What is the APP_GUARD / APP_PIPE token pattern in NestJS?

0-2 years experience

1. 1How would you add a global authentication guard to all routes in a NestJS app using APP_GUARD?
2. 2What happens if you register a validation pipe using APP_PIPE but forget to import ValidationPipe from '@nestjs/common'?

2-5 years experience

1. 1A user reports that POST requests are failing with 400 errors, but the validation pipe logs show no output — how would you debug whether APP_PIPE is even being applied?
2. 2You added a global guard to restrict access to admin routes, but now some public endpoints are blocked — what’s likely wrong, and how would you fix it?

5-8 years experience

1. 1You’re migrating a legacy NestJS app to use modular guards instead of global ones — what edge cases might break when replacing APP_GUARD with module-scoped guards, and how would you test for them?
2. 2How would you design a system where different microservices use different APP_PIPE implementations for request validation, while avoiding duplication and ensuring consistent error responses?

8+ years experience

1. 1You’re leading a cross-team migration from global APP_GUARD/APP_PIPE to per-module implementations across 12 services — what architectural tradeoffs do you consider for maintainability, rollout risk, and observability?
2. 2How would you design a long-term strategy to phase out legacy APP_PIPE logic that’s tightly coupled to deprecated DTOs, while ensuring zero downtime and backward compatibility for external clients?