Describes flow of messages from Subscribers to Publishers , that is, in opposed direction compared to the flow of messages from Publishers to Subscribers in java.util.concurrent.Flow.

ReverseFlow.java

import java.util.concurrent.Flow;

/**
 * Describes flow of messages from Subscribers to Publishers
 * (as opposed to the flow of messages from Publishers to Subscribers in {{@link Flow}}).
 * Subribers here act as masters/clients, and Publishes as slaves/servers, just like in {{@link Flow}}). 
 */
public class ReverseFlow {

    private ReverseFlow() {}

    /**
     * Consumes messages.
     *
     * A {@link Publisher} can serve multiple {@link Subscriber}s subscribed dynamically
     * at various points in time.
     *
     * @param <T> the type of messages.
     */
    public interface Publisher<T> {

        /**
         * @param producer the {@link Subscriber} which offers messages for this {@link Publisher}
         */
        void subscribe(Subscriber<T> producer);
    }

    /**
     * Produces messages.
     *
     * A {@link Subscriber} is a provider of a series of tokens, transmitting them to a {@link Publisher}(s).
     * <p>
     * @param <T>  type of messages.
     */
    public interface Subscriber<T> {
        /**
         * Method invoked prior to invoking any other Subscriber
         * methods for the given Subscription. If this method throws
         * an exception, resulting behavior is not guaranteed, but may
         * cause the Subscription not to be established or to be cancelled.
         *
         * <p>Typically, implementations of this method invoke {@code
         * subscription.request} to enable sending items.
         *
         * @param subscription a new subscription
         */
        void onSubscribe(Flow.Subscription subscription);


        /**
         * 
         * @return true if producer has completed the flow of items 
         */
        default boolean isCompleted() {
            return false;
        }

        /**
         * 
         * @return the cause of exceptional completion, if any
         *        - null otherwise
         */
        default Throwable getCompletionException() {
            return null;
        }

        /**
         * {@link Publisher} gets data from {@link Subscriber}
         * when it has room to save the data.
         * @return the data from  {@link Subscriber}
         */
        T remove();

        /**
         * called by {@link Publisher} when it is completed and asks to not disturb.
         *
         */
        void onComplete();
    }
}