Extending KIWI NG with Custom Operations

Note

Abstract

Extension plugins in KIWI NG offer a mechanism for adding information outside the standard KIWI NG schema.

This document describes how to create an extension plugin for the KIWI NG schema as well as how to add and validate additional information in the KIWI NG image description.

The described schema extension can be used in an additional KIWI NG task plugin to provide a new subcommand for KIWI NG. At the present moment, there is no other plugin interface except for providing additional KIWI NG commands.

Depending on the demand for custom plugins, the interface to hook into code in other parts of the KIWI NG processing needs to be extended.

This description applies to version 10.2.42.

The <extension> Section

The main KIWI NG schema supports an extension section that allows you to specify any XML structure and attributes, as long as they are attached to a namespace. This means that any custom XML structure can be implemented similar to the example below:

<image>
    ...
    <extension xmlns:my_plugin="http://www.my_plugin.com">
        <my_plugin:my_feature>
            <my_plugin:title name="cool stuff"/>
        </my_plugin:my_feature>
    </extension>
</image>

• Any top-level namespace must be unique.

• Multiple different top-level namespaces are allowed, for example, my_plugin_a, my_plugin_b.

RELAX NG Schema for the Extension

If an extension section is found, KIWI NG looks up its namespace and uses the main XML catalog for the schema file to validate the extension data. The schema file must be a RELAX NG schema in the rng format. We recommend saving the schema as /usr/share/xml/kiwi/my_plugin.rng.

For the example above, the RELAX NG Schema in the compressed format, my_plugin.rnc, looks as follows:

namespace my_plugin = "http://www.my_plugin.com"

start =
    k.my_feature

div {
    k.my_feature.attlist = empty
    k.my_feature =
        element my_plugin:my_feature {
            k.my_feature.attlist &
            k.title
        }
}

div {
    k.title.name.attribute =
        attribute name { text }
    k.title.attlist = k.title.name.attribute
    k.title =
        element my_plugin:title {
            k.title.attlist
        }
}

To convert this schema to the rng format, just call:

$ trang -I rnc -O rng my_plugin.rnc /usr/share/xml/kiwi/my_plugin.rng

Extension schema in XML catalog

As mentioned above, the mapping from the extension namespace to the correct RELAX NG schema file is handled by an XML catalog file. The XML catalog for the example is as follows:

<?xml version="1.0"?>
<catalog xmlns="urn:oasis:names:tc:entity:xmlns:xml:catalog">
    <system
        systemId="http://www.my_plugin.com"
        uri="file:////usr/share/xml/kiwi/my_plugin.rng"/>
</catalog>

For resolving the catalog, KIWI NG uses the xmlcatalog command and the main XML catalog from the system, /etc/xml/catalog.

Note

How the main catalog is informed about the existence of the KIWI NG extension catalog file depends on the distribution and its version. Refer to the distribution documentation for information on adding XML catalogs.

If the following command provides the information for the correct RELAX NG schema file, you are ready for a first test:

$ xmlcatalog /etc/xml/catalog http://www.my_plugin.com

Using the extension

To test the extension, insert the example extension into one of your image description’s config.xml file.

The following example reads the name attribute from the title section of the my_feature root element and prints it:

import logging

from kiwi.xml_description import XMLDescription

description = XMLDescription('path/to/kiwi/XML/config.xml')
description.load()

my_plugin = description.get_extension_xml_data('my_plugin')

print(my_plugin.getroot()[0].get('name'))