How do you implement idempotency keys for POST requests to prevent duplicate operations in NestJS?

Q: How do you implement idempotency keys for POST requests to prevent duplicate operations in NestJS?

@Injectable() export class IdempotencyInterceptor implements NestInterceptor { constructor(private cache: CacheService) {} async intercept( ctx: ExecutionContext, next: CallHandler, ): Promise > { const req = ctx.switchToHttp().getRequest (); const idempotencyKey = req.headers['idempotency-key'] as string; // Only apply to mutating methods if (!idempotencyKey || !['POST', 'PUT', 'PATCH'].includes(req.method)) { return next.handle(); } const cacheKey = `idempotency:${idempotencyKey}`; const cached = await this.cache.get(cacheKey); if (cached) { ctx.switchToHttp().getResponse() .setHeader('Idempotency-Replayed', 'true'); return of(cached); // handler is skipped entirely } return next.handle().pipe( tap(async response => { await this.cache.set(cacheKey, response, 86_400); // 24h TTL }), ); } } @Post() @UseInterceptors(IdempotencyInterceptor) create(@Body() dto: CreateOrderDto) { return this.ordersService.create(dto); } The Idempotency-Key header must be unique per logical operation — clients generate it (e.g. UUID v4). Cache the full response object — replay must return the exact same response body and status. Set Idempotency-Replayed: true header on replayed responses so clients can detect duplicates. Only apply to mutating methods (POST, PUT, PATCH) — GET is already idempotent by definition. Use a distributed cache (Redis) in production — in-memory cache breaks on multi-instance deployments.

Difficulty: 6/10

idempotency, NestJS interceptors, persistent storage

Build an IdempotencyInterceptor that reads the Idempotency-Key request header, checks a cache for a stored response, and returns it if found — short-circuiting the handler. On the first request, let the handler run and store the response in cache with tap(). Fire-and-forget on cache writes so the response is never delayed.

Idempotency interceptor with cache short-circuit

Idempotency key implementation rules:

1
The Idempotency-Key header must be unique per logical operation — clients generate it (e.g. UUID v4).
2

How do you implement idempotency keys for POST requests to prevent duplicate operations in NestJS?

Difficulty: 6/10

idempotency, NestJS interceptors, persistent storage

Build an IdempotencyInterceptor that reads the Idempotency-Key request header, checks a cache for a stored response, and returns it if found — short-circuiting the handler. On the first request, let the handler run and store the response in cache with tap(). Fire-and-forget on cache writes so the response is never delayed.

Idempotency interceptor with cache short-circuit

Idempotency key implementation rules:

1
The Idempotency-Key header must be unique per logical operation — clients generate it (e.g. UUID v4).
2

How do you implement idempotency keys for POST requests to prevent duplicate operations in NestJS?

How do you implement idempotency keys for POST requests to prevent duplicate operations in NestJS?

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