The Reactive Streams specification defines four primary interfaces: Publisher, Subscriber, Subscription, and Processor. Since that initiative grew independently from any organization, it became available as a separate JAR file where all interfaces live within the org.reactivestreams package.

In general, the specified interfaces are similar to what we had earlier (for example, in RxJava 1.x). In a way, these reflect the well-known classes from RxJava. The first two of those interfaces are similar to Observable-Observer, which resemble the classic Publisher-Subscriber model. Consequently, the first two were named Publisher and Subscriber. To check whether these two interfaces are similar to Observable and Observer, let’s consider the declaration of those:

package org.reactivestreams;

public interface Publisher<T> {...