Address Space

The Address Space models data as a network of Nodes rather than a simple flat list. Each Node represents a real-world entity like a device, a sensor, or a process. Nodes are defined by their Attributes (properties like Value, DataType, and Timestamp) and connected by References (typed relationships). This allows a TemperatureSensor Node to be a child of a Boiler Node, inheriting context and making the system's structure self-describing.

Every Node is an instance of a specific NodeClass, which defines its fundamental behavior:

• Object Nodes: Represent physical or logical system components (e.g., 'Pump_01').
• Variable Nodes: Hold actual process values (e.g., 'Current_Speed').
• Method Nodes: Represent callable server functions that a Client can invoke (e.g., 'EmergencyStop').
• View Nodes: Provide filtered, task-specific subsets of the full Address Space, simplifying client navigation for specific user roles.

Relationships are first-class citizens. References are not just parent-child hierarchies; they are typed pointers that create a semantic web. Common reference types include:

• Organizes: Defines a standard hierarchical structure for browsing.
• HasComponent: Indicates a part-of relationship (e.g., a Motor is a component of a Conveyor).
• HasTypeDefinition: Links an instance Node to its template (Type Node), enabling instant discovery of its full semantic contract. This graph of typed connections allows a Client to traverse from a production line down to a specific sensor's alarm state.

The base Address Space is extended by Information Models, which are formal, pre-defined schemas. The OPC UA specification provides core models, but industry-specific Companion Specifications (e.g., for Robotics or Machinery) add domain semantics. This means a 'Robot' Node from Vendor A and Vendor B will expose the same standardized structure and identifiers, enabling true plug-and-produce interoperability without custom mapping for each device.

The Address Space is designed for dynamic exploration. A Client can browse the hierarchy to discover unknown Nodes and their relationships. Crucially, it can also query in reverse: a Client can ask, 'Which other Nodes reference this specific Node?' This bidirectional traversal is critical for impact analysis, allowing a system to instantly identify all processes that depend on a failing sensor by following incoming HasComponent or HasEffect references.

The Address Space is not just for static data. It integrates a sophisticated Alarms and Conditions (A&C) model. Alarm states (e.g., 'HighTemperature') are represented as Nodes with specific state machines. A Client can browse the Address Space to find all currently active alarms, their severity, and the specific Node they are associated with. This stateful model supports acknowledgment, confirmation, and shelving workflows directly within the information structure.