RXJS observable vs subject, Observable is unicast vs subject is mutli cast

 In stream programming there are two main interfaces: Observable and Observer.

Observable is for the consumer, it can be transformed and subscribed:

observable.map(x => ...).filter(x => ...).subscribe(x => ...)

Observer is the interface which is used to feed an observable source:

observer.next(newItem)

We can create new Observable with an Observer:

var observable = Observable.create(observer => { 
    observer.next('first'); 
    observer.next('second'); 
    ... 
});
observable.map(x => ...).filter(x => ...).subscribe(x => ...)

Or, we can use a Subject which implements both the Observable and the Observer interfaces:

var source = new Subject();
source.map(x => ...).filter(x => ...).subscribe(x => ...)
source.next('first')
source.next('second')

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 Observable are unicast(observer emits on each subscription) where as subjects are multi cast, different subscription receives same value, and subscription has to come first)
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Observables are unicast by design and Subjects are multicast by design.
if you look at the below example - each subscription recieves the diffrent values as observables developed as unicast by design.
import {Observable} from 'rxjs';

let obs = Observable.create(observer=>{
   observer.next(Math.random());
})

obs.subscribe(res=>{
  console.log('subscription a :', res); //subscription a :0.2859800202682865
});

obs.subscribe(res=>{
  console.log('subscription b :', res); //subscription b :0.694302021731573
});
this could be weird if you are expecting the same values on both the subscription.
we can overcome this issue using Subjects. Subjects is similar to event-emitter and it does not invoke for each subscription. consider the below example.
import {Subject} from 'rxjs';

let obs = new Subject();

obs.subscribe(res=>{
  console.log('subscription a :', res); // subscription a : 0.91767565496093
});

obs.subscribe(res=>{
  console.log('subscription b :', res);// subscription b : 0.91767565496093
});

obs.next(Math.random());

https://stackoverflow.com/questions/47537934/what-is-the-difference-between-a-observable-and-a-subject-in-rxjs