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OPc UA Object Oriented Information Model
To make applications interoperable, the data transfer mechanism must be associated with a consistent information representation model and a description and discovery mechanism. OPC UA uses an object as a fundamental notion to represent data and activity of an underlying processes system. The objects are placeholders of variables, events and methods and are interconnected by references. This concept is similar to well-known object oriented programming (OOP) that is a programming paradigm using "objects" – data structures consisting of fields, events and methods – and their interactions to design applications and computer programs. The OPC UA Information Model (visit OPC UA eBook to get more details), provides features such as data abstraction, encapsulation, polymorphism, and inheritance.
The OPC UA object model allows servers to provide type definitions for objects and their components. Type definitions may be abstract, and may be inherited by new types to reflect polymorphism. They may also be common or they may be system-specific. Object types may be defined by standardization organizations, vendors or end users. Each type must have a globally unique identifier that can be used to provide description of the information meaning, i.e. semantics from the defining body or organization. Using the type definitions to describe the information exposed by the server allows:
- Development against type definition
- Unambiguous assignment of the semantics to the data
The Information Model is a very powerful concept, but it is abstract and hence, in a real environment, it must be implemented in terms of bit streams (to make information transferable) and addresses (to make information selectively available). To meet this requirement, OPC UA introduces a Node notion as an atomic addressable entity that consists of attributes (value-holders) and references (address-holders of coupled nodes). The set of Nodes that an OPC UA server makes available to clients is referred to as its Address Space, which enables representation of both real process environment and real-time process behavior.
The Information Model and Address Space concepts are described in depth in:
OPC Main Technology fetures