Number of Sessions per Client-Server Connection

[dominik0071]

I use Milo Client 0.6.15 to connect to the OPC UA server on the device, where the open62541 library is used.

Milo Client is used for the repetitive read, call methods requests - just gathering data based on scheduled operations.

When the device is disconnected from the network and then connected again, the server is overwhelmed with several sessions created by one instance of Milo Client. Other applications, e.g., UA Expert, can't successfully connect to the OPC UA server. Usually, there is an error: Bad Session ID invalid, but not every time.

Is there an option to set up a maximum number of sessions created by one client in Milo?

What is the recommended settings (timeout, keep-alive-interval etc.) for the long-running connection on the expected bad network?

Do asynchronous requests create sessions per each?

[kevinherron]

Each OpcUaClient creates one Session at a time. It deletes the Session when you call disconnect().

What may be happening is that you lose your connection to this server (often because it stops responding) and then the Milo Client reconnects and creates a new Session.

If you turn on DEBUG logging for org.eclipse.milo.opcua.sdk.client.SessionFsm and/or get a Wireshark capture I can get a better sense for what is happening.

[dominik0071]

I will look at it.

Is there a way how to check that connection is active? Some method like "isConnected" ?

[kevinherron]

You get notified whether your Session is active/inactive via SessionActivityListener.

If you get the underlying transport, check if it's a TCP transport, then you can get the ChannelFsm and get its State.

But the idea you might be "connected" is transport-dependent.

Your application should not be trying to manage connection loss / reconnect logic on its own. This is entirely handled by the SDK.

[dominik0071]

I understand, but on the other hand you want to see in your UI that your client has active connection. Imagine system component with REST API with method, whether client isConnected ... I don't want to handle connection loss on my own

[kevinherron]

Keep track of whether you're "connected" based on whether your Session is active or inactive.

The Session status is what matters.

[kevinherron]

Example where you get the status from the underlying transport (against 1.0 API, but similar concept)

    if (client.getTransport() instanceof OpcTcpClientTransport tcp) {
      State state = tcp.getChannelFsm().getState();
      boolean connected = state == State.Connected;

      System.out.println("state=" + state);
      System.out.println("connected=" + connected);
    }