Technical note-OPC UA Discovery in QuickOPC

This article summarizes the OPC UA discovery features available, and how they work internally. Currently, it describes the status as of QuickOPC 2016.2.

Main methods and properties

Methods on the IEasyUAClient interface

• UAApplicationElementCollection DiscoverApplications(UADiscoveryQuery discoveryQuery). Discovers available OPC-UA applications and returns information about them, using the specified discovery query.

IEasyUAClient Extension methods

• UAApplicationElementCollection DiscoverApplications(this IEasyUAClient client, string discoveryHost, UAApplicationTypes applicationTypes). Discovers available OPC-UA applications and returns information about them, using the specified discovery host, and filtering by specified application types.
• UAApplicationElementCollection FindApplications(this IEasyUAClient client, string[] discoveryUriStringArray, UAApplicationTypes applicationTypes). Discovers available OPC-UA applications and returns information about them, using the specified discovery URI strings and filtering by specified application types.
• UAApplicationElementCollection DiscoverServers(this IEasyUAClient client, string discoveryHost). Discovers available OPC-UA servers and returns information about them, using the specified discovery host.

Properties and methods on the EasyUAClient object

• Property InstanceParameters.DiscoveryHost. The name of the machine that runs the discovery server that will be used for discovery tasks.
• Method UAApplicationElementCollection DiscoverServers(). Discovers available OPC-UA servers and returns information about them, using the default discovery host ("localhost", unless specifically modified by the developer).

COM interface

The _EasyUACllient interface contains a projection of the DiscoverApplications method of the IEasyUAClient, and projections of the DiscoverServers and FindApplications extension methods.

Design considerations

The design allows to provide for multiple types of discovery in an extensible way. The thinking behind having the current two types was roughly as follows:

• We wanted a method where the developer can pass in just the discovery host name, as it is the most common use case.
• On the other hand, we needed a way for the developer to be able to specify a discovery URI explicitly, for use cases where the LDS runs on an unusual port or uses an unusual protocol. While at this, we should not lose the built-in parallel ability, hence not just one, but multiple discovery URIs needed to be passed in.
• The above goals were not achievable with one method being just an extension to the other - at least not without introducing some mandatory parameters (such as discovery URI template strings) to the IEasyUAClient interface, which is what we want to avoid.

In the future, we expect more types of discovery queries to become available - e.g. for mDNS (FindServersOnNetwork), or OPC UA Globsl Discovery Server (GDS).

Functionality

When the UALocalDiscoveryQuery is used

1. Determines and uses the effective host parameters (UAHostParameters) and discovery parameters (UADiscoveryParameters).
2. Creates a *virtual* host session to the discovery host (note that this involves no connection to the target at this time).
3. Determines the discovery URI strings for a given host. This is based on the HostParameters.DiscoveryUriTemplateStrings setting.
4. Continues with #Steps common for both discovery methods.

When the UAEndpointDiscoveryQuery is used

1. Determines and uses the effective discovery parameters (UADiscoveryParameters).
2. Continues with #Steps common for both discovery methods.

Steps common for both types of discovery queries

1. Depending on the UADiscoveryParameters.ParallelDiscovery setting, performs the following operations serially or in parallel, on each discovery URI string, until a first operation success.
2. Creates a *virtual* discovery session to the discovery URI string (note that this involves no connection to the target at this time).
3. Determines the SDK endpoint configuration, setting its operation timeout to be UADiscoveryParameters.DiscoveryTimeout.
4. Creates SDK discovery client. This uses the given discovery URI string.
5. Calls the OPC UA FindServers service, using the SDK discovery client.
6. Performs various checks on the returned application descriptions.
7. Converts the returned application descriptions to UAApplicationElementCollection.

Parameters on the EasyUAClient

The UAHostParameters object

The UAHostParameters object contains:

• Property StringSet DiscoveryUriTemplateStrings. The URL templates of discovery endpoints that will be used for discovery tasks.

The UADiscoveryParameters object

The UADiscoveryParameters object contains:

• Property bool CheckApplicationDescription. Check the application description.
• Property int DiscoveryTimeout. A maximum time a server discovery can take (in milliseconds).
• Property bool ParallelDiscovery. When set to true, the server discovery will be attempted on configured discovery endpoints simultaneously.