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Once you have the OPC Gateway running, it's possible access the OPC Legacy server from any system using pure Java code.  Here's a sample that shows how to do that.

<code java>
import com.ergotech.util.SimulationManager;
import com.ergotech.util.TargetLicenseManager;
import com.ergotech.vib.exceptions.BadParameterException;
import com.ergotech.vib.exceptions.VIBUpdateFailedException;
import com.ergotech.vib.servers.SimpleDataSource;
import com.ergotech.vib.servers.opc.opcclient.OPC;
import com.ergotech.vib.utils.DataSourceContainer;
import com.ergotech.vib.valueobjects.ValueChangedEvent;
import com.ergotech.vib.valueobjects.ValueObjectInterface;

import java.io.InputStream;
import java.nio.file.Files;
import java.nio.file.Path;

public class OPCDemo {

    public static void main(String[] args) throws InterruptedException {
        OPCDemo opcDemo = new OPCDemo();

        // Configure the license, required for OPC component to function.
        opcDemo.setupLicense();

        // Disable simulation mode to connect to the actual OPC server.
        SimulationManager.getSimulationManager().setSimulating(false);

        // Initialize and start the OPC setup.
        opcDemo.setupOpc();

        // Keep the application running for 5 minutes to receive updates from the OPC server.
        Thread.sleep(1000 * 60 * 5);
    }

    public void setupLicense() {
        try (InputStream license = Files.newInputStream(Path.of("resources/MIX.trg"))) {
            // Initialize the license manager with the appropriate target.
            final TargetLicenseManager manager = new TargetLicenseManager("MIX",
                    license,
                    OPCDemo.class.getClassLoader());

            // Set the license manager for components.
            SimpleDataSource.setClassLicenseManager(manager);
        } catch (Exception e) {
            // Handle any failure during license setup.
            throw new RuntimeException("Failed to setup license", e);
        }
    }

    public void setupOpc() {
        DataSourceContainer container;

        try {
            // Create a new OPC container to hold the OPC components.
            container = DataSourceContainer.getRootContainer().createNewChildContainer("OPCExample");
        } catch (BadParameterException e) {
            // Occurs if a container with the same name already exists.
            throw new RuntimeException("Failed to create OPC container", e);
        }

        OPC opc = new OPC();

        try {
            // Assign a unique, case-insensitive name to the OPC component.
            opc.setName("OPC");

            // Assign the host and item to connect to in the OPC server.
            opc.setHost("localhost"); // the host that is running the OPCGateway
            opc.setItem("GEMToolSimulator.VIDs.SVIDs.Temperature");
        } catch (BadParameterException e) {
            // Handle invalid configuration for the OPC component.
            throw new RuntimeException("Invalid OPC configuration", e);
        }

        try {
            // Add the configured OPC component to the container.
            container.addComponent(opc, opc.getName());
        } catch (BadParameterException e) {
            // Occurs if a component with the same name is already added to the container.
            throw new RuntimeException("Failed to add OPC component to container", e);
        }

        // Register a listener to handle value change events from the OPC server.
        opc.addValueChangedListener(new ValueChangedCallback());

        try {
            // Initialize and start the OPC component.
            opc.init();
            opc.start();
        } catch (BadParameterException | VIBUpdateFailedException e) {
            // Handle any failure during initialization or startup.
            throw new RuntimeException("Failed to start OPC component", e);
        }

        // Access current values as various types
        // Force the server to register for value updates even though there are no listeners
        opc.setAutoSuspend(SimpleDataSource.AUTOSUSPEND_NEVER);
        // read the data
        System.out.println("Current value as String: " + opc.getValueObject().getStringValue());
        System.out.println("Current value as Double: " + opc.getValueObject().getDoubleValue());
        System.out.println("Current value as Float: " + opc.getValueObject().getFloatValue());
        System.out.println("Current value as Integer: " + opc.getValueObject().getIntValue());
        System.out.println("Current value as Long: " + opc.getValueObject().getLongValue());
        System.out.println("Current value as Boolean: " + opc.getValueObject().getBoolValue());
    }

    // Callback to process value changes from the OPC component.
    public static class ValueChangedCallback {

        // Any object with a method "valueInput(ValueChangedEvent) can be used a callback
        public void valueInput(ValueChangedEvent valueChangedEvent) {
            // Logic to handle changes in OPC data, triggered whenever the OPC value changes.
            ValueObjectInterface valueObject = valueChangedEvent.getValueObject();
            System.out.println("Value Changed: " + valueObject + " [quality: " + valueObject.getQuality() + "]");
        }
    }

}
</code>

The OPC DA standard is optimized for a subscription model. Here's an example of reading multiple servers.  Each will update when the value changes at the server and the updated value will be stored in a HashMap.  Multiple listeners/HashMaps could be used to build "groups" of data, especially if you're looking to use ErgoTech's "TriggeredHistorical" component to periodically save these straight to a database.

