We are faced again and again with the problem of interfacing various constructs which boil down to addressing YANG-modeled data.

The usual solution involves YangInstanceIdentifier, but that has non-trivial mapping to YANG due to its addressing mode of operation, which takes into account keyed list entries as well as augmentations. It also accumulated weird things like:

• wildcards, i.e. the use of NodeIdentifier to say 'match all MapEntryNodes'
• partial matches, i.e. NodeIdentifierWithPredicates without all the keys corresponding to schema present

In yang-model-api we have had a similar (but different) problem with SchemaPath, which we solved through introduction of SchemaNodeIdentifier and EffectiveStatementInference, each of which has a narrow scope of what it solves and how it operates.

At the end of the day, at least where NETCONF/RESTCONF is concerned, we really need an identifier which works strictly on YANG data tree addressing, pure and simple. At the heart of it is something like:

class DataTreeStep {
  QName nodeId;
  Map<QName, Object> predicates; // may be empty
}

class DataTreeIdentifier {
  List<DataTreeStep> steps; // at least one
}

Taking the YangInstanceIdentifier/wildcard case to heart, though, we need two dedicated specializations of that structure:

1. one for read requests, where it is treated like a NETCONF subtree filter
2. one for read responses, where it is treated like a proper instance identifier, i.e. predicates for list keys are fully populated.
   The specializations must not share a (publicly visible) superclass, so that there is no way of confusing the request and response side. The DataTreeIdentifier -> DataTreeFilter conversion is trivial.

We then should provide schema-assisted utilities to covert:

• DataTreeFilter -> DataTreeIdentifier, which validates the filter specifies all key list predicates
• DataTreeIdentifier <-> exact YangInstanceIdentifier
• DataTreeFilter -> wildcard YangInstanceIdentifier which matches the superset of the DataTreeFilter (plus a structure describing what are the parts where a DataTreeChangeListener is providing a superset)
• partial/wildcard YangInstanceIdentifier -> DataTreeFilter

This toolkit should make life a lot easier for our downstreams.