Subjects & Multicasting

A Subject is both an Observable and an Observer: you can push values into it with next(), and many subscribers can listen to it at the same time. Subjects are the standard way to multicast in RxJS, and they power most state-sharing patterns in Angular services.

Pick the right Subject

Type	New subscriber receives	Typical Angular use
Subject	Only values emitted after subscribing	Event buses, one-off notifications
BehaviorSubject	The current value, then updates	State that always has a value (auth status, selected item)
ReplaySubject	The last N values, then updates	Late subscribers that need recent history
AsyncSubject	Only the final value, at completion	One-shot results shared with all consumers (rare)

Not sure which one fits? See the Subject vs BehaviorSubject vs ReplaySubject comparison.

Sharing a stream without a Subject

Operators can multicast an existing Observable for you:

• share: one shared subscription to the source, no replay for late subscribers.
• shareReplay: one shared subscription plus a replay buffer, commonly used to cache HTTP results.

These are the tools behind "make this HTTP call once and let every component use the result", one of the most common Angular interview scenarios.