data-validation

# Data Validation

Schema-based data validation across languages and formats. Covers JSON Schema, Zod (TypeScript), Pydantic (Python), API boundary validation, data contracts, and integrity checking.

## When to Use

- Defining the shape of API request/response bodies
- Validating user input before processing
- Setting up data contracts between services
- Checking CSV/JSON file integrity before import
- Migrating data (did the ETL preserve everything?)
- Generating types or documentation from schemas

## JSON Schema

### Basic schema

```json
{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "type": "object",
  "required": ["name", "email", "age"],
  "properties": {
    "name": {
      "type": "string",
      "minLength": 1,
      "maxLength": 100
    },
    "email": {
      "type": "string",
      "format": "email"
    },
    "age": {
      "type": "integer",
      "minimum": 0,
      "maximum": 150
    },
    "role": {
      "type": "string",
      "enum": ["user", "admin", "moderator"],
      "default": "user"
    },
    "tags": {
      "type": "array",
      "items": { "type": "string" },
      "uniqueItems": true,
      "maxItems": 10
    },
    "address": {
      "type": "object",
      "properties": {
        "street": { "type": "string" },
        "city": { "type": "string" },
        "zip": { "type": "string", "pattern": "^\\d{5}(-\\d{4})?$" }
      },
      "required": ["street", "city"]
    }
  },
  "additionalProperties": false
}
```

### Common patterns

```json
// Nullable field
{ "type": ["string", "null"] }

// Union type (string or number)
{ "oneOf": [{ "type": "string" }, { "type": "number" }] }

// Conditional: if role is admin, require permissions
{
  "if": { "properties": { "role": { "const": "admin" } } },
  "then": { "required": ["permissions"] }
}

// Pattern properties (dynamic keys)
{
  "type": "object",
  "patternProperties": {
    "^env_": { "type": "string" }
  }
}

// Reusable definitions
{
  "$defs": {
    "address": {...