Angular RxJS: Prevent Duplicate HTTP Requests

In Angular RxJS, duplicate HTTP requests happen when multiple observables like searchInput$ and clearFilterBtn$ both trigger the same load() method, especially since Angular’s HttpClient creates cold observables that refire on each subscription. The best fix? Merge them into one stream with merge rxjs, apply debounceTime and distinctUntilChanged to throttle emissions, then pipe the load() HTTP call through shareReplay to cache and multicast responses—preventing redundant API hits while keeping things reactive.

Contents

• Understanding Duplicate HTTP Requests in Angular RxJS
• Why Angular HttpClient Triggers Multiple Calls
• Merging Observables with RxJS merge
• Using RxJS shareReplay for Caching HTTP Responses
• Debouncing and Filtering Emissions
• Complete Code Example: Refactored load Method
• Advanced Tips and Best Practices
• Sources
• Conclusion

Understanding Duplicate HTTP Requests in Angular RxJS

Ever hit that frustrating moment where your Angular app fires off the same API call twice—once from a search input change, then again from a clear button click? That’s classic RxJS territory with angular observable streams like searchInput$ and clearFilterBtn$. Both subscribe separately and yell “load!” at your service, but since load() uses HttpClient under the hood, each call spins up a fresh, cold observable. Boom—duplicate requests clog your network tab.

Your code nails the issue:

this.testService.searchInput$.pipe(takeUntil(this.ngUnsubscribe$)).subscribe(() => {
 console.log('test searchInput$');
 this.load();
});

this.testService.clearFilterBtn$.pipe(takeUntil(this.ngUnsubscribe$)).subscribe(() => {
 console.log('test backBtn$');
 this.load();
});

When clearFilterBtn$ emits, it might cross wires with searchInput$ (maybe a shared state reset?), triggering both logs and double load(). RxJS operators fix this without hacks. No more polling your backend like it’s 2010.

Why Angular HttpClient Triggers Multiple Calls

Angular HttpClient returns cold observables by default. Subscribe once? It fires the request. Subscribe again? It fires another request, even if params are identical. Your dual subscriptions amplify this—load() gets called back-to-back, each birthing its own HTTP trip.

Picture it: searchInput$ emits “foo”, calls load(). Then clearFilterBtn$ emits, calls load() again. If they’re near-simultaneous, debounce won’t fully save you without sharing. As the Angular University blog points out, this is a top RxJS pitfall in angular http flows. Cold obs are great for one-offs, but multicasting demands hot ones.

Quick test? Log inside load():

load() {
 console.log('load called');
 return this.http.get('/api/data').pipe(tap(() => console.log('HTTP fired')));
}

You’ll see multiples. Time to RxJS operators like shareReplay.

Merging Observables with RxJS merge

Why separate subs when you can merge rxjs them? Combine searchInput$ and clearFilterBtn$ into one observable stream. Any emission from either triggers a single downstream effect—no more duplicate load() triggers.

Here’s the swap:

merge(
 this.testService.searchInput$.pipe(mapTo('search')),
 this.testService.clearFilterBtn$.pipe(mapTo('clear'))
).pipe(
 takeUntil(this.ngUnsubscribe$)
).subscribe((action) => {
 console.log('Merged trigger:', action);
 this.load();
});

mapTo tags the source (optional, for logging). Now one sub handles both. But if emissions cluster (e.g., rapid typing + clear), add debounce later. This cuts your subs from two to one, slashing rxjs angular boilerplate. Stack Overflow discussions swear by it for component-level merging.

What if you need to distinguish actions? Pipe payloads through instead of mapTo.

Using RxJS shareReplay for Caching HTTP Responses

Merging triggers is step one. The real magic? Make load()'s HTTP observable hot with shareReplay. Subscribers get the latest (or replayed) value without refiring the request.

Refactor load() in your service:

private data$?: Observable<any>;

load() {
 if (!this.data$) {
 this.data$ = this.http.get('/api/data').pipe(
 shareReplay({ bufferSize: 1, refCount: true })
 );
 }
 return this.data$;
}

From your component:

merge(/* ... */).pipe(
 switchMap(() => this.testService.load()),
 takeUntil(this.ngUnsubscribe$)
).subscribe(data => {
 // Use shared data
});

First sub creates the request. Later ones replay it. refCount: true auto-cleans when unsubbed—memory leak? Nah. Official RxJS docs recommend { bufferSize: 1, refCount: true } for HTTP. The BitsRC blog demos this exact lazy-cache pattern. Perfect for rxjs sharereplay in services.

