001package org.reactivestreams;
002
003/**
004 * A {@link Subscription} represents a one-to-one lifecycle of a {@link Subscriber} subscribing to a {@link Publisher}.
005 * <p>
006 * It can only be used once by a single {@link Subscriber}.
007 * <p>
008 * It is used to both signal desire for data and cancel demand (and allow resource cleanup).
009 *
010 */
011public interface Subscription {
012    /**
013     * No events will be sent by a {@link Publisher} until demand is signaled via this method.
014     * <p>
015     * It can be called however often and whenever needed—but the outstanding cumulative demand must never exceed Long.MAX_VALUE.
016     * An outstanding cumulative demand of Long.MAX_VALUE may be treated by the {@link Publisher} as "effectively unbounded".
017     * <p>
018     * Whatever has been requested can be sent by the {@link Publisher} so only signal demand for what can be safely handled.
019     * <p>
020     * A {@link Publisher} can send less than is requested if the stream ends but
021     * then must emit either {@link Subscriber#onError(Throwable)} or {@link Subscriber#onComplete()}.
022     * 
023     * @param n the strictly positive number of elements to requests to the upstream {@link Publisher}
024     */
025    public void request(long n);
026
027    /**
028     * Request the {@link Publisher} to stop sending data and clean up resources.
029     * <p>
030     * Data may still be sent to meet previously signalled demand after calling cancel as this request is asynchronous.
031     */
032    public void cancel();
033}