OPC UA back-ends for quasar

Author: Piotr P. Nikiel

Date: ~2016, rewritten 03-Oct-2022

quasar for C++ can create OPC UA software components (servers and clients) on top of OPC UA libraries (SDK - software development kits) which follow the Unified Automation C++ SDK API.

As of the writing, two such libraries exist:

The UA C++ SDK, in different versions, including the evaluation version, detailed below.
The open62541-compat, internally using open62541, both of them being free and open-source, detailed below.

Please note that quasar can also be used for Python-based OPCUA software (in a bit limited fashion), through the MilkyWay project.

Using quasar with the Unified Automation C++ SDK

quasar-made software can be easily build against the UA SDK. The supported UA SDK version generally depends on versions of quasar used; 2022-made versions (e.g. 1.5.12) are fine with UA SDK 1.5.x, 1.6.x and 1.7.x, earlier versions also covered UA SDK 1.3.x and 1.4.x.

To achieve the goal, it is only necessary to point quasar to where the UA SDK is, that is done by either:

using the FrameworkInternals/default_configuration.cmake and passing the paths via environment
writing a new build configuration file

Using quasar with open62541-compat and open62541 (the free and open-source approach)

The quasar team has created a compatibility layer which permits using open62541 in quasar projects. The compatibility layer is called open62541-compat, it is free & open-source and quickly available to quasar developers as one of supported “optional modules”.

Activating open62541 is a simple procedure which is best described in the quasar tutorials on YouTube, see e.g. https://www.youtube.com/watch?v=6MsWUQ0OXbY .

OPC UA Back-end dependent features: a comparison table

Feature		UA SDK 1.5+	open62541- compat 1.4.2
Cache variables		✓	✓
	Data Types: Boolean, SByte (~signed char), Byte (~unsigned char), Int16, UInt16, Int32, UInt32, Int64, UInt64, Float, Double, String, ByteString	✓	✓
	Variant Data Type	✓	✓
	Null Data Type	✓	✓
	Arrays	✓	✓
	Access rights (r/w/rw control)	✓	✓
	No-sampling mode for cache-variables	✓	x
Source variables			x
	Support for same data types as with cache-variables	✓	x
	Synchronous read/write handling	✓	x
	Asynchronous read/write handling	✓	x
Methods
	Support for same data types as with cache-variables	✓	✓
	Synchronous invocation	✓	✓
	Asynchronous invocation	✓	x
Calculated variables		✓	✓
Standardized back-end configuration ServerConfig.xml		✓	✓