<code java>
import com.ergotech.util.SimulationManager;
import com.ergotech.util.TargetLicenseManager;
import com.ergotech.vib.exceptions.BadParameterException;
import com.ergotech.vib.exceptions.VIBUpdateFailedException;
import com.ergotech.vib.servers.SimpleDataSource;
import com.ergotech.vib.servers.opc.opcclient.OPC;
import com.ergotech.vib.utils.DataSourceContainer;
import com.ergotech.vib.valueobjects.ValueChangedEvent;
import com.ergotech.vib.valueobjects.ValueObjectInterface;

import java.io.InputStream;
import java.nio.file.Files;
import java.nio.file.Path;
import java.util.HashMap;

public class OPCDemo {

    // HashMap to store the latest values for each item
    private HashMap<String, ValueObjectInterface> itemValueMap = new HashMap<>();

    public static void main(String[] args) throws InterruptedException {
        OPCDemo opcDemo = new OPCDemo();

        // Configure the license, required for OPC component to function.
        opcDemo.setupLicense();

        // Disable simulation mode to connect to the actual OPC server.
        SimulationManager.getSimulationManager().setSimulating(false);

        // Initialize and start the OPC setup.
        opcDemo.setupOpc();

        // Keep the application running for 5 minutes to receive updates from the OPC server.
        Thread.sleep(1000 * 60 * 5);
    }

    public void setupLicense() {
        try (InputStream license = Files.newInputStream(Path.of("resources/MIX.trg"))) {
            // Initialize the license manager with the appropriate target.
            final TargetLicenseManager manager = new TargetLicenseManager("MIX",
                    license,
                    OPCDemo.class.getClassLoader());

            // Set the license manager for components.
            SimpleDataSource.setClassLicenseManager(manager);
        } catch (Exception e) {
            // Handle any failure during license setup.
            throw new RuntimeException("Failed to setup license", e);
        }
    }

    public void setupOpc() {
        DataSourceContainer container;

        try {
            // Create a new OPC container to hold the OPC components.
            container = DataSourceContainer.getRootContainer().createNewChildContainer("OPCExample");
        } catch (BadParameterException e) {
            // Occurs if a container with the same name already exists.
            throw new RuntimeException("Failed to create OPC container", e);
        }

        // List of items to monitor
        String[] items = {
                "GEMToolSimulator.VIDs.SVIDs.Temperature",
                "GEMToolSimulator.VIDs.SVIDs.Pressure",
                "GEMToolSimulator.VIDs.SVIDs.Humidity"
        };

        // Create a single listener instance
        ValueChangedCallback listener = new ValueChangedCallback();

        for (String item : items) {
            OPC opc = new OPC();

            try {
                // Assign a unique, case-insensitive name to the OPC component.
                opc.setName("OPC_" + item);

                // Assign the host and item to connect to in the OPC server.
                opc.setHost("localhost"); // the host that is running the OPCGateway
                opc.setItem(item);
            } catch (BadParameterException e) {
                // Handle invalid configuration for the OPC component.
                throw new RuntimeException("Invalid OPC configuration", e);
            }

            try {
                // Add the configured OPC component to the container.
                container.addComponent(opc, opc.getName());
            } catch (BadParameterException e) {
                // Occurs if a component with the same name is already added to the container.
                throw new RuntimeException("Failed to add OPC component to container", e);
            }

            // Register the same listener to handle value change events from the OPC server.
            opc.addValueChangedListener(listener);

            try {
                // Initialize and start the OPC component.
                opc.init();
                opc.start();
            } catch (BadParameterException | VIBUpdateFailedException e) {
                // Handle any failure during initialization or startup.
                throw new RuntimeException("Failed to start OPC component", e);
            }

            // Force the server to register for value updates even though there are no listeners
            opc.setAutoSuspend(SimpleDataSource.AUTOSUSPEND_NEVER);
        }

        // Access current values from the HashMap
        for (String item : items) {
            ValueObjectInterface valueObject = itemValueMap.get(item);
            if (valueObject != null) {
                System.out.println("Current value of " + item + " as String: " + valueObject.getStringValue());
                System.out.println("Current value of " + item + " as Double: " + valueObject.getDoubleValue());
                System.out.println("Current value of " + item + " as Float: " + valueObject.getFloatValue());
                System.out.println("Current value of " + item + " as Integer: " + valueObject.getIntValue());
                System.out.println("Current value of " + item + " as Long: " + valueObject.getLongValue());
                System.out.println("Current value of " + item + " as Boolean: " + valueObject.getBoolValue());
            } else {
                System.out.println("No value yet for " + item);
            }
        }
    }

    // Callback to process value changes from the OPC component.
    public class ValueChangedCallback {

        // Any object with a method "valueInput(ValueChangedEvent)" can be used as a callback
        public void valueInput(ValueChangedEvent valueChangedEvent) {
            // Logic to handle changes in OPC data, triggered whenever the OPC value changes.
            ValueObjectInterface valueObject = valueChangedEvent.getValueObject();
            OPC opc = (OPC) valueChangedEvent.getSource();
            String itemName = opc.getItem();

            // Update the HashMap with the new value
            itemValueMap.put(itemName, valueObject);

            System.out.println("Value Changed for item " + itemName + ": " + valueObject + " [quality: " + valueObject.getQuality() + "]");
        }
    }
}
</code>