The schema is the contract

Key points

• Every GraphQL API publishes a schema: the types, their fields, and what is nullable.
• String! means non-null; [Order!]! is a non-null list of non-null orders.
• The server rejects a query naming a field that does not exist - before running anything.
• That schema powers editor autocomplete, client codegen and introspection for free.
• The trade: the schema is a contract, so removing a field is a breaking change for every client.

Example

type Query {
  customer(id: ID!): Customer
  orders(status: OrderStatus, first: Int = 20): [Order!]!
}

type Customer {
  id: ID!
  name: String!
  email: String          # nullable: may be hidden
  orders: [Order!]!
}

enum OrderStatus { PENDING SHIPPED CANCELLED }

The bang is non-null, and its position matters: [Order!]! and [Order]! mean different things.