if we need to do things in sequence while waiting for completion, then concatMap is the right choice

for doing things in parallel, mergeMap is the best option

in case we need cancellation logic, switchMap is the way to go

for ignoring new Observables while the current one is still ongoing, exhaustMap does just that

    const subject = new Subject<{ url: string; delay: number; }>();

    subject
      .pipe(
        //mergeMap(concurrent?: number)  concatMap   switchMap exhaustMap
        exhaustMap( httpGet
        ),
        map((res: string) => JSON.parse(res).ResultData)
      )
      .subscribe((value) => {
        let dateTime = new Date();
        console.log(value + "  " + dateTime.toLocaleTimeString());
      });

    let dateTime = new Date();
    console.log(dateTime.toLocaleTimeString());
    console.log("start http://foo ");
    subject.next({ url: 'http://foo', delay: 5000 });
    console.log("start http://bar ");
    subject.next({ url: 'http://bar', delay: 5000 });
    console.log("start http://baz ");
    subject.next({ url: 'http://baz', delay: 5000 });

    function httpGet(pram :{url: string, delay: number}) {
      return interval(1000).pipe(delay(pram.delay),take(4), map(val => {
        const obj = { ResultData: pram.url + ' -> ' + val };
        //console.log(JSON.stringify(obj));
        return JSON.stringify(obj);
      }))
    }