xpath/

XPath support with YANG

XPath 1.0 is a referenced standard as part of the YANG specification. It is used as a query language for specifying many inter-node references and dependencies.

To bring XPath support to YGOT, this project uses a Go XPath implementation from Antchfx. Specifically it implements the NodeNavigator interface, as YangNodeNavigator thereby allowing it to reuse the Select() and Evaluate() methods on parsed XPath statements.

The implementation consists of 3 major parts:

Creating the YangNodeNavigator

A YangNodeNavigator is created by merging together the YANG Schema with a ValidatedGoStruct.

The Schema (created by YGOT) is a tree of yang.Entrys reflecting the hierarchy of the YANG model. In building the YangNodeNavigator, a recursive function steps through every part of the tree, adding new Annotation entries to hold the related Go Struct.

List entries are treated specially with each instance of a list, being given a new Dir entry in the tree with the name format <listname>__<listindex>.

To ensure the consistent navigation of the tree, a slice of orderedkeys is added to the yang.Entry as an Annotation. If a leaf has no value, it will be omitted from the orderedkeys list.

Querying the YangNodeNavigator

Once created, the YangNodeNavigator can be traversed using the interface methods (from NodeNavigator) like MoveToChild(), MoveToNext() etc.

The orderedkeys list ensures a consistent order of traversal.

XPath queries (once Compile()ed) can be executed on the YangNodenavigator in 2 ways:

1. Evaluate() - returns the result of the expression - this can handle XPath functions like count, sum etc. and can return scalar values (number, boolean, string) or a NodeNavigator.
2. Select() - selects a node set using the specified XPath expression - this node set can be then be navigated to extract values.

There are many examples of both types of query in the unit test.

For example

count(/t1:cont1a/t1:list2a[number(t1:tx-power) < number(t1:rx-power)])

This can be interpreted as: count the number of instances of list2a whose attribute tx-power is less than its rx-power.

Each attribute and container should be prefixed with its namespace prefix. Also any comparison will be a text comparison by default, and hence numbers must be cast explicitly.

Another example is:

sum(/t1:cont1a/t1:list2a[contains(@t1:name,'l2')]/t1:rx-power)

This can be interpreted as: add together all of the rx-power values from list2a entries that have a name attribute that contains the substring l2.

Because name is the key of the list2a list in YANG, it must be prefixed with @ when referring to it. Also text strings should be wrapped in single quotes. Predicates are given in [] and any number can be given, allowing querying of double keyed lists.

Evaluating 'must' statements in YANG model

One of the main use cases for XPath within YANG is the must statement. This is specified as an XPath query that must evaluate to true.

To validate a ValidatedGoStruct, it is transformed to a YangNodeNavigator and the WalkAndValidateMust() method can then be used to iterate through the tree, finding must statements and Evaluate()ing them. If any must statement does not evaluate to true, then an error is thrown and the validation stops.

An example of a must statement, is given below as a statement on a container that enforces some relationship between leafs in that container, or its parent, peers or children:

container cont2a {
  must "string(t1:leaf2g) = 'true' or number(./t1:leaf2a) < 4" {
    error-message "If Leaf 2a is above 4 then leaf2g must be true for the validation to pass";
  }
  ...

This can be interpreted as: either leaf2g (which is a boolean) must be true or else the value of leaf2a must be less than 4. When this is not the case, the must validation will fail, as the configuration is not valid.

Care must be taken with data types when doing comparisons - above the leaf2a is a number and must be converted before comparison with the number 4. Also comparing a boolean leaf (leaf2g) is best done by conversion to a string before comparison.

In the above XPath query, the relative operator . is used to refer to the container context - it could be omitted from the ./t1:leaf2a, as it is assumed by default (as happens in t1:leaf2g). Similarly

• The .. operator can be used to refer to the parent of the container
• The / refers to the root of the tree
• The // refers to a child at any level beneath the root