Debouncing and Filtering Emissions

Merges alone won’t stop spam from fast typing. Enter debounceTime and distinctUntilChanged—RxJS operators that throttle and dedupe.

Enhance the merge:

merge(
 this.testService.searchInput$.pipe(
 debounceTime(300),
 distinctUntilChanged()
 ),
 this.testService.clearFilterBtn$
).pipe(
 takeUntil(this.ngUnsubscribe$)
).subscribe(() => this.load());

Search gets debounced (wait 300ms post-keystroke). Clear fires instantly—use debounceTime(0) if needed. distinctUntilChanged skips identical sequential emissions (e.g., same search term). For objects/params, add a comparator:

distinctUntilChanged((prev, curr) => prev.value === curr.value)

This duo slims observable rxjs noise. Per another Stack Overflow thread, selective debounce per stream rocks.

Complete Code Example: Refactored load Method

Tying it together. Service first:

@Injectable()
export class TestService {
 private data$?: Observable<any>;

 load() {
 if (!this.data$) {
 this.data$ = this.http.get('/api/data').pipe(
 shareReplay(1),
 takeUntilDestroyed() // Angular 16+ auto-unsub
 );
 }
 return this.data$;
 }

 searchInput$ = new Subject<string>();
 clearFilterBtn$ = new Subject<void>();
}

Component:

ngOnInit() {
 merge(
 this.testService.searchInput$.pipe(debounceTime(300)),
 this.testService.clearFilterBtn$
 ).pipe(
 switchMap(() => this.testService.load()),
 takeUntilDestroyed()
 ).subscribe(data => {
 console.log('Shared data:', data);
 this.items = data;
 });
}

One sub. Cached HTTP. Debounced search. When clear emits, it grabs the replayed response instantly—no dupe. Test in RxJS playground to see the flow. Matches your setup perfectly, as in the Angular University example.

Advanced Tips and Best Practices

Scale up? Use switchMap over mergeMap to cancel in-flight requests on new emissions. For multiple params:

merge(search$, clear$).pipe(
 map(() => ({ query: this.searchTerm || '' })),
 distinctUntilChanged((a, b) => a.query === b.query),
 switchMap(params => this.http.get(`/api/data?query=${params.query}`))
)

refCount: false? Keeps replaying forever—risky for dynamic data. Alternatives like manual BehaviorSubject caches work but violate RxJS purity, per Daniele Ghidoli’s blog.

Pitfalls: Errors replay too—add retry(1). Params change? Reset data$. Angular 16+ takeUntilDestroyed() simplifies unsubs. Reddit threads like this one echo: service-level shareReplay wins for shared state.

Sources

1. Angular RxJS Common Pitfalls — Detailed merge + shareReplay fix for duplicate HTTP triggers: https://blog.angular-university.io/angular-2-rxjs-common-pitfalls/
2. Angular RxJS 6: How to prevent duplicate HTTP requests — Service caching with shareReplay and cold observable explanation: https://stackoverflow.com/questions/50864978/angular-rxjs-6-how-to-prevent-duplicate-http-requests
3. Avoiding Duplicate HTTP Requests with shareReplay — Lazy service observable with debounceTime and refCount config: https://blog.bitsrc.io/avoiding-angular-duplicate-http-requests-with-the-rxjs-sharereplay-operator-773ba1b4ad5e
4. shareReplay — Official RxJS operator docs with bufferSize and refCount options: https://rxjs.dev/api/operators/shareReplay
5. Debounce on one observable in merge — Selective debouncing in merged RxJS streams: https://stackoverflow.com/questions/49649469/debounce-on-one-observable-in-merge

Conclusion

Mastering RxJS operators like merge, shareReplay, debounceTime, and distinctUntilChanged eliminates duplicate Angular HTTP requests effortlessly—turning chaotic subs into a clean, performant stream. Start with merging your triggers, cache the load() observable, and debounce where needed. Your app (and backend) will thank you with snappier loads and zero waste.