How to get data from failed forkJoin request? - rxjs

Question

Using Angular Rxjs and ngrx
I have an action that dispatch 4 API and I am doing the following =>
#Effect()
getAllModels$ = this.actions$.pipe(
ofType<featureActions.GetAllModelsRequest>(featureActions.ActionTypes.GetAllModelsRequest),
switchMap((action) =>
forkJoin([
this.dataService.GetAllModelFromServer(),
this.dataService.GetAllModelFromHost(),
this.dataService.GetAllModelFromCache(),
this.dataService.GetAllModelFromPreference(),
]).pipe(
map(
([server, host, cache, preference]) =>
new featureActions.GetAllModelsSuccess({
//...
})
),
catchError((error: HttpErrorResponse) => {
return of(new featureActions.GetAllModelsFailed({ error: error.message }));
})
)
)
);
The problem is, when one of those API fail, everything fail and I am in fail action. all the data that got retrieved (before the one endpoint that failed) is lost.
Is there a way to get the data retrieved in the catchError or the only solution is to chain the api one after the other ?

Answer 1

You can write your own implementation of forkJoin. Here is a simple example sourced from the original (https://github.com/ReactiveX/rxjs/blob/master/src/internal/observable/forkJoin.ts):
export function forkJoin2(...args: any[]): Observable<any> {
const resultSelector = popResultSelector(args);
const { args: sources, keys } = argsArgArrayOrObject(args);
if (resultSelector) {
// deprecated path.
return forkJoinInternal(sources, keys).pipe(map((values: any[]) => resultSelector!(...values)));
}
return forkJoinInternal(sources, keys);
}
function forkJoinInternal(sources: ObservableInput<any>[], keys: string[] | null): Observable<any> {
return new Observable((subscriber) => {
const len = sources.length;
if (len === 0) {
subscriber.complete();
return;
}
const values = new Array(len);
let completed = 0;
let emitted = 0;
for (let sourceIndex = 0; sourceIndex < len; sourceIndex++) {
const source = innerFrom(sources[sourceIndex]);
let hasValue = false;
subscriber.add(
source.subscribe({
next: (value) => {
if (!hasValue) {
hasValue = true;
emitted++;
}
values[sourceIndex] = value;
},
error: (err) => { return subscriber.error({ error: err, values }) },
complete: () => {
completed++;
if (completed === len || !hasValue) {
if (emitted === len) {
subscriber.next(keys ? keys.reduce((result, key, i) => (((result as any)[key] = values[i]), result), {}) : values);
}
subscriber.complete();
}
},
})
);
}
});
}
Notice, when an error occurs, you are returning the error along with the values:
error: (err) => { return subscriber.error({ error: err, values }) }

Answer 2

I went with this solution found here : https://medium.com/better-programming/rxjs-error-handling-with-forkjoin-3d4027df70fc
#Effect()
getAllModels$ = this.actions$.pipe(
ofType<featureActions.GetAllModelsRequest>(featureActions.ActionTypes.GetAllModelsRequest),
switchMap((action) =>
forkJoin([
this.dataService.GetAllModelFromServer().pipe(catchError(() => of({ data: [] }))),
this.dataService.GetAllModelFromHost().pipe(catchError(() => of({ data: [] }))),
this.dataService.GetAllModelFromCache().pipe(catchError(() => of({ data: [] }))),
this.dataService.GetAllModelFromPreference().pipe(catchError(() => of({ data: [] }))),
]).pipe(
map(
([server, host, cache, preference]) =>
new featureActions.GetAllModelsSuccess({
//...
})
),
catchError((error: HttpErrorResponse) => {
return of(new featureActions.GetAllModelsFailed({ error: error.message }));
})
)
)
);