Polling server after each server response + delay

Question

I am working on an effect, that will be polling server.

What I want to achieve is as follows:

1) Send GET request to server

2) After response is received, wait 3 seconds

3) Send same GET request

4) After response is received, wait 3 seconds

5) Send same GET request

... and so on.

The code that I have now, doesn't quite work, as it polls server every 3 seconds, no matter if response was received or not:

@Effect()
pollEntries$ = this.actions$.pipe(
  ofType(SubnetBrowserPageActions.SubnetBrowserPageActionTypes.StartPollingSubnetEntries),
  switchMap(() => {
    return timer(0, 3000);
  }),
  takeUntil(this.actions$.pipe(ofType(SubnetBrowserPageActions.SubnetBrowserPageActionTypes.StopPollingSubnetEntries))),
  switchMap(() => {
    return this.subnetBrowserService.getSubnetEntries();
  }),
  map((entries) => {
    return new SubnetBrowserApiActions.LoadEntriesSucces({ entries });
  }),
  catchError((error) => {
    return of(new SubnetBrowserApiActions.LoadEntriesFailure({ error }));
  }),
);

Another thing that I am struggling with is how to stop polling. If I emit StopPollingSubnetEntries action before request is sent to server, then it works fine - however if I emit it after request is sent, then I receive one more subsequent response, before polling stops.

Answer 1

You could use expand to continuously map to the next http request and add a timer beforehand.

const stopPolling$ = this.actions$.pipe(
  ofType(SubnetBrowserPageActions.SubnetBrowserPageActionTypes.StopPollingSubnetEntries)
);

const httpRequest$ = this.subnetBrowserService.getSubnetEntries().pipe(
  map(entries => new SubnetBrowserApiActions.LoadEntriesSucces({ entries })),
  catchError(error => of(new SubnetBrowserApiActions.LoadEntriesFailure({ error })))
)

const pollEntries$ = this.httpRequest$.pipe(
  expand(_ => timer(3000).pipe(
    mergeMap(_ => this.httpRequest$),
  )),
  takeUntil(this.stopPolling$)
);

To start polling you have to subscribe to pollEntries$.

startPolling() {
  this.pollEntries$.subscribe(entries => console.log(entries));
}

Or map to the pollEntries whenever your action emits.

const pollEntriesOnAction$ = this.actions$.pipe(
  ofType(SubnetBrowserPageActions.SubnetBrowserPageActionTypes.StartPollingSubnetEntries),
  switchMap(() => this.pollEntries$)
);

this.pollEntriesOnAction$.subscribe(entries => console.log(entries));

https://stackblitz.com/edit/angular-cdtwoc?file=app/app.component.ts

Answer 2

I think you were close just instead of switchMap and timer you can use and delay(), take(1) and repeat():

const stop$ = this.actions$.pipe(ofType(SubnetBrowserPageActions.SubnetBrowserPageActionTypes.StopPollingSubnetEntries));

@Effect()
pollEntries$ = this.actions$.pipe(
  ofType(SubnetBrowserPageActions.SubnetBrowserPageActionTypes.StartPollingSubnetEntries),
  switchMap(() => this.subnetBrowserService.getSubnetEntries().pipe(
    catchError(...),
    delay(3000),
    take(1),
    repeat(),
    takeUntil(stop$),
  )),
);

Answer 3

I wrote a blog post on this topic - https://bbonczek.github.io/jekyll/update/2018/03/01/polling-with-ngrx.html.

I've decided to create a few small effects, that when working together - will poll server. Code:

@Injectable()
export class SubnetEffects {
  constructor(
    private actions$: Actions<SubnetActions.SubnetActionsUnion>,
    private http: HttpClient
  ) {}

  private isPollingActive = false;

  @Effect()
  startPolling$ = this.actions$.pipe(
    ofType(SubnetActions.SubnetActionTypes.StartPollingSubnetDevices),
    map(() => this.isPollingActive = false), // switch flag to true
    switchMap(() => {
      return this.http.get<SubnetEntry>('http://localhost:5000/api/subnet').pipe(
        switchMap(entries => new SubnetActions.GetSubnetDevicesSucceded({ entries })),
        catchError(error => of(new SubnetActions.GetSubnetDevicesFailed({ error })))
      ),
    }),
  );

  @Effect()
  stopPolling$ = this.actions$.pipe(
    ofType(SubnetActions.SubnetActionTypes.StopPollingSubnetDevices),
    map(() => this.isPollingActive = false) // switch flag to false
  );

  @Effect()
  continuePolling$ = this.actions$.pipe(
    ofType(
      SubnetActions.SubnetActionTypes.GetSubnetDevicesSucceded,
      SubnetActions.SubnetActionTypes.GetSubnetDevicesFailed
    ),
    takeWhile(() => this.isPollingActive), // do this only as long as flag is set to true
    switchMap(() => {
      return this.http.get<SubnetEntry>('http://localhost:5000/api/subnet').pipe(
        delay(5000),
        switchMap(entries => new SubnetActions.GetSubnetDevicesSucceded({ entries })),
        catchError(error => of(new SubnetActions.GetSubnetDevicesFailed({ error })))
      );
    })
  );
}