Vaadin23 Server Push for specific other user - vaadin

Question

There is a Vaadin 23 tutorial which shows how to send messages to all users (broadcast) https://vaadin.com/docs/latest/advanced/server-push
But what if I need to send a Push message from me to only one other specific user? Is it possible with server push and Vaadin and if so - how ? For example, an Admin user updated something for another User and would like to immediately notify such user about that.
UPDATED
Based on the answer and comments, I updated the Broadcaster from the example to the following:
public class Broadcaster {
static Executor executor = Executors.newSingleThreadExecutor();
static Map<String, List<Consumer<String>>> listeners = new ConcurrentHashMap<>();
public static synchronized Registration register(String userUuid, Consumer<String> listener) {
addListener(userUuid, listener);
return () -> {
synchronized (Broadcaster.class) {
listeners.remove(listener);
}
};
}
private static synchronized void addListener(String userUuid, Consumer<String> listener) {
List<Consumer<String>> consumers = listeners.get(userUuid);
if (consumers == null) {
consumers = new LinkedList<>();
}
consumers.add(listener);
listeners.put(userUuid, consumers);
}
public static synchronized void broadcast(String userUuid, String message) {
List<Consumer<String>> consumers = listeners.get(userUuid);
if (CollectionUtils.isNotEmpty(consumers)) {
for (Consumer<String> consumer : consumers) {
executor.execute(() -> consumer.accept(message));
}
}
}
}
Will such implementation properly work in case I'd like to push a message to the listeners of the specific user?

Answer 1

You need some PubSub in place.
Push is pretty much agnostic to distribution events or what you do with
it in general. It just allows the server to notify the client-side
out-of-band. What means you use this features for, is up to you.
E.g. each client could register to the pub-sub on session-init with
their user-name or -group (or a subject in general) and later some admin
publishes notifications with the target. Only clients registered to
that target react by e.g. pushing.
The poor-mans version would be all clients listening to the same stream
of messages but only react if they are mentioned as the target. This is
most likely less efficient.