Error fetching data from Rockwell OPC UA Server using Eclipse Milo JAva Client

Question

I have built a Java Application to fetch data from OPC UA Server. I have used Eclipse Milo UA Client SDK to build the client. I am facing a problem in OpcUaClient.readValues method. I am passing a list of NodeId created from Tags defined and if I traverse the DataValue array returned by readValues method, I get follwing error for some of the elements

DataValue{value=Variant{value=null}, status=StatusCode{name=Bad_NodeIdUnknown, value=0x80340000, quality=bad},

Sudo code looks like

List<NodeId> nodeIds = new ArrayList<NodeId>();
List<Parameter> parameters = device.getParameters();
for (int i = 0; i < parameters.size(); i++) {
    Parameter parameter = parameters.get(i);
                            
    nodeIds.add(new NodeId(device.getNsID(),  parameter.getTagName()));
                            
}
List<DataValue> values = client.readValues( 0.0, TimestampsToReturn.Both, nodeIds).get();
for (int i = 0; i < values .size(); i++) {
    DataValue dvalue = values.get(i);
    if( dvalue.getValue().getValue() != null) {
        Integer intValue =  (Integer) values.get(i).getValue().getValue();
                            
        logger.info("captured Value is  {}", value);
                                        
    }
    else
    {
       logger.error("Error fetching value for {} , {}", parameter.getName(),dvalue.toString());
    }
}

The error is flagged from else clause.

My application is running on a separate computer than the SCADA Server ( OPC UA). A peculiar behavior observed is, If I use ProSys Client to check value of failing tag, and if I re-run my application, data is fetched correctly. I am completely clueless about how ProSys client and my application is related. Is prosys client triggering something on the server side to load the tags in some cache... Any clues/pointers will help. Thanks in advance.

Answer 1

It sure sounds like the Rockwell server doesn't actually "build" the Node instances until you've browsed it first.

You aren't doing anything wrong in the client; this is stupid behavior on the server's part. You may have to program your client to recursively browse the hierarchy you're interested in before reading or creating monitored items.