Custom Data Subscriptions

The custom data subscriptions are typically used when the underlying data source has some kind of subscription mechanism, and you would like to use it for performance and data accuracy reasons, instead of using the default "data polling" mechanism (as described in Data Provision And Consumption Models). OPC Wizard allows you to override the data subscription behavior, and replace it by your own.

In order to take over the data subscriptions mechanism for a certain data variable, you should first set its UseDataPolling Property to false. This turns off the data polling algorithm provided by the OPC Wizard for this data variable. You then become responsible for handling the data subscriptions to this data variable.

Each data subscription (there can be multiple data subscriptions to the same data variable) is represented by an internal OPC Wizard object with IUADataSubscription Interface. The OPC Wizard creates and removes these data subscription objects for you. The OPC Wizard maintains a current set of data subscriptions on a data variable in its DataSubscriptionSet Property. Whenever a data subscription is added, modified, or removed, an OnDataSubscriptionChanged Method is called that you can override, and consequently the DataSubscriptionChanged Event is also raised. This works analogically to other requests described in Data Provision And Consumption Models, i.e. the request propagates (bubbles) to parent levels and can also be handled there, you can choose between handling the event or overriding the method, etc.

The argument passed to the OnDataSubscriptionChanged Method or the DataSubscriptionChanged Event is an instance of UADataSubscriptionChangedEventArgs Class. The member that distinguishes how the data subscription set has changed is the Action Property. Depending on its value, your code will do different things:

If the action is Add, a new data subscription has been added to the data variable. Your code should activate periodic updates of the data subscription. The new data subscription is passed to you in the DataSubscription Property of the event arguments. You can obtain its sampling interval from the SamplingInterval Property.
If the action is Modify, an existing data subscription has been modified. Your code should adjust to the new SamplingInterval Property of the data subscription.
If the action is Remove, an existing data subscription has been removed from the data variable. Your code should deactivate periodic updates of the data subscription.

The data subscription object implements the ISink<UAAttributeData> Interface, which you use to provide updated data to the data subscription, by calling its OnNext Method, with an instance of UAAttributeData Class that contains the new data. There are also extension method overloads for the OnNext method; the extension methods allow you to simplify the code in cases when you just want to construct a "Good" attribute data with given value and current time as a timestamp, or to construct the attribute with specific status code, etc.

The data subscription has a State Property, which can hold a reference to any object. You can use it to associate your own data with the data subscription.

Your code is responsible for supplying the new data to the data variable when the data changes, in accordance with the SamplingInterval Property of the data subscription. This needs to be done for each of the data subscriptions that currently exist on the data variable. Note that since the data subscriptions may have different sampling intervals, the update mechanism needs to be able to supply data to each of them based on their own schedule. This can be achieved by treating each data subscription completely separately, or (in an advanced scenario) by devising an optimization algorithm to merge the subscriptions, but also properly handle their updates.

Due to OPC protocol and the underlying library requirements, you cannot provide data to the data subscription sooner than the sampling interval elapses. If you attempt to do so, this premature data update will be silently dropped.

The example below illustrates how a custom data subscription can be implemented.

// This example shows how to implement own handling of data subscriptions.
// You can use any OPC UA client, including our Connectivity Explorer and OpcCmd utility, to connect to the server. 
//
// Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .
// OPC client, server and subscriber examples in C# on GitHub: https://github.com/OPCLabs/Examples-OPCStudio-CSharp .
// Missing some example? Ask us for it on our Online Forums, https://www.opclabs.com/forum/index ! You do not have to own
// a commercial license in order to use Online Forums, and we reply to every post.

using System;
using OpcLabs.EasyOpc.UA;
using OpcLabs.EasyOpc.UA.NodeSpace;
using Timer = System.Timers.Timer;

namespace UAServerDocExamples._UAServerNode
{
    class DataSubscriptionChanged
    {
        public static void Main1()
        {
            // Instantiate the server object.
            // By default, the server will run on endpoint URL "opc.tcp://localhost:48040/".
            var server = new EasyUAServer();

            // Create a read-only data variable.
            var random = new Random();
            var dataVariable = UADataVariable.CreateIn(server.Objects, "SubscribeToThisVariable")
                .ValueType<int>()
                .Writable(false);

            dataVariable.UseDataPolling = false;    // Recommended, but not strictly necessary.
            dataVariable.DataSubscriptionChanged += (sender, args) =>
            {
                switch (args.Action)
                {
                    case UADataSubscriptionChangedAction.Add:
                    {
                        // Obtain the sampling interval from the data subscription.
                        int samplingInterval = args.DataSubscription.SamplingInterval;
                        Console.WriteLine($"Data subscription added, sampling interval: {samplingInterval}");

                        // Create a timer that will provide the data variable with a new data. In a real server the activity
                        // may also come from other sources.
                        var timer = new Timer { AutoReset = true, Interval = samplingInterval };
                        args.DataSubscription.State = timer;

                        // Set the read attribute data of the data variable to a random value whenever the timer interval elapses.
                        timer.Elapsed += (s, e) => args.DataSubscription.OnNext(random.Next());

                        // Start the subscription timer.
                        timer.Start();
                        break;
                    }

                    case UADataSubscriptionChangedAction.Remove:
                    {
                        Console.WriteLine("Data subscription removed");

