3 Terms, definitions, and abbreviations ToC Previous Next

3.2 Terms and definitions ToC Previous Next

For the purposes of this document, the following terms apply.

3.3 AddressSpace

collection of information that a Server makes visible to its Clients

Note 1 to entry: See OPC 10000-3 for a description of the contents and structure of the Server AddressSpace.

3.4 Aggregate

a function that calculates derived values from Raw data

Note 1 to entry: Raw data may be from a historian or buffered real time data. Common Aggregates include averages over a given time range, minimum over a time range and maximum over a time range.

3.5 Alarm

type of Event associated with a state condition that typically requires acknowledgement

Note 1 to entry: See OPC 10000-9 for a description of Alarms.

3.6 Attribute

primitive characteristic of a Node

Note 1 to entry: All Attributes are defined by OPC UA, and may not be defined by Clients or Servers. Attributes are the only elements in the AddressSpace permitted to have data values.

3.7 Broker

intermediary program module that routes NetworkMessages from Publishers to Subscribers

Note 1 to entry:  Brokers are building blocks of Message Oriented Middleware.

3.8 Certificate

digitally signed data structure that contains a public key and the identity of a Client or Server

3.9 Client

software application that sends Messages to OPC UA Servers conforming to the Services specified in this set of specifications

3.10 Condition

generic term that is an extension to an Event

Note 1 to entry: A Condition represents the conditions of a system or one of its components and always exists in some state.

3.11 Communication Stack

layered set of software modules between the application and the hardware that provides various functions to encode, encrypt and format a Message for sending, and to decode, decrypt and unpack a Message that was received

3.12 Complex Data

data that is composed of elements of more than one primitive data type, such as a structure

3.13 DataSet

list of named data values

Note 1 to entry:  A DataSet typically consists of Event fields or Variable values..

3.14 DataSetMessage

payload of a NetworkMessage created from a DataSet

Note 1 to entry:  The DataSetMessage is an immutable payload of the NetworkMessage handed off to the Message Oriented Middleware (transport layer) for delivery by the Publisher. The Subscriber receives the DataSetMessage as the payload of a NetworkMessage from the Publisher with additional headers that may be supplied by the Message Oriented Middleware along the way.

3.15 Discovery

process by which Client obtains information about Servers, including endpoint and security information

3.16 Event

generic term used to describe an occurrence of some significance within a system or system component

3.17 EventNotifier

special Attribute of a Node that signifies that a Client may subscribe to that particular Node to receive Notifications of Event occurrences

3.18 Information Model

organizational framework that defines, characterizes and relates information resources of a given system or set of systems.

Note 1 to entry: The core address space model supports the representation of Information Models in the AddressSpace. See OPC 10000-5 for a description of the base OPC UA Information Model.

3.19 Message

data unit conveyed between Client and Server that represents a specific Service request or response

3.20 Message Oriented Middleware

infrastructure supporting sending and receiving NetworkMessages between distributed systems

Note 1 to entry: An OPC UA Application may support different types of Message Oriented Middleware infrastructures and protocols like AMQP or UADP with IP multicast. Other types like DDS, MQTT or XMPP can also be integrated into the OPC UA PubSub model.

3.21 Method

callable software function that is a component of an Object

3.22 MonitoredItem

Client-defined entity in the Server used to monitor Attributes or EventNotifiers for new values or Event occurrences and that generates Notifications for them

3.23 NetworkMessage

DataSetMessages and header to facilitate delivery, routing, security and filtering

Note 1 to entry: The Publisher hands off the NetworkMessage to the Message Oriented Middleware (transport layer) to deliver DataSetMessages to the Subscribers.

Note 2 to entry: The term message is used with various connotations in the messaging world. The Publisher might like to think of the message as an immutable payload handed off to the Message Oriented Middleware for delivery. The Subscriber often thinks of the message as not only that immutable payload from the sender, but also various annotations supplied by the Message Oriented Middleware along the way. To avoid confusion the term DataSetMessage is used to mean the message as supplied by the Publisher for a DataSet and the term NetworkMessage is used to mean the DataSetMessage plus sections for annotation at the head and tail of the DataSetMessage.

