Observables, Flowables, Observers, and Subscriptions

In ReactiveX, an observer subscribes to an Observable. When the Observable emits data, the observer reacts by consuming or transforming the data. This pattern facilitates concurrent operations because it does not need to block while waiting for the Observable to emit objects. Instead, it creates a sentry in the form of an observer that stands ready to react appropriately whenever new data in the form of an Observable is available. This model is referred to as the reactor pattern. The following diagram, taken from http://reactivex.io/assets/operators/legend.png, explains the flow of Observables:

Reactive's Observable is similar to the imperative Iterable. It addresses the same problem but the strategy is different. The Observable works by pushing changes, once available, asynchronously, while the Iterable pulls the changes mechanism in a synchronous way. The way to deal with the errors is different too; one uses an error callback while the...