A Subscription abstracts the notion of a subscriber's communication channel back to the publisher. This channel is what enables the subscriber to either cancel the subscription or signal demand. Its definition is as follows.
public interface Subscription {
public void request(long n);
public void cancel();
}
The request( ) method signals to the publisher that the subscriber can receive more items, also specifying the quantity.
The cancel( ) method notifies the publisher that the subscriber is no longer interested in receiving items.
This channel between a publisher and a subscriber is what enables to achieve non blocking back-pressure in a Reactive Streams.