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Composition vs Inheritance in OPCUA

· 4 min read
Etienne Rossignon
Etienne Rossignon
CEO sterfive

When modeling industrial systems with OPC UA, the debate between composition and inheritance often arises In object modeling, composition frequently proves to be a more flexible and powerful alternative to derivation (inheritance). While inheritance can result in complex and rigid hierarchies, composition facilitates the creation of flexible and reusable components.

OPC UA Version 1.04, released in 2018, introduced the concept of Interfaces. This feature allows for the definition of property sets or methods that can be implemented by multiple ObjectTypes, enhancing modularity and reusability.

Building on this, OPC UA Version 1.05, released in 2022, introduced the concept of AddIns. Although object composition has been a core aspect of OPC UA modeling since Version 1.02 in 2012 with the introduction of ObjectTypes, Version 1.05 has added new capabilities that make object composition and reuse even more straightforward.

Composition in OPC UA: AddIns to the Rescue​

AddIns embody the principle of composition, enabling the development of modular and extensible OPC UA systems.

Benefits of AddIns​

  • Flexibility: AddIns allow you to add custom components to your objects without altering their core structure. This makes your system more adaptable to changes.

  • Reusability: Define AddIn ObjectTypes once and reuse them across multiple objects, promoting consistency and reducing redundancy.

  • Modularity: Break down complex objects into smaller, manageable components. This makes your system easier to understand, maintain, and extend.

Good Practices with AddIns​

  • Modular Design: Use AddIns to create modular components that can be easily added to or removed from objects. This promotes a plug-and-play architecture.

  • Consistent Naming: Utilize the DefaultInstanceBrowseName property to ensure consistent naming conventions across your AddIn instances.

  • Optional vs. Mandatory: Specify whether an AddIn is optional or mandatory using the modellingRule property. This helps in defining clear and enforceable rules for your object types.

Example​

Imagine you are modeling a manufacturing system with various devices. Instead of creating a complex hierarchy of derived types, you can use AddIns to compose your devices:

This is how you can achieve this easily using the YAML syntax of sterfive opcua-modeler.

objectTypes:
    - browseName: SensorType
      defaultInstanceBrowseName: Sensor
    - browseName: ActuatorType
      defaultInstanceBrowseName: Actuator
    - browseName: DeviceType
      addIns:
           - typeDefinition: SensorType
              modellingRule: Mandatory
           - typeDefinition: ActuatorType
              modellingRule: Optional

In this example, DeviceType is composed of SensorType and ActuatorType AddIns. The SensorType is mandatory, ensuring that every device has a sensor, while the ActuatorType is optional, allowing for devices without actuators.

Using the opcua-modeler tool, this YAML code can then be converted to the NodeSet2.XML format that most OPCUA servers can directly consume.

Conclusion​

The introduction of AddIns and Interfaces has significantly enhanced the flexibility and modularity of OPC UA systems. These features empower system designers and object modelers to create more maintainable and extensible information models, making it easier to adapt to changing requirements and integrate new functionalities.

As the OPC UA Companion specifications continue to evolve, we are witnessing a trend towards richer and more sophisticated models where AddIns and Interfaces are becoming increasingly prevalent. This evolution not only makes the models more robust but also easier to understand and navigate.

In the context of Industry 4.0, the structure of the model is paramount. These advancements in OPC UA modeling ensure that industrial systems are well-prepared to meet the demands of the future, fostering greater efficiency, interoperability, and innovation.

Get Started​

Check out our detailed documentation to learn more about AddIns and how to implement them in your OPC UA models: OPC UA Modeler Documentation

Stay tuned for more exciting updates and features from Sterfive! 🔧✨

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