How to run web request (Observable) in `finalize()` or achieve similar behavior?

Question

I want to somehow chain two web requests such that:

1. Request A (Observable<void>) is always run.
2. Request B (also Observable<void>) is also always run, after A and even when A completes with errors (like finalize()).
3. If A errors with E_A, I want the whole pipeline to complete with E_A.
4. If A succeeds, but B errors with E_B, I want the whole pipeline to complete with E_B.
   • I could also do with the pipeline just succeeding in this case.
5. If both error, then I want the whole pipeline to complete with E_A.

---

This is the solution I could come up with, but it feels very clumsy:

// Changes different parts of backend state in multiple steps,
// some of which might fail.
const reqA: Observable<void> = this.mutateSystemState();

// Loads & renders the system state, updating the parts that
// could be changed.
const reqB: Observable<void> = this.updateSystemStateDisplay();

const pipeline = reqA.pipe(
    catchError(e => of({ errA: e })),
    mergeMap(resultA => reqB.pipe(
        catchError(e => of({ errB: e })),
        map(resultB => {
            if (resultA instanceof Object) {
                throw resultA.errA;
            } else if (resultB instanceof Object) {
                throw resultB.errB;
            }
        })
    ))
);

Answer 1

The materialize operator converts next, error, or completion 'events' into ObservableNotification objects.

Assuming you want pipeline$ to either error or complete without emitting a value, you can do the following:

const pipeline$ = reqA.pipe(
  materialize(),
  concatMap(a => reqB.pipe(
    materialize(),
    map(
      // Both have executed.
      // Both a & b are ObservableNotification objects
      // Emit a if ErrorNotification
      b => a.kind === 'E' ? a
      // Otherwise emit CompleteNotification if b is a NextNotification
        : b.kind === 'N' ? {kind: 'C'}
      // Otherwise emit b
        : b
    )
  )),
  // At this point, we've got reqA's ErrorNotification, reqB's ErrorNotification or a CompleteNotification.
  // Either way, this observable is closing before the first materialize gets a chance to emit reqA's CompleteNotification
  dematerialize()
);

Answer 2

onErrorResumeNext()

whatever reqA$ error or not, will subscribe reqB$

reqA$.pipe(
  onErrorResumeNext(reqB$)
).subscribe(...);

https://stackblitz.com/edit/rxjs-ps5f1j

---

const pipeline$ = of(reqA$, reqB$).pipe(
  concatMap(materialize()),
  filter(({kind}) => kind === 'E'), // filter Error
  reduce(identity), // collect then return first value
  dematerialize(),
);

or

const pipeline$ = of(reqA$, reqB$).pipe(
  concatMap(materialize()),
  max(({kind}) => +(kind === 'E')),
  dematerialize(),
);

https://stackblitz.com/edit/rxjs-jnqhjw