Qt OPC UA Overview

OPC UA is a protocol for data modelling and exchange of data in industrial applications.

An OPC UA server offers access to data that is organized in a mesh of nodes connected by references. The use of different reference types and nodes containing metadata enable a client to navigate and interpret the data without knowing their structure in advance.

Each node has a unique identifier and attributes that can be read and written. Among others, there are Variable nodes that store values and callable Method nodes with attached nodes describing parameters and return values. Notifications in case of events and monitoring of Variable nodes for value changes are offered too.

Complex objects can be created by combining nodes using references. Inheritance is also possible. OPC UA offers support for pre-made models that can be extended to fit special needs.

OPC UA is the platform-independent successor of OPC Classic intended for usage on all levels, from embedded sensors up to manufacturing execution and enterprise resource planning systems. It has a service-oriented architecture based on standardized messages for service requests and responses. There are different ways for these messages to be encoded and transported over the network. The most common way is binary encoding over TCP.

Implemented Features from the OPC UA Protocol

The following table summarizes the features of OPC UA that are supported by Qt OPC UA.

FeatureOpen62541 pluginUACpp plugin
Batch readXX
Batch writeXX
Multidimensional arraysXX
Data change subscriptionsXX
Event subscriptionsXX
Modify subscriptions / monitored ItemsXX
Method callsXX
Browse path resolutionXX

Data Types

A subset of the OPC UA data types is currently supported in Qt OPC UA. Most of them are available with all backends.

Data typeOpen62541 pluginUACpp pluginQt OPC UA data type
Int16, Int32, Int64XXDirectly used
UInt16, UInt32, UInt64XXDirectly used
BooleanXXDirectly used
DoubleXXDirectly used
FloatXXDirectly used

Classes and Ownership

Two important classes are exposed to the user: QOpcUaClient and QOpcUaNode.

Objects of both classes are owned by the user and must be deleted when they are no longer needed.

Logging Categories

The following table summarizes the logging categories that are used by Qt OPC UA.

Logging CategoryDescription
qt.opcuaPlugin independent messages generated by QOpcUaProvider and QOpcUaClient
qt.opcua.plugins.open62541Messages generated by the open62541 plugin
qt.opcua.plugins.uacppMessages generated by the UACpp plugin

Open62541 specific Logging Categories

The open62541 sdk logging categories depend on the log level for open62541, which is configured at compile time. The default is 300, which enables: info, warning, error and critical. To obtain less or more verbose output, the open62541 sdk needs to be rebuilt with a different configuration.

The following table shows the logging categories used by open62541:

Open62541 Logging CategoryDescription
qt.opcua.plugins.open62541.sdk.networkMessages generated by the open62541 network
qt.opcua.plugins.open62541.sdk.securechannelMessages generated by the open62541 channel
qt.opcua.plugins.open62541.sdk.sessionMessages generated by the open62541 session
qt.opcua.plugins.open62541.sdk.serverMessages generated by open62541 server components
qt.opcua.plugins.open62541.sdk.clientMessages generated by the open62541 client
qt.opcua.plugins.open62541.sdk.userlandMessages generated by the open62541 userland
qt.opcua.plugins.open62541.sdk.securitypolicyMessages generated by the open62541 security policies


The Qt OPC UA module is available under commercial licenses from The Qt Company. In addition, it is available under free software licenses. These free software licenses are GNU Lesser General Public License, version 3, or the GNU General Public License, version 2. See Qt Licensing for further details.

Qt OPC UA in Qt 6.6.2 may contain third party modules under following permissive licenses:

Open62541, version 1.3.6

Mozilla Public License 2.0 and Creative Commons Zero v1.0 Universal and Creative Commons Attribution Non Commercial Share Alike 4.0 International and BSD 3-Clause "New" or "Revised" License and Apache License 2.0 and MIT License

Linking to external libraries involves licenses from the backend providers.


The Open62541 plugin is available under the same licenses as Qt OPC UA. The Open62541 library itself is licensed under Mozilla Public License v2.0.

The security support is provided by the OpenSSL plugins of the open62541 library. If the open62541 plugin is built with security support and OpenSSL is not available on the target system, the open62541 plugin will fail to load.


The UA CPP plugin is available under commercial licenses from The Qt Company. In addition, it is available under the GNU General Public License, version 3. The UA CPP library itself is available under commercial licenses from Unified Automation.

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