Reactive Streams
A specification for asynchronous streams with non-blocking backpressure, developed 2013–2015 by people from Netflix, Lightbend and Pivotal. Four interfaces: Publisher, Subscriber, Subscription, Processor.
The whole point is Subscription.request(n): the consumer states how much it can handle, and the producer may not send more. Without that mechanism a fast producer fills a slow consumer's memory. The spec was absorbed into JDK 9 as java.util.concurrent.Flow — identical interfaces, different package name. Implementations: RxJava 2+, Project Reactor (Spring WebFlux), Akka Streams, Vert.x, Mutiny. Interoperability is the entire selling point — you can wire a Reactor source to an Akka sink.