What is a GraphQL schema?

A GraphQL schema is the contract between client and server. It defines every type and its fields, how the types relate, and which operations a client can run.

The server checks each request against the schema before running it, so a query for a missing field fails early. That makes the schema the single source of truth for what data can be fetched and how.

Schemas are usually written in the Schema Definition Language (SDL). Here Query lists the entry points for reading, and User.posts links each user to their posts.

type User {
  id: ID!
  name: String!
  email: String!
  posts: [Post!]!
}

type Query {
  user(id: ID!): User
  users: [User!]!
}