Essential Concepts

Data modelling and schema languages is a complex business – that’s why JSON folks try to avoid it like plague. We believe, however, that data modelling greatly helps interoperability and unification of management interfaces. From a programmer’s perspective, data models not only document the data that our programs process, but they can be also be used for automating many difficult and/or laborious tasks such as validating data or building user interfaces.

YANG tries to be easy to read and understandable. Very often, people are able to understand what YANG data models mean without having read YANG language specification. Nevertheless, some concepts and rules in YANG are less than intuitive, and some are perhaps even slightly peculiar. This section gives an overview of fundamental YANG concepts and terms the are needed for understanding the documentation of the Yangson library. However, it is no substitute for studying YANG documentation, especially [RFC7950] and [Bie16].

Another factor that may confuse users of the Yangson library is conflicting terminology: some terms, such as module or instance, are used, with different meanings, in both Python and YANG. Therefore, when reading the following sections, it is important to distinguish whether a given text discusses programming language or data modelling stuff.

Data Models

Data models define and describe some data. A complete YANG data model usually corresponds to a particular device, physical virtual, with a dedicated configuration and management system.

YANG distinguishes four different sorts of data:

  • configuration,
  • state data and statistics,
  • parameters of RPC (remote procedure call) operations,
  • asynchronous event notifications.

YANG assumes that data of all the sorts listed above are hierarchically organised, i.e. they form a tree. For example, data encoded in JSON can be modelled nicely with YANG.

For the specification of a data model, YANG uses both formal means and textual descriptions that may specify additional rules and constraints. Such textual descriptions are considered an integral part of the data model and cannot be ignored!

The formal means include:

  • The hierarchical structure of data is described via containment of YANG. For example, a container node is defined through the container statement, and child nodes of this container are defined through its substatements.
  • YANG also allows for defining which nodes are mandatory and which are optional. For lists (sequences of entries of the same type), it is possible to specify the minimum and maximum number of entries.
  • All scalar parameters have a type. YANG offers a wide variety of built-in types, such as string, boolean and int32. It is also possible to define derived types by taking an existing type (built-in or derived), giving it a new name, and optionally specifying some restrictions. For example, a restriction that may be applied to the string type is the pattern statement that specifies a regular expression that strings belonging to that type must match.
  • For scalar parameters, it is also possible to define a default value.

For the Yangson library, a fully specified data model is the baseline from which any further processing starts. That’s why operations with isolated YANG modules are not “officially” supported, i.e. not available through the public API.

Trees and Nodes

In most practical applications of the Yangson library, a programmer needs to work with two types of trees:

  • data tree contains real data such as configuration, state data, RPC input/output parameters, or notifications. For our purposes, a data tree is a JSON document, or a parsed in-memory representation thereof.
  • schema tree allows us to decide which data trees are valid and which are not.

Each node in the data tree corresponds to a data node in the schema tree. This looks confusing but in fact it is quite logical: data nodes are special schema nodes that have counterparts in the data tree. There are other schema nodes, namely choice and case, that don’t have this property – they are used in the schema for specifying possible alternatives of which only one can appear in the data tree.

YANG Modules

YANG data models consist of modules. Each module defines the schema for some (usually related) parts of the data trees. Typically, a YANG module covers a certain subsystem or function. Every module defines a namespace that needs to be locally unique in a given data model. In Yangson, the namespace is identified by the YANG module name.

YANG modules may also offload parts of their contents into submodules. One can then have one (main) module and any number of submodules that are included from the main module. The main module and all its submodules share the same namespace identified by the main module name.

In order to create a particular data model, one has to decide which YANG modules will become part of it. The selection is recorded in YANG library data [RFC7895]. And since YANG modules may exist in multiple revisions, a revision also needs to be specified for each module.

YANG also offers two mechanisms that allow for finer-grain control of data model content:

  • features are essentially boolean flags that indicate whether an optional subsystem or function is supported or not. Parts of the schema tree can be labelled as being dependent on a feature: such a part exists only if the feature is supported.
  • deviations allow for specifying that a given implementation doesn’t exactly follow what’s written in a YANG module. In effect, a deviation can be understood as a “patch” of the original YANG module.

Support for individual features and/or deviations are also indicated in YANG library data.

Content Types

YANG distinguishes configuration from state data (see sec. 4.2.3 in [RFC7950]), and the config statement can be used to specify to which of the two categories a given schema node belongs. A schema node whose definition doesn’t contain the config statement inherits this property from its parent schema node. State data may be embedded inside configuration, but not vice versa. Finally, for schemas of RPC operations, actions and notifications, the distinction between configuration and state data makes no sense at all, and config statements, if present, are ignored there.

The approach adopted by the Yangson library is to assign a content type to every SchemaNode. The values are members of the enumeration ContentType:

All non-terminal schema nodes (container, list, choice and case) that represent configuration have the content type all because they may have both configuration and state data nodes as descendants.

Content type of terminal data nodes (leaf, leaf-list, anydata and anyxml) reflects their config, i.e. it is either config or nonconfig.

Other nodes always have content type nonconfig.

The method SchemaNode.content_type() returns the content type of the receiver.

The above rules allow for a straightforward implementation of content filtering in RESTCONF based on the content query parameter, see sec. 4.8.1 in [RFC8040].