Terms and definitions – OPC Unified Architecture

2.1 Terms and definitions

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

ISO and IEC maintain terminology databases for use in standardization at the following addresses:

IEC Electropedia: available at https://www.electropedia.org/

ISO Online browsing platform: available at https://www.iso.org/obp

2.1.1 AddressSpace

collection of information that a Server makes visible to its Clients

2.1.2 Aggregate

function that calculates derived values from Raw data

2.1.3 Alarm

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

2.1.4 Attribute

primitive characteristic of a Node

2.1.5 Broker

intermediary program module that routes NetworkMessages from Publishers to Subscribers

2.1.6 Certificate

digitally signed data structure that contains a public key and an identity

2.1.7 Client

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

2.1.8 ClientServer

OPC UA variant of the client-server messaging pattern

2.1.9 Condition

generic term that is an extension to an Event

2.1.10 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

2.1.11 Complex Data

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

2.1.12 DataSet

list of named data values

2.1.13 DataSetMessage

payload of a NetworkMessage created from a DataSet

2.1.14 Discovery

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

2.1.15 Event

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

2.1.16 EventNotifier

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

2.1.17 Information Model

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

2.1.18 Message

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

2.1.19 Message Oriented Middleware

infrastructure supporting sending and receiving NetworkMessages between distributed systems

2.1.20 Method

callable software function that is a component of an Object

2.1.21 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

2.1.22 NetworkMessage

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

Note 1

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.

2.1.23 Node

fundamental component of an AddressSpace

2.1.24 NodeClass

class of a Node in an AddressSpace

2.1.25 Notification

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

2.1.26 NotificationMessage

Message published from a Subscription that contains one or more Notifications

2.1.27 Object

Node that represents a physical or abstract element of a system

2.1.28 Object Instance

synonym for Object

2.1.29 ObjectType

Node that represents the type definition for an Object

2.1.30 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.

2.1.31 Profile

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

2.1.32 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

2.1.33 Publisher

entity sending NetworkMessages to a Message Oriented Middleware

2.1.34 PubSub

OPC UA variant of the publish subscribe messaging pattern

2.1.35 Reference

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

2.1.36 ReferenceType

Node that represents the type definition of a Reference

2.1.37 Secure Channel

in OPC UA, a communication path established between an OPC UA Client and Server that have authenticated each other using certain OPC UA services and for which security parameters have been negotiated and applied

2.1.38 Server

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

2.1.39 Service

Client-callable operation in a Server

2.1.40 Service Set

group of related Services

2.1.41 Session

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

2.1.42 Subscriber

entity receiving DataSetMessages from a Message Oriented Middleware

2.1.43 Subscription

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

2.1.44 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.

2.1.45 Variable

Node that contains a value

2.1.46 View

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