Variables are used to represent values. Two types of Variables are defined, Properties and DataVariables. They differ in the kind of data that they represent and whether they can contain other Variables.
Properties are Server-defined characteristics of Objects, DataVariables and other Nodes. Properties differ from Attributes in that they characterise what the Node represents, such as a device or a purchase order. Attributes define additional metadata that is instantiated for all Nodes from a NodeClass. Attributes are common to all Nodes of a NodeClass and only defined by this specification whereas Properties can be Server-defined.
For example, an Attribute defines the DataType of Variables whereas a Property can be used to specify the engineering unit of some Variables.
To prevent recursion, Properties are not allowed to have Properties defined for them. To easily identify Properties, the BrowseName of a Property shall be unique in the context of the Node containing the Properties (see 5.6.3 for details).
A Node and its Properties shall always reside in the same Server.
DataVariables represent the content of an Object. For example, a file Object may be defined that contains a stream of bytes. The stream of bytes may be defined as a DataVariable that is an array of bytes. Properties may be used to expose the creation time and owner of the file Object.
For example, if a DataVariable is defined by a data structure that contains two fields, “startTime” and “endTime” then it might have a Property specific to that data structure, such as “earliestStartTime”.
As another example, function blocks in control systems might be represented as Objects. The parameters of the function block, such as its setpoints, may be represented as DataVariables. The function block Object might also have Properties that describe its execution time and its type.
DataVariables may have additional DataVariables, but only if they are complex. In this case, their DataVariables shall always be elements of their complex definitions. Following the example introduced by the description of Properties in 4.4.2, the Server could expose “startTime” and “endTime” as separate components of the data structure.
As another example, a complex DataVariable may define an aggregate of temperature values generated by three separate temperature transmitters that are also visible in the AddressSpace. In this case, this complex DataVariable could define HasComponent References from it to the individual temperature values that it is composed of.