The set of Objects and related information that the OPC UA Server makes available to Clients is referred to as its AddressSpace. The model for Objects is defined by the OPC UA Object Model (see 4.2).

Objects and their components are represented in the AddressSpace as a set of Nodes described by Attributes and interconnected by References. Figure 3 illustrates the model of a Node and the remainder of 4.3 discusses the details of the Node Model.


Figure 3 – AddressSpace Node Model

NodeClasses are defined in terms of the Attributes and References that shall be instantiated (given values) when a Node is defined in the AddressSpace. Attributes are discussed in 4.3.3 and References in 4.3.4.

Clause 5 defines the NodeClasses for the OPC UA AddressSpace. These NodeClasses are referred to collectively as the metadata for the AddressSpace. Each Node in the AddressSpace is an instance of one of these NodeClasses. No other NodeClasses shall be used to define Nodes, and as a result, Clients and Servers are not allowed to define NodeClasses or extend the definitions of these NodeClasses.

Attributes are data elements that describe Nodes. Clients can access Attribute values using Read, Write, Query, and Subscription/MonitoredItem Services. These Services are defined in OPC 10000-4.

Attributes are elementary components of NodeClasses. Attribute definitions are included as part of the NodeClass definitions in Clause 4.9 and, therefore, are not included in the AddressSpace.

Each Attribute definition consists of an attribute id (for attribute ids of Attributes, see OPC 10000-6), a name, a description, a data type and a mandatory/optional indicator. The set of Attributes defined for each NodeClass shall not be extended by Clients or Servers.

When a Node is instantiated in the AddressSpace, the values of the NodeClass Attributes are provided. The mandatory/optional indicator for the Attribute indicates whether the Attribute has to be instantiated.

References are used to relate Nodes to each other. They can be accessed using the browsing and querying Services defined in OPC 10000-4.

Like Attributes, they are defined as fundamental components of Nodes. Unlike Attributes, References are defined as instances of ReferenceType Nodes. ReferenceType Nodes are visible in the AddressSpace and are defined using the ReferenceType NodeClass (see 5.3).

The Node that contains the Reference is referred to as the SourceNode and the Node that is referenced is referred to as the TargetNode. The combination of the SourceNode, the ReferenceType and the TargetNode are used in OPC UA Services to uniquely identify References. Thus, each Node can reference another Node with the same ReferenceType only once. Any subtypes of concrete ReferenceTypes are considered to be equal to the base concrete ReferenceTypes when identifying References (see 5.3 for subtypes of ReferenceTypes). Figure 4 illustrates this model of a Reference.


Figure 4 – Reference Model

The TargetNode of a Reference may be in the same AddressSpace or in the AddressSpace of another OPC UA Server. TargetNodes located in other Servers are identified in OPC UA Services using a combination of the remote Server name and the identifier assigned to the Node by the remote Server.

OPC UA does not require that the TargetNode exists, thus References may point to a Node that does not exist.