GraphQL Schema Design

GraphQL schema design is the practice of defining the type system, queries, mutations, and subscriptions that form the API contract using Schema Definition Language (SDL). Unlike REST, where the server dictates the response shape, a GraphQL schema lets clients request exactly the fields they need. The schema acts as both documentation and runtime validation -- any query that doesn't conform to the schema is rejected before execution.

Effective schema design follows several principles: use domain-driven naming that reflects business concepts rather than database tables, prefer explicit types over generic JSON blobs, use interfaces and unions for polymorphism, and design input types separately from output types to control what clients can write versus read. Connection-based pagination (the Relay specification) is the standard pattern for list fields, using edges and nodes rather than simple arrays to allow attaching metadata to relationships.

Schema evolution in GraphQL differs from REST versioning. Because clients select specific fields, new fields can be added without breaking anyone. Fields are deprecated using the @deprecated directive with a reason string, and tooling (GraphiQL, Apollo Studio) surfaces these deprecations to consumers. The goal is to maintain a single evolving schema rather than multiple versioned schemas, which is one of GraphQL's key advantages over REST.