What is a cold versus hot Observable?

Answer

A cold Observable creates its producer for each subscriber, such as an HTTP request. A hot Observable shares a producer independent of subscribers, such as a DOM event or Subject.

Context

RxJS models asynchronous values as composable streams. The important design choice is the source lifetime and concurrency rule: cancel, merge, queue, or ignore competing work deliberately.

Example

const results$ = query$.pipe(
  debounceTime(250),
  distinctUntilChanged(),
  switchMap(query => api.search(query))
)

switchMap ensures an older search result cannot overwrite a newer query.

Practical considerations

1. Choose the approach from the requirement and constraints, not from habit.
2. Include validation, error handling, and cleanup where the boundary requires them.
3. Verify the observable result with focused tests or measurement.

Follow-up prompts

• What failure mode would you expect if this were implemented incorrectly?
• How would you test this behaviour?
• What changes when the feature must scale to a larger application or team?

In practice

For this hard-level topic, make assumptions explicit, choose the smallest safe implementation, and verify the behavior at the relevant boundary.