Variable
In OPC UA, a Variable is the NodeClass that holds a value. Its Value attribute is a DataValue - value, StatusCode and timestamps - described by the DataType, ValueRank and ArrayDimensions attributes and governed by AccessLevel. Variables are what clients read, write, subscribe to and historise; everything else is structure.
The attributes that define a Variable
DataType names the DataType node the value conforms to. ValueRank and ArrayDimensions say whether it is a scalar, a fixed-size array or a matrix. AccessLevel and UserAccessLevel are bitmasks for readable, writable and history access, the second one after the current user's Role permissions are applied. MinimumSamplingInterval tells you the fastest rate the server can actually sample the source, and Historizing whether history is being collected.
DataVariables and Properties
Two roles exist, distinguished only by the reference that reaches the Variable. A DataVariable, reached by HasComponent, is process data: a temperature, a counter, a state. A Property, reached by HasProperty and typed PropertyType, is metadata about its parent: EngineeringUnits, EURange, a serial number. Properties are leaves - they must not have DataVariable children - and are typically static, so subscribing to them at a 100 ms interval wastes server resources.
Typed Variables in Part 8
Data Access adds the VariableTypes that make analogue data self-describing: AnalogItemType with EURange and EngineeringUnits properties, AnalogUnitType, DiscreteItemType and its two-state and multi-state variants. A client that reads these properties can render 42.5 degrees C with a valid range of 0 to 100 instead of a bare double, which is exactly what opcua-commander does.
Example
The attributes of one AnalogItemType Variable as UaExpert shows them
NodeId ns=4;s=Mixer_01.Temperature
NodeClass Variable
BrowseName 4:Temperature
DataType Double (ns=0;i=11)
ValueRank -1 (Scalar)
ArrayDimensions null
AccessLevel CurrentRead | HistoryRead
UserAccessLevel CurrentRead
MinimumSamplingInterval 100 ms
Historizing true
Value 42.5
StatusCode Good (0x00000000)
SourceTimestamp 2026-09-07T08:14:22.310Z
ServerTimestamp 2026-09-07T08:14:22.318Z
HasTypeDefinition -> AnalogItemType
HasProperty -> EURange { Low = 0, High = 100 }
HasProperty -> EngineeringUnits EUInformation, UNECE code for degree CelsiusDefined in: Part 3 - Address Space Model, Variable NodeClass and its attributes; Part 8 - Data Access for AnalogItemType and the discrete item types.
Last verified 7 September 2026 against OPC UA 1.05.07. OPC UA Connect is an educational resource and is not affiliated with the OPC Foundation.