How are disconnects/re-connects handled in Eclipse Milo on the client side?

Question

I am writing an OPC UA client using Eclipse Milo and stumbled over the following question: how does the client handle the loss of connections.

For monitoring values I do this using a subscription with the SubscriptionManager:

OpcUaClient client = myCreateClient();
List<MonitoredItemCreateRequest> items = myCreateMonitoredItems();

UaSubscription subscription = client.getSubscriptionManager().createSubscription(1_000.0).get();
List<UaMonitoredItem> result = subscription.createMonitoredItems(TimestampsToReturn.Both, items).get();

for (UaMonitoredItem item : result) {
   if (!item.getStatusCode().isBad()) {
     item.setValueConsumer(value -> System.out.println("Update: " + value));
   }
}

Now when I restart my OPC UA server, which is also implemented using Eclipse Milo, then I do see the client reconnecting, but the subscriptions don't get any more updates. In the log I get the following output:

09:11:15.734 [ua-shared-pool-0] DEBUG o.e.m.o.s.c.s.OpcUaSubscriptionManager - Publish service failure: StatusCode{name=Bad_NoSubscription, value=0x80790000, quality=bad}
java.util.concurrent.CompletionException: UaServiceFaultException: status=Bad_NoSubscription, message=There is no subscription available for this session.
    <stack-trace-omitted>
    …

So it seems that OpcUaSubscriptionManager is aware of the situation, but does not try to re-register those items. Is that to be done manually?

Answer 1

There is a flowchart that describes the reconnect sequence a client should follow in OPC-UA Part 4, Section 6.5. It involves attempting to re-use the same secure channel, attempting to re-activate the prior session, and even attempting to transfer subscriptions to a new session if that fails. The Milo client SDK does all of this.

Restarting the server is worst-case because it throws away all the state in the process, unlike a normal network interruption. In this case the client SDK will notify via callback that its attempts to restore state after reconnect have failed and that subscriptions must be re-created manually.

Add a SubscriptionListener to UaSubscriptionManager and if you receive the onSubscriptionTransferFailed callback then it's time to re-create the subscription and monitored items.