typescript 43 lines · 6 steps

Deduping in-flight requests in NestJS

A NestJS interceptor that collapses identical concurrent GET requests into a single shared upstream call.

Explained by highlit
NestJS Advanced interceptor request-deduplication rxjs caching concurrency
1import {
2 CallHandler,
3 ExecutionContext,
4 Injectable,
5 NestInterceptor,
6} from '@nestjs/common';
7import { Request } from 'express';
8import { createHash } from 'crypto';
9import { Observable, from } from 'rxjs';
10import { finalize, shareReplay } from 'rxjs/operators';
11 
12@Injectable()
13export class DedupeInterceptor implements NestInterceptor {
14 private readonly inFlight = new Map<string, Observable<unknown>>();
15 
16 intercept(context: ExecutionContext, next: CallHandler): Observable<unknown> {
17 const req = context.switchToHttp().getRequest<Request>();
18 
19 if (req.method !== 'GET') {
20 return next.handle();
21 }
22 
23 const key = this.buildKey(req);
24 const existing = this.inFlight.get(key);
25 if (existing) {
26 return existing;
27 }
28 
29 const shared = next.handle().pipe(
30 finalize(() => this.inFlight.delete(key)),
31 shareReplay({ bufferSize: 1, refCount: false }),
32 );
33 
34 this.inFlight.set(key, shared);
35 return shared;
36 }
37 
38 private buildKey(req: Request): string {
39 const userId = (req as Request & { user?: { id?: string } }).user?.id ?? 'anon';
40 const raw = `${userId}:${req.originalUrl}`;
41 return createHash('sha1').update(raw).digest('hex');
42 }
43}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Sharing one Observable across concurrent callers turns N duplicate requests into a single upstream execution.
  2. 2shareReplay with a cleanup finalize keeps the in-flight map self-pruning once the work completes.
  3. 3Scoping the dedupe key to user plus URL prevents leaking one caller's response to another.

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