How to resume RxJs Observable Interval on Error

Question

I am merging two Observables. The first one gets the current temperature on init. The second one polls at a certain interval the API. If the Api call fails, then the Observable interval is not resumed.

How could I resume it?

getCurrentTemp(): Observable<number> {
    return this.http.get(this.environmentService.getTemperatureUrl())
      .map((res: Response) => res.json())
      .switchMap(() => res.temp);
  }

  pollCurrentTemperature(): Subscription {
    const temp$ = this.getCurrentTemp();
    const tempInterval$ = Observable
      .interval(3000)
      .flatMap(() => this.getCurrentTemp());

    return temp$
      .take(1)
      .merge(tempInterval$)
      .subscribe((temp: number) => {
        console.log('temp', temp);
      }, (err) => {
        console.log('error', err);
        // When the api fails my interval does not resume. How can I retry it?
      });
  }

Any ideas? Ty

maybe this could help: https://stackoverflow.com/a/34999441/2829204 — CozyAzure, Jul 03 '17 at 16:29
You want the interval to continue from the last `index` that didn't fail? — martin, Jul 03 '17 at 17:17

score 1 · Answer 1 · answered Jul 04 '17 at 12:26

Use catch.

Catch: recover from an onError notification by continuing the sequence without error

getCurrentTemp(): Observable<number> {
    return this.http.get(this.environmentService.getTemperatureUrl())
      .map((res: Response) => res.json())
      .catch(error => {
          console.log('Error occured');
          return Observable.empty();
       });
      .switchMap(() => res.temp);
}

It will catch the error and silently return an empty observable in its place. In effect, the switchmap will skip over the failed api call silently as it will not emit for the empty observable.

Of course, you could have an alternate behaviour on error, but you need to catch it to avoid the problem you are facing.

score 0 · Answer 2 · answered Jul 03 '17 at 16:51

Using the http status codes you can retrieve an observable only if its a 200 let's say:

 getCurrentTemp(): Observable<number> {
    return Observable.from(
      [
        { value: 1, status: 200 },
        { value: 2, status: 200 },
        { value: 3, status: 200 },
        { value: 4, status: 200 },
        { value: 5, status: 200 },
        { value: 6, status: 400 }])
      .switchMap((x: any) => {
        if (x.status === 200) {
          return Observable.of(x.value);
        }
        return Observable.onErrorResumeNext();
      });
  }

  pollCurrentTemperature(): Subscription {
    const temp$ = this.getCurrentTemp();
    const tempInterval$ = Observable
      .interval(3000)
      .flatMap(() => this.getCurrentTemp());

    return temp$
      .take(1)
      .merge(tempInterval$)
      .subscribe((temp: number) => {
        console.log('temp', temp);
      }, (err) => {
        console.log('error', err);
        // When the api fails my interval does not resume. How can I retry it?
      });
  }

The important bit is this return Observable.onErrorResumeNext();

How to resume RxJs Observable Interval on Error

2 Answers2