SDL is language for defining GraphQL schemas. Define types, queries, mutations. Features: type system, field definitions, relationships. Essential for API structure definition.
Custom scalars through scalar type definition, serialization methods. Implement parsing, validation logic. Features: type safety, custom validation. Handle specialized data types.
Unions combine multiple types, interfaces define shared fields. Implement type resolution, conditional fields. Features: polymorphic queries, type flexibility. