Cold vs. Hot Observables

Cold observable

An Observable is considered cold if it is not actively emitting items. It only starts emitting items when it is subscribed to. Each subscription to a cold Observable will cause it to emit the underlying sequence from beginning to end for each Observer.

Note: The exact values of the sequence may differ for each Observer, depending on the underlying action the Observable encapsulates as we’ll see more of this in the section Lazy Emissions.

Generally, a cold Observable is what we would want if we want a complete copy of events emitted in the stream. It’s what we want for operations that should only be invoked when subscribed to: network operations, database queries, file I/O, and so on.

Looking at .just() as an example, we see that the sequence is repeated on each subscription: