Validating JSON with JSON Schema

What it is

A JSON Schema is itself a JSON document with keywords such as type, properties, required, enum, minimum/maximum, minLength/maxLength, pattern, items and additionalProperties. Validators exist for every major language; OpenAPI request and response schemas are a dialect of it.

Why it matters

A schema is a contract that both sides can check mechanically and that generates documentation; it catches missing keys, wrong types and out-of-range values before business logic sees them and produces field-level errors (paths) for the client.

How to apply

• Write the schema first for any payload that crosses a trust boundary; keep it next to the code that consumes the data.
• Set additionalProperties: false (or list them) for inputs, so that typos in keys and injected fields are rejected instead of ignored.
• Use enum for closed sets and bounds for numbers and lengths; use format (date-time, uri) as a hint, knowing that validators differ in enforcing it.
• Validate examples in tests, and validate live responses against the published schema in the pipeline.

Pitfalls

Schemas can grow into a second implementation of the business rules; keep them structural and validate semantics in code. Draft versions differ (2020-12 versus draft-07); pin one. Schema-generated errors can leak values into messages; sanitise before returning.