Node

The NodeId is the unambiguous identifier for a Node within a Server. It consists of a namespace index (identifying the naming authority) and an identifier (a numeric, string, GUID, or opaque value). This structure allows multiple information models to coexist without naming collisions.

• Namespace 0: Reserved for the OPC UA core specification.
• Namespace 1+: Assigned to vendor-specific or Companion Specification models.
• A Client must always treat the combination of NamespaceUri and identifier as the true unique key.

The NodeClass attribute defines the fundamental kind of Node, determining its structure and purpose. The eight core NodeClasses form the building blocks of every Information Model.

• Object: Represents a physical or logical system component (e.g., a pump).
• Variable: Holds a value, such as a sensor reading or configuration parameter.
• Method: A callable function that triggers an action on the Server.
• ObjectType & VariableType: Templates defining the structure of Objects and Variables.
• ReferenceType, DataType, View: Define relationship semantics, value structures, and organizational subsets.

Attributes are data elements that describe a Node. While mandatory attributes like NodeId, NodeClass, and BrowseName exist on every Node, optional attributes provide domain-specific context.

• Value: The current data held by a Variable Node, accompanied by a StatusCode and timestamp.
• DisplayName: A human-readable, localizable name for user interfaces.
• Description: A localized text field providing engineering context.
• AccessLevel: Specifies whether a Variable is readable, writable, or subscribable.
• Historizing: A boolean indicating if the Server archives changes to the Value.

References are the directed edges that connect Nodes, forming the graph structure of the Address Space. They define hierarchical (organizational) and non-hierarchical (semantic) relationships.

• HasComponent: Indicates a parent-child containment relationship.
• Organizes: Groups Nodes for browsing without strict containment.
• HasTypeDefinition: Links an Object to its ObjectType, defining its semantic contract.
• HasProperty: Connects an Object to a characteristic Variable that cannot exist independently.
• References enable Clients to browse from a root Node to any leaf in the system.

The BrowseName is a qualified name used for programmatic path navigation. Unlike the NodeId, it is not globally unique but must be unique within the context of its parent Node.

• Composed of a namespace index and a string name.
• Enables Clients to traverse the Address Space using human-readable paths like /Objects/ProductionLine1/ConveyorBelt.
• Remains stable across Server restarts, providing a reliable navigation handle even if the NodeId is dynamically assigned.

Every Object and Variable Node is an instance of a specific Type Node (ObjectType or VariableType). This mechanism enforces semantic consistency by guaranteeing that all instances of a type share the same structure.

• A MotorType might define mandatory Variables for Speed and Temperature.
• Every Motor instance automatically inherits these Variables.
• This enables type-safe programming, where a Client can expect a known interface on any Node of a given type, enabling true plug-and-produce interoperability.