Attach DataLoader instances to the GraphQL context in the GraphQLModule.forRootAsync() context factory. The factory is called per request, so each request gets a fresh loader. Resolvers access the loader via the @Context() decorator instead of injecting the loader service directly into every resolver class.

DataLoader in GraphQL context

Context-based DataLoader benefits:

1
Context factory is called per request — each request gets its own DataLoader instance automatically.
2
Avoids injecting the loader service into every resolver that needs it.
3
A single GqlContext interface documents all available loaders in one place.
4
Resolvers stay clean — access loaders via @Context() without constructor injection overhead.
5
Multiple loaders for different entities can all be attached to the same context object.

Difficulty: 6/10

Topics: GraphQL context, DataLoader integration, NestJS providers

Scenario Questions

0-2 years experience

1
You need to add a DataLoader for batching user lookups in a NestJS GraphQL resolver. Walk me through how you would expose that DataLoader through the GraphQL context so each resolver can use it.
2
If you forget to mark the DataLoader provider as request‑scoped, what behavior would you observe when multiple requests hit the server concurrently?
3
Where would you place the code that attaches the DataLoader to the context in a typical NestJS GraphQL module setup?

2-5 years experience

1
We have a resolver that fetches posts and their authors. After adding a DataLoader for authors, we notice that author data is sometimes duplicated across requests. What could cause that, and how would you debug it?
2
Explain the trade‑offs between creating a separate DataLoader per resolver versus sharing a single DataLoader instance via the GraphQL context in NestJS.
3
During a code review you see the DataLoader being instantiated inside the resolver method. What problems does this cause, and how would you refactor it?

5-8 years experience

1
Our service handles thousands of concurrent GraphQL queries, each using several DataLoaders. How would you design the DataLoader registration in NestJS to avoid memory leaks and ensure per‑request isolation?
2
If we need to add caching to a DataLoader while still keeping it request‑scoped, what patterns would you use in NestJS to inject a cache service without breaking the DataLoader lifecycle?
3
Describe how you would test that the DataLoader is correctly attached to the context and that batching works across multiple resolvers.

8+ years experience

1
We are migrating a monolithic NestJS GraphQL API to a micro‑service architecture, and each service will expose its own GraphQL schema. How would you standardize DataLoader provisioning across services to maintain consistency and avoid duplicated implementations?
2
Consider a scenario where multiple teams need to share DataLoaders for cross‑domain entities (e.g., users, permissions). What governance and architectural strategies would you put in place to manage versioning, testing, and deployment of shared DataLoader modules?
3
If we decide to replace DataLoader with a custom batching layer that lives outside NestJS (e.g., a dedicated batching service), what impact does that have on the GraphQL context design, and how would you transition without breaking existing resolvers?

Follow-up Questions

❓What would change if you made the DataLoader a singleton instead of request‑scoped?
❓How does the choice of where you attach the loader in the context affect resolver testability?
❓Can you walk me through how you would profile the performance impact of your DataLoader setup?