Are NestJS modules singletons? What does that mean practically?

0-2 years experience

1. 1If you create two services inside the same NestJS module and inject them into two different controllers, will they share the same instance? Explain what happens.
2. 2How would you register a provider so that each request gets a new instance instead of the default singleton?
3. 3What would happen if you accidentally add the same provider to two different modules?

2-5 years experience

1. 1You notice that a stateful service is leaking data between requests. Walk me through how you would debug this in NestJS and what module singleton behavior might be causing it.
2. 2Suppose you need two different implementations of a logging service in different parts of the app. How would you configure modules to avoid the singleton conflict?
3. 3Why might moving a provider from a global module to a feature module fix a bug you’re seeing?

5-8 years experience

1. 1Design a multi‑tenant NestJS application where each tenant should have its own instance of a configuration service. How would you handle the singleton nature of modules to achieve isolation?
2. 2Discuss the performance implications of having many globally‑scoped singleton providers versus lazy‑loaded feature modules.
3. 3If you need to hot‑reload a module in production without restarting the server, how does the singleton nature affect your approach?

8+ years experience

1. 1Your organization is migrating a monolithic NestJS codebase to a microservices architecture. How would you refactor module boundaries and singleton providers to minimize coupling and support independent deployment?
2. 2When multiple teams share a common library of NestJS modules, what governance practices would you put in place to manage singleton state and avoid hidden side effects?
3. 3Consider a scenario where a globally‑scoped provider holds a cache that grows unbounded. How would you redesign the module/provider structure to mitigate memory risks while preserving performance?