Observer & Observable Streams

The Observable-Observer contract forms the backbone of every reactive system, formalized by reactivex.io, with a single Observable emitting items via three lifecycle methods: onNext, onError, and onComplete. Cold Observables emit only when subscribed, offering each subscriber its own execution—ideal for independent HTTP fetches—while hot Observables emit regardless of subscribers, suiting shared WebSocket feeds. Reactor's Flux and Mono implement this contract directly, integrating with the Reactive Streams specification, while Java 9's Flow API mirrors it with a default buffer of 256 items. Kotlin Flow is cold by default, with SharedFlow and StateFlow for hot scenarios, now dominating Android development over RxJava. The pattern's formal invariants, such as RxJava's prohibition of null emissions, prevent NullPointerExceptions and ensure type safety. A concrete data point: RxJS 7 processes 1 million events per second in browser benchmarks, demonstrating the pattern's performance and reliability.