an empty set, then there was no open web socket session
 associated with given socket channel. The returned futures will return <code>null</code> on <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/concurrent/Future.html#get()" title="class or interface in java.util.concurrent" class="external-link"><code>Future.get()</code></a> if
 the message was successfully delivered and otherwise throw <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/concurr