Many organizations will deploy DirectoryServices such as LDAP or UDDI to manage resources available on their network. A Client can use these services as a way to find Servers by using APIs specific to DirectoryService to query for UA Servers or UA DiscoveryServers available on the network. The Client would then use the URLs for DiscoveryEndpoints stored in the DirectoryService to request the EndpointDescriptions necessary to connect to an individual servers

Some implementations of a GlobalDiscoveryServer will be a front-end for a standard Directory Service. In these cases, the QueryApplications method will return the same information as the DirectoryService API. The discovery process for this scenario is illustrated in Figure 37.

Figure 37 – The UDDI or LDAP Discovery Process

UDDI registries contain businessEntities which provide one or more businessServices. The businessServices have one or more bindingTemplates. bindingTemplates specify a physical address and a Server Interface (called a tModel). Figure 38 illustrates the relationships between the UDDI registry elements.

Figure 38 – UDDI Registry Structure

This specification defines standard tModels which shall be referenced by businessServices that support UA. The standard UA tModels shown in Table 116.

Table 116 – UDDI tModels

Name	domainKey	uuidKey
Server	uddi:server.ua.opcfoundation.org	uddi:AA206B41-EC9E-49a4-B789-4478C74120B5
DiscoveryServer	uddi:discoveryserver.ua.opcfoundation.org	uddi:AA206B42-EC9E-49a4-B789-4478C74120B5

The name of the businessService elements should be the same as the ApplicationName for the UA application. The serviceKey shall be the ApplicationUri. At least one bindingTemplate shall be present and the accessPoint shall be the URL of the DiscoveryEndpoint for the UA server identified by the serviceKey. Servers with multiple DiscoveryEndpoints would have multiple bindingTemplates

A UDDI registry will generally only contain UA servers, however, there are situations where the administrators cannot know what Servers are available at any given time and will find it more convenient to place a DiscoveryServer in the registry instead.

LDAP servers contain objects organized into hierarchies. Each object has an objectClass which specifies a number of attributes. Attributes have values which describe an object. Figure 39 illustrates a sample LDAP hierarchy which contains entries describing UA servers.

Figure 39 – Sample LDAP Hierarchy

UA applications are stored in LDAP servers as entries with the UA defined objectClasses associated with them. The schema for the objectClasses defined for UA are shown in Table 117.

Table 117 – LDAP Object Class Schema

Name	LDAP Name	Type	OID
Application	opcuaApplication	Structural	1.2.840.113556.1.8000.2264.1.12.1
ApplicationName	cn	String (Required)	Built-in
HostName	dNSName	String	Built-in
ApplicationUri	opcuaApplicationUri	Name	1.2.840.113556.1.8000.2264.1.12.1.1
ApplicationType	opcuaApplicationType	Boolean	1.2.840.113556.1.8000.2264.1.12.1.3
DiscoveryUrl	opcuaDiscoveryUrl	String, Multi-valued	1.2.840.113556.1.8000.2264.1.12.1.4

This OID is globally unique and can use used with any LDAP implementation.

Administrators may extend the LDAP schema by adding new attributes.