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
Cache the full response object — replay must return the exact same response body and status.
3
Set Idempotency-Replayed: true header on replayed responses so clients can detect duplicates.
4
Only apply to mutating methods (POST, PUT, PATCH) — GET is already idempotent by definition.
5
Use a distributed cache (Redis) in production — in-memory cache breaks on multi-instance deployments.

Difficulty: 6/10

Topics: idempotency, NestJS interceptors, persistent storage

Scenario Questions

0-2 years experience

1
We have a NestJS controller that creates an order via POST /orders. How would you add support for an idempotency key so that if the client retries the same request, only one order is created?
2
If a client sends the same idempotency key twice within a minute, what should your service return, and how would you implement that logic?
3
Where would you store the idempotency key and its result in a typical NestJS app, and why?

2-5 years experience

1
You notice duplicate orders are still being created after adding an idempotency interceptor. Walk me through how you would debug the issue.
2
Explain the trade‑offs between storing idempotency keys in Redis versus a relational database in a NestJS microservice.
3
How would you handle expiration of idempotency keys for a high‑traffic payment endpoint, and what changes would you make to the NestJS middleware?

5-8 years experience

1
Design a reusable idempotency solution that works across multiple NestJS modules and can be toggled per route. What components would you create and how would they interact?
2
Consider a scenario where the downstream service fails after you have recorded the idempotency key. How would you ensure consistency and avoid lost updates?
3
What performance implications does checking idempotency keys have at scale, and how would you mitigate them in a NestJS application serving thousands of requests per second?

8+ years experience

1
Your organization is migrating legacy monolith endpoints to a new NestJS platform. How would you introduce idempotency handling without breaking existing clients, and what governance processes would you put in place?
2
Across several teams, some services use Redis and others use PostgreSQL for idempotency storage. How would you standardize the approach and what architectural guidelines would you propose?
3
If you need to guarantee exactly‑once processing across distributed services, how does the idempotency‑key pattern fit, and what additional mechanisms would you combine with NestJS to achieve that?

Follow-up Questions

❓Can you walk me through the request flow from controller to storage?
❓What would you do if the storage layer is temporarily unavailable?
❓How would you test that your idempotency implementation works correctly?