3.24 Node

fundamental component of an AddressSpace

3.25 NodeClass

class of a Node in an AddressSpace

Note 1 to entry: NodeClasses define the metadata for the components of the OPC UA Object Model. They also define constructs, such as Views, that are used to organize the AddressSpace.

3.26 Notification

generic term for data that announces the detection of an Event or of a changed Attribute value; Notifications are sent in NotificationMessages.

3.27 NotificationMessage

Message published from a Subscription that contains one or more Notifications

3.28 Object

Node that represents a physical or abstract element of a system

Note 1 to entry: Objects are modelled using the OPC UA Object Model. Systems, subsystems and devices are examples of Objects. An Object may be defined as an instance of an ObjectType.

3.29 Object Instance

synonym for Object

Note 1 to entry: Not all Objects are defined by ObjectTypes.

3.30 ObjectType

Node that represents the type definition for an Object

3.31 OPC UA Application

Client, which calls OPC UA Services, or a Server, which performs those Services, or an OPC UA Publisher or an OPC UA Subscriber.

3.32 Publisher

entity sending NetworkMessages to a Message Oriented Middleware

Note 1 to entry: A Publisher can be a native OPC UA Application or an application that only has knowledge about the Message Oriented Middleware and the rules for encoding the NetworkMessages and DataSetMessages.

3.33 PubSub

OPC UA variant of the publish subscribe messaging pattern

3.34 Profile

specific set of capabilities to which a Server may claim conformance.

Note 1 to entry: Each Server may claim conformance to more than one Profile

Note 2 to entry: The set of capabilities are defined in OPC 10000-7

3.35 Program

executable Object that, when invoked, immediately returns a response to indicate that execution has started, and then returns intermediate and final results through Subscriptions identified by the Client during invocation

3.36 Reference

explicit relationship (a named pointer) from one Node to another

Note 1 to entry: The Node that contains the Reference is the source Node, and the referenced Node is the target Node. All References are defined by ReferenceTypes.

3.37 ReferenceType

Node that represents the type definition of a Reference

Note 1 to entry: The ReferenceType specifies the semantics of a Reference. The name of a ReferenceType identifies how source Nodes are related to target Nodes and generally reflects an operation between the two, such as “A Contains B”.

3.38 Server

software application that implements and exposes the Services specified in this set of specifications

3.39 Service

Client-callable operation in a Server

Note 1 to entry: Services are defined in OPC 10000-4. A Service is similar to a method call in a programming language or an operation in a Web services WSDL contract.

3.40 Service Set

group of related Services

3.41 Session

logical long-running connection between a Client and a Server.

Note 1 to entry: A Session maintains state information between Service calls from the Client to the Server.

3.42 Subscriber

entity receiving DataSetMessages from a Message Oriented Middleware

Note 1 to entry:  A Subscriber can be a native OPC UA Application or an application that has just knowledge about the Message Oriented Middleware and the rules for decoding the NetworkMessages and DataSetMessages. A Subscription in the OPC UA Client Server model has a different meaning than the Subscriber in the PubSub model.

3.43 Subscription

Client-defined endpoint in the Server, used to return Notifications to the Client

Note 1 to entry: ”Subscription” is a generic term that describes a set of Nodes selected by the Client (1) that the Server periodically monitors for the existence of some condition, and (2) for which the Server sends Notifications to the Client when the condition is detected.

3.44 Underlying System

Hardware or software platforms that exist as an independent entity. UA Applications are dependent on an entity’s existence in order to perform UA services. However, the entity is not dependent on UA Applications.

Note 1 to entry: Hardware and software platforms include physical hardware, firmware, operating system, networking, non-UA applications, as well as other UA Applications. A Distributed Control System, PLC/Device, and UA Server are examples of an Underlying System.

3.45 Variable

Node that contains a value

3.46 View

specific subset of the AddressSpace that is of interest to the Client.

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