Schema

In a database, a schema defines the structure and organization of data. In GraphQL, it specifies the capabilities of the API and how to query it.

A schema is a formal definition of the structure, constraints, and relationships of data in a system. The term is used in multiple contexts: in relational databases, a schema defines tables, columns, data types, and relationships; in GraphQL, a schema specifies the types, queries, mutations, and subscriptions available in the API; in JSON Schema, it defines the expected structure and validation rules for JSON documents.

API schemas serve as contracts between API producers and consumers. OpenAPI specifications and JSON Schema definitions describe the expected format of request bodies, response payloads, query parameters, and headers. These schemas enable automated validation, documentation generation, and client SDK generation.

In API gateway architectures, schemas are used for request validation -- the gateway can reject requests that do not conform to the defined schema before they reach backend services. This protects backends from malformed input and provides clear error messages to API consumers. Schema validation at the gateway level is especially valuable in serverless environments, where it prevents unnecessary function invocations for invalid requests.