Web & Networking › API Styles & Formats

GraphQL Schema and Resolvers

Types describing the data, and functions that fetch each field.

Also known as: graphql schema, resolvers, graphql types

A GraphQL API is its schema: type definitions declaring what exists (object types, queries, mutations, subscriptions), and resolvers — one function per field — that fetch the data. The schema is both contract and documentation; resolvers are the implementation.

type User { id: ID! name: String! orders: [Order!]! }
type Query { user(id: ID!): User }
# resolver: Query.user → fetch user; User.orders → fetch their orders

Design pressure concentrates in two places: shape (types clients can rely on — evolve by addition, deprecate rather than remove) and fetching (each field’s resolver runs per parent object, so nested selections multiply database calls unless batched).

The classic mistakes:

• N+1 resolvers. users { orders } firing one query per user is the signature GraphQL performance bug. Batch per-request with a loader (see DataLoader).
• Resolvers with business logic. Resolvers should fetch and shape; rules belong in services. Logic in resolvers is untestable and unreusable.
• Nullable chaos. Everything nullable by default produces defensive ?. everywhere. Mark genuinely-required fields non-null (!) deliberately.
• Mutations returning nothing useful. A mutation should return the changed object (and errors in-band) so clients update caches without refetching.
• Breaking changes disguised as edits. Renaming types, changing nullability, removing enum values — all break clients. additive evolution plus @deprecated.
• One giant schema, no ownership. Federated or modular schemas need clear ownership per type, or the schema rots into everyone’s-second-priority.
• Ignoring complexity. Deeply nestable types let clients build expensive queries; bound depth and cost.

How to build it: schema-first design with consumers, non-null where guaranteed, batched resolvers over thin data access, additive evolution. The schema is the product; resolvers are just fetching done well.