Configuration Automations Part 2.

This article is a continuation of the previous article on Configuration Automations and describes the syntax of the configuration templates.  

The templates can declare variables and logic that expand to actual configurations when substituted. As common to declarative syntax across declarative frameworks such as for user interface resources, infrastructure resources, and container orchestration frameworks, most begin and end with a specific delimiter.

The W3 organization specifies a template language for xml and xquery that involves a QName. Template arguments can be described in a key-value pairs where the key is a proper key name. These must be uniquely identified for their usage.

Templates can also import modules and open data files. The import modules take a namespace to avoid conflict from other modules. The data files can be opened as a string or an xml node using a built-in intrinsic. This can be used to assign the content to a variable. With the help of this variable, it is possible to browse an element by walking the hierarchy checking for each level if it exists and then finding the value corresponding to the name. A text file can be opened as a sequence of strings as well.

XPath is the language for navigating the xml hierarchy to identify a node. It uses curly braces as delimiters and those delimiters so those cannot be used inside the XPath embedded in a template. XQuery is the language for querying the nodes with the criteria specified. XQuery embedded in an attribute value of the template will call the builtin implicitly and this will return the node itself if it were an attribute that was queried or the concatenated text of the node and its descendants or if the input contains multi-nodes, their return value will be joined and delimited by white space. If the XQuery is embedded in an element text, it will keep its original result and in the case of attribute node will return the surrounding elements’ attribute while in the case of element node will become the surrounding elements’ child. As an example, a

<root>

<item name=”A” value=“1”>X</item>

<item name=”B” value=”2” >Y</item>

<item name=”C” value=”3”>Z</item>

</root> 

where xml content can be used with a template specifying an attribute as:

<xml><test result=”{root/item/@*}”</test>

and this will result in

“A 1 B 2 C 3”

And an element as:

<test result=”{root/item}” ></test>

And this will result in

<test result=”X Y Z”></test>

Templates can also include references to other entities in element texts and attribute values and there’s some difference between this format and X-Query.

One of the challenges with template syntax that is often encountered is the use of quotes for enclosing literals. Single quotes and double quotes cannot be used together in a string literal but concat operator can be used to build a string from parts that are enclosed in both.

There are extensions available that can take a template in one form and translate it into another. For example, a JSONiq extension is designed to write JSON templates naturally.