured to inherit elements from other element sets. For example, Flex might inherit from the Default set.

An element set may also contain a file with default templates, called templates.kds. This file must be a valid Krang::Dataset file containing only Krang::Template objects. If present, it will be loaded into the database by krang_createdb.

Finally, elements sets may include a lists.conf file describing lists available for use by the element set and in the user interface. The lists.conf file is an XML file containing a description of each list group. For example:

    <list_group list_group_name="Cars">
      <list list_name="Make" />
      <list list_name="Model" />
      <list list_name="Year" />
    <list_group list_group_name="Segments" description="This list group just contains one list">
      <list list_name="Segments" />

This configuration file describes two list groups. A list group is a group of linked lists. The first list group on the example describes a three-level tree of make/model/year values- meaning each 'Make' can be associated with a list of 'Model's, and each 'Model' can be associated with a list of 'Year's. The second list group in the example contains just a single flat list. Note that a default description for the list group can be set within the list_group tag.

You may also specify initial list items via lists.conf by using the optional element, ``list_item'', which expects two attributes, ``item_name'' and (optional) ``parent_item_name'':

  <list_group list_group_name="Make/Model/Year" description="blah blah blah">
    <list list_name="Make">
      <list_item item_name="Ford" />
      <list_item item_name="Honda" />
      <list_item item_name="GMAC" />
    <list list_name="Model">
      <list_item parent_item_name="Ford" item_name="Explorer" />
      <list_item parent_item_name="Ford" item_name="Escort" />
      <list_item parent_item_name="Ford" item_name="Focus" />
      <list_item parent_item_name="Honda" item_name="Element" />
      <list_item parent_item_name="Honda" item_name="Civic" />
      <list_item parent_item_name="Honda" item_name="Accord" />
      <list_item parent_item_name="GMAC" item_name="Hummer" />
      <list_item parent_item_name="GMAC" item_name="Pontiac" />
      <list_item parent_item_name="GMAC" item_name="Humongulous" />
    <list list_name="Year">
      <list_item parent_item_name="Explorer" item_name="1998" />
      <list_item parent_item_name="Explorer" item_name="1999" />
      <list_item parent_item_name="Explorer" item_name="2000" />
      <list_item parent_item_name="Explorer" item_name="2001" />
      <list_item parent_item_name="Civic" item_name="1999" />
      <list_item parent_item_name="Civic" item_name="2000" />
      <list_item parent_item_name="Civic" item_name="2001" />
      <list_item parent_item_name="Civic" item_name="2002" />

This allows you to intuitively set up lists when you run a ``make db''.

Below is an example of how a ListGroup would be added to an element in your element library:

  Krang::ElementClass::ListGroup->new(  name => 'auto_segments',
                                        list_group => 'Segments',
                                        multiple => 1 )

'multiple' allows for multiple selections from a flat list.

See Krang::ElementClass::ListGroup, Krang::ListGroup, Krang::List and Krang::ListItem for further details.

Element Classes

An element class is a Perl module that controls how the element data will be edited, stored and published. Element class modules inherits from Krang::ElementClass or one of Krang::ElementClass's sub-classes like Krang::ElementClass::Textarea.

Here's an example container element, Flex::page:

  package Flex::page;
  use base qw(Krang::ElementClass);
  sub new {
      my $pkg = shift;
      my %opt = (
         name         => "page",  # unique name for the element
         display_name => "Page",  # display name for the UI
         min          => 1,       # always have one page
         max          => undef,   # allow any number of pages
         # list of potential child elements, specified as
         # names or with Krang::ElementClass objects
         children     => [
             # looks for a paragraph element across defined element
             # sets
             # instantiate using a direct reference to an
             # element class, with parameters for the constructor
             Flex::keywords->new(fields => 6),
             # create sub-elements dynamically without
             # requiring new module
             Krang::ElementClass::MediaLink->new(name => "inline_image"),
          # allow code that creates new pages to override any of the
          # available parameters
      return $pkg->SUPER::new(%opt); 

Flex::page is considered a 'container element' because its purpose is to contain other elements - paragraphs, images and keywords in this case.

As you can see above, data elements are also classes in the element set. A data element allows users to enter data, and usually does not contain sub-elements. Here's an example data element, Flex::paragraph, which inherits from Krang::ElementClass::TextBox:

  package Flex::paragraph;
  use base qw(Krang::ElementClass::TextBox);
  sub new {
      my $pkg = shift;
      my %opt = (
                 name           => "paragraph",
                 rows           => 4,
                 cols           => 40,
                 bulk_edit      => 1,
      return $pkg->SUPER::new(%opt);

Most usages of Krang will not require more code in the element classes than is shown above. In most cases it will be enough to inherit from one of the provided element classes. However, overriding methods inside the element class can give you control over every operation involving the element. See Krang::ElementClass for more details.


An element is a piece of a story. Every element is a Krang::Element object and belongs to a particular element class. The element class of an object is accessible through the class() method.

When you create a new story you define the class of its root element with the class parameter. For example, to create a new story based on the Article element class:

  $story = Krang::Story->new(class      => 'article',
                             title      => "Title",
                             slug       => "slug",
                             categories => [1]);

This prints ``Article'' because $story->element is an element assigned to the ``Article'' class:

  print $story->element->class->display_name;

Elements proxy element class methods to their element classes, mimicking inheritance. So this also prints ``Article'':

  print $story->element->display_name;

Given an element, you can access a list of children with the children() method. These children are also elements with their own class assignments. For example, if the Article has three paragraph elements as children then this will print ``Paragraph'' three times:

  print $_->display_name . "\n" for $story->element->children;

You can add child elements to add data to the story. For example, to add another paragraph with the content ``foo'':

  $story->element->add_child(class => "paragraph",
                             data  => "foo");

See Krang::Element for more information about how to work with elements.

List Groups and Lists