                        // Dispose of the subscription timer (stopping it too).
                        var timer = (Timer)args.DataSubscription.State;
                        timer.Dispose();
                        break;
                    }

                    case UADataSubscriptionChangedAction.Modify:
                    {
                        int samplingInterval = args.DataSubscription.SamplingInterval;
                        Console.WriteLine($"Data subscription modified, sampling interval: {samplingInterval}");

                        // Change the interval of the subscription timer.
                        var timer = (Timer)args.DataSubscription.State;
                        timer.Interval = samplingInterval;
                        break;
                    }
                }
                args.Handled = true;    // Do not forget to indicate that your code has handled the event.
            };

            // The read behavior of the data variable needs to be defined as well, separately from the data subscriptions.
            dataVariable.ReadValueFunction(() => random.Next());
            
            // Start the server.
            Console.WriteLine("The server is starting...");
            server.Start();

            Console.WriteLine("The server is started.");
            Console.WriteLine();

            // Let the user decide when to stop.
            Console.WriteLine("Press Enter to stop the server...");
            Console.ReadLine();

            // Stop the server.
            Console.WriteLine("The server is stopping...");
            server.Stop();

            Console.WriteLine("The server is stopped.");
        }
    }
}

' This example shows how to implement own handling of data subscriptions.
' You can use any OPC UA client, including our Connectivity Explorer and OpcCmd utility, to connect to the server. 
'
' Find all latest examples here: https://opclabs.doc-that.com/files/onlinedocs/OPCLabs-OpcStudio/Latest/examples.html .
' OPC client and subscriber examples in VB.NET on GitHub: https://github.com/OPCLabs/Examples-QuickOPC-VBNET .
' Missing some example? Ask us for it on our Online Forums, https://www.opclabs.com/forum/index ! You do not have to own
' a commercial license in order to use Online Forums, and we reply to every post.

Imports System
Imports OpcLabs.EasyOpc.UA
Imports OpcLabs.EasyOpc.UA.NodeSpace
Imports Timer = System.Timers.Timer

Namespace _UAServerNode
    Partial Friend Class DataSubscriptionChanged
        Shared Sub Main1()
            ' Instantiate the server object.
            ' By default, the server will run on endpoint URL "opc.tcp://localhost:48040/".
            Dim server = New EasyUAServer()

            ' Create a read-only data variable.
            Dim random = New Random()
            Dim dataVariable = UADataVariable.CreateIn(server.Objects, "SubscribeToThisVariable") _
                .ValueType(Of Integer)() _
                .Writable(False)

            dataVariable.UseDataPolling = False ' Recommended, but not strictly necessary.
            AddHandler dataVariable.DataSubscriptionChanged, Sub(sender, args)
                                                                 Select Case args.Action
                                                                     Case UADataSubscriptionChangedAction.Add
                                                                         ' Obtain the sampling interval from the data subscription.
                                                                         Dim samplingInterval As Integer = args.DataSubscription.SamplingInterval
                                                                         Console.WriteLine($"Data subscription added, sampling interval: {samplingInterval}")

                                                                         ' Create a timer that will provide the data variable with a new data. In a real server the activity
                                                                         ' may also come from other sources.
                                                                         Dim timer = New Timer With {.AutoReset = True, .Interval = samplingInterval}
                                                                         args.DataSubscription.State = timer

                                                                         ' Set the read attribute data of the data variable to a random value whenever the timer interval elapses.
                                                                         AddHandler timer.Elapsed, Sub(s, e) args.DataSubscription.OnNext(random.Next())

                                                                         ' Start the subscription timer.
                                                                         timer.Start()

                                                                     Case UADataSubscriptionChangedAction.Remove
                                                                         Console.WriteLine("Data subscription removed")

                                                                         ' Dispose of the subscription timer (stopping it too).
                                                                         Dim timer As Timer = CType(args.DataSubscription.State, Timer)
                                                                         timer.Dispose()

                                                                     Case UADataSubscriptionChangedAction.Modify
                                                                         Dim samplingInterval As Integer = args.DataSubscription.SamplingInterval
                                                                         Console.WriteLine($"Data subscription modified, sampling interval: {samplingInterval}")

                                                                         ' Change the interval of the subscription timer.
                                                                         Dim timer As Timer = CType(args.DataSubscription.State, Timer)
                                                                         timer.Interval = samplingInterval
                                                                 End Select
                                                             End Sub

            ' The read behavior of the data variable needs to be defined as well, separately from the data subscriptions.
            dataVariable.ReadValueFunction(Function() random.Next())

            ' Start the server.
            Console.WriteLine("The server is starting...")
            server.Start()

            Console.WriteLine("The server is started.")
            Console.WriteLine()

            ' Let the user decide when to stop.
            Console.WriteLine("Press Enter to stop the server...")
            Console.ReadLine()

            ' Stop the server.
            Console.WriteLine("The server is stopping...")
            server.Stop()

            Console.WriteLine("The server is stopped.")
        End Sub
    End Class
End Namespace

Reference

UADataSubscriptionChangedEventArgs Class

IUADataSubscription Interface

DataSubscriptionChanged Event

OnDataSubscriptionChanged Method

UseDataPolling Property