What is the difference between Observable and Flowable in RxJava 2.0?

As stated in the documentation:

A small regret about introducing backpressure in RxJava 0.x is that
instead of having a separate base reactive class, the Observable
itself was retrofitted. The main issue with backpressure is that many
hot sources, such as UI events, can’t be reasonably backpressured and
cause unexpected MissingBackpressureException (i.e., beginners don’t
expect them).

We try to remedy this situation in 2.x by having
io.reactivex.Observable non-backpressured and the new
io.reactivex.Flowable be the backpressure-enabled base reactive class.

Use Observable when you have relatively few items over time (<1000) and/or there’s no risk of producer overflooding consumers and thus causing OOM.

Use Flowable when you have relatively large amount of items and you need to carefully control how Producer behaves in order to to avoid resource exhaustion and/or congestion.

Backpressure
When you have an observable which emits items so fast that consumer can’t keep up with the flow leading to the existence of emitted but unconsumed items.

How unconsumed items, which are emitted by observables but not consumed by subscribers, are managed and controlled is what backpressure strategy deals with.

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