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API / Swagger Interview questions

What is a schema in OpenAPI?

A schema is a description of the shape and constraints of a piece of JSON (or other media type) data — the fields an object has, their types, which are required, and validation rules like string formats or numeric ranges — used throughout an OpenAPI document to describe request bodies, response bodies, and individual parameters.

User:
  type: object
  required: [id, email]
  properties:
    id:
      type: integer
    email:
      type: string
      format: email
    nickname:
      type: string

OpenAPI's schema syntax is based on JSON Schema, though earlier OpenAPI versions supported only a subset of full JSON Schema's capabilities; OpenAPI 3.1 aligned much more closely with JSON Schema itself, closing most of those historical gaps.

Schemas are commonly defined once in a reusable components/schemas section and referenced from multiple places via $ref, so a User schema used in both a create-user request body and a get-user response doesn't need to be duplicated and kept in sync by hand in two places.

What standard is OpenAPI's schema syntax based on?
Why are schemas commonly placed in components/schemas and referenced with $ref?

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