Update OPC XML DA to OPC UA|OPC UA Standard|Forum|OPC Foundation

An OPC UA client is significantly different from OPC XML DA so you will need to rework a lot of code in the Client to migrate to OPC UA. You will likely find that everything below your internal abstraction layer needs to be rewritten.

You should probably start by identifying an OPC UA SDK that works with your development environment because that would give you the fastest/cheapest path to OPC UA. If you are using Java then Milo (https://github.com/eclipse/milo) or ProSys (https://www.prosysopc.com/prod.....-java-sdk/) are good choices.

Once you have identified the SDK you will get all of the protocols that the SDK supports so you will no longer need to use SOAP XML calls (UA TCP is more widely supported and tested).

All of the options you suggest will involve rewriting all of your code.

With an SDK it would only take a day or two to implement a simple UA client in your server that only calls a method/writes a value.

All of the other options sound like a lot more effort - especially the REST/MQTT option which would involve developing custom messages.

Here is a simple program generated by ChatGPT that writes a value using Milo:

---

import org.eclipse.milo.opcua.sdk.client.OpcUaClient;
import org.eclipse.milo.opcua.sdk.client.api.identity.UsernameProvider;
import org.eclipse.milo.opcua.stack.core.types.builtin.DataValue;
import org.eclipse.milo.opcua.stack.core.types.builtin.NodeId;
import org.eclipse.milo.opcua.stack.core.types.builtin.StatusCode;
import org.eclipse.milo.opcua.stack.core.types.builtin.Variant;

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;

public class OPCUAWriter {

public static void main(String[] args) throws Exception {
   String endpointUrl = "opc.tcp://localhost:53530/OPCUA/SimulationServer";
   OpcUaClient client = createClient(endpointUrl);

   NodeId nodeId = new NodeId(2, "MyVariable");
   Variant value = new Variant("Hello World!");

   StatusCode statusCode = writeValue(client, nodeId, value).get();
   System.out.println("Write status: " + statusCode);

  client.disconnect();
}

private static OpcUaClient createClient(String endpointUrl) throws Exception {
   OpcUaClient client = new OpcUaClient(endpointUrl);

   // set username and password if necessary
  client.setIdentityProvider(new UsernameProvider("username", "password"));

  // connect to the server 
  CompletableFuture future = client.connect();
  future.get();

  return client;
}

private static CompletableFuture writeValue(OpcUaClient client, NodeId nodeId, Variant value) {
   DataValue dataValue = new DataValue(value);

   return client.writeValue(nodeId, dataValue);
}

}

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OPC UA has a standard mechanisms built in for redundancy.

https://reference.opcfoundatio.....5/docs/6.6

Even if the server has no special redundancy support you can create 2 identical instances and design your client to automatically failover if the primary fails. You have the option of HOT, WARM or COLD redundancy which trade off higher overhead for faster failover time.

That said, you make not need it because you do not need subscriptions. Just calling the method on multiple servers may be sufficient.

Develop only a REST Client app that using an REST API of IOT

This does not mean much. It is too generic. There is no "REST API of IOT" there are only custom REST interfaces that people can develop which usually come with more cost and complexity in the long run.

And implementing UA redundancy in your case should be trivial because you only need to call methods.