OPC UA Application URI Derivation

Introduction

Every OPC UA application (client or server) needs an application URI, which uniquely identifies the application instance in the system.

Applications based on QuickOPC determine their application URI in the following way:

1. If the application manifest contains a non-empty ApplicationUriString property, it will be used as the application URI.
2. Otherwise, the application URI is automatically generated, using a template string from the ApplicationUriTemplateString property in the application parameters.

Template string syntax

The application URI template string consists of characters that are copied literally to the output string, and macros, which are replaced by values provided by QuickOPC. When the macro is expanded, it is URL-escaped before it is inserted into the output string, so that a correct URI can be formed. Macro replacements have the syntax $(name), where name is one of the names defined in the following table.

Category	Name	Description
Common	CanonicalizedHostName	Canonicalized (lowercased) host name of the local computer.
	HostName	Host name of the local computer.
	ModuleName	Name of the main module for the current process.
	ProcessId	Numeric Id of the current process.
	ProcessName	The name of the current process.
Application Assembly	SimpleName	Simple name of the application assembly.
	Version	Version number of the application assembly.
	VersionMajor	Major version part of the Version.
	Culture	Supported culture of the application assembly.
	PublicKeyToken	Public key token of the application assembly.
	AssemblyName	Display name of the application assembly (such as "ExampleAssembly, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null").
	AssemblyTokens	A combination of SimpleName, VersionMajor, and Culture, separated by colons. The trailing tokens are left out together with the colons that precede them, if they are at default values ("0" for the VersionMajor, and "neutral" for the Culture).
Application Version Info	CompanyName	Name of the company from the application version info.
	FileDescription	Description of the file, from the application version info.
	FileVersion	File version number from the application version info.
	FileVersionMajor	Major version part of the FileVersion.
	Language	Default language string from the application version info.
	NameToken	Internal name from the application version info, without the ".exe" extension if present.
	ProductVersion	Product version number from the application version info.
	ShortFileVersion	The part of FileVersion before the first space character (' ').
	VersionInfoTokens	A combination of NameToken, FileVersionMajor and Language, separated by colons. The trailing tokens are left out together with the colons that precede them, if they are at default values ("0" for the FileVersionMajor, and "Language Neutral" for the Language).

The information in the "Application Assembly" and "Application Version Info" categories may not always be available - it depends on factors such as the type of application and the hosting process. For this reason, the template string can have one or more sections, separated by a semicolon (';'). If three or more sections are present, they are interpreted like this:

• If the application assembly information is available, the last section from the end is used.
• Otherwise, if the application version info is available, the second section from the end is used.
• Otherwise, the third section from the end is used.

If there are fewer sections, the first available section that would be to the right of the position indicated by the above rules is used. This algorithm also means that you can use just one section, and this section will then always be used.

Template string defaults

The default application URI template string (in OPC Studio 2024.2 and newer) is as follows:

 http://$(CanonicalizedHostName)/$(ModuleName);http://$(CanonicalizedHostName)/$(VersionInfoTokens);http://$(CanonicalizedHostName)/$(AssemblyTokens)

The "http" scheme was chosen because OPC Foundation LDS (Local Discovery Server) only accepts registrations from servers whose application URI starts with "urn:" or "http://", and there is currently no way to generate technically correct Uniform Resource Names for OPC UA applications, because no suitable freely usable NID (namespace identifier) has been registered with IANA. In spite of this fact, the "urn" scheme appears to be quite common among the OPC UA implementations, and host name is used in place of the namespace identifier.

The default application URI template string (in QuickOPC 2023.1 - 2024.1) was as follows:

 uri:$(HostName):$(ModuleName);uri:$(HostName):$(VersionInfoTokens);uri:$(HostName):$(AssemblyTokens)