Resolver

Every field in a GraphQL schema is backed by a resolver — a function that knows how to produce that field's value, whether that means querying a database, calling another API, or just reading a value off a parent object. A query's execution is really a tree of resolver calls, one per requested field, each one able to fetch from a completely different source.

• —Resolvers are where GraphQL meets your actual data sources — the schema is just the contract, resolvers are the implementation.
• —A naive resolver per nested field can trigger the "N+1 query" problem — batching (e.g. via DataLoader) is the standard fix.
• —Resolvers can pull from anywhere — a SQL database, a REST API, another GraphQL service — the client never knows or cares.

This is the exact lesson that covers this term in depth — with examples, diagrams, and a hands-on exercise.