OpcUa client throws SocketException when disconnected from a server with failover mechanism

[ikaro143]

I have implemented a client to connect to a server with a failover mechanism.
There are 2 identical servers, one acting as leader and the second as backup. When the leader is disconnected, the backup will immediately take over in the same endpoint.

Currently if I disconnect the leader, my client doesn't recover the subscriptions listeners, I have the following stack trace:

2025-10-15 08:30:07,169 ERROR [org.ecl.mil.opc.sta.tra.cli.uas.UascClientMessageHandler] (milo-netty-event-loop-1) [remote=globalstate/10.32.98.5:52520] Exception caught: Connection reset: java.net.SocketException: Connection reset
at [email protected]/sun.nio.ch.SocketChannelImpl.throwConnectionReset(SocketChannelImpl.java:394)
at [email protected]/sun.nio.ch.SocketChannelImpl.read(SocketChannelImpl.java:426)
at io.netty.buffer.PooledByteBuf.setBytes(PooledByteBuf.java:255)
at io.netty.buffer.AbstractByteBuf.writeBytes(AbstractByteBuf.java:1132)
at io.netty.channel.socket.nio.NioSocketChannel.doReadBytes(NioSocketChannel.java:356)
at io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:151)
at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:788)
at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:724)
at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:650)
at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:562)
at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:997)
at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)
at [email protected]/java.lang.Thread.run(Thread.java:840)
at org.graalvm.nativeimage.builder/com.oracle.svm.core.thread.PlatformThreads.threadStartRoutine(PlatformThreads.java:885)
at org.graalvm.nativeimage.builder/com.oracle.svm.core.thread.PlatformThreads.threadStartRoutine(PlatformThreads.java:860)

[kevinherron]

Do you set a SubscriptionListener on each of your OpcUaSubscription instances and implement onTransferFailed?

milo/opc-ua-sdk/sdk-client/src/main/java/org/eclipse/milo/opcua/sdk/client/subscriptions/OpcUaSubscription.java

Lines 1679 to 1686 in 00170d2

	/**
	* Called when a new Session is established after reconnecting, but transferring this
	* Subscription to the new Session was unsuccessful.
	*
	* @param subscription the Subscription that failed to transfer to the new Session.
	* @param status the {@link StatusCode} for the transfer failure.
	*/
	default void onTransferFailed(OpcUaSubscription subscription, StatusCode status) {}

Upon reconnecting, if the Session cannot be re-activated, a Subscription transfer is attempted, and if this fails the callback is made and you are then responsible for re-creating your Subscriptions.

[ikaro143]

Hi Kevin, thanks for the quick reply. I'll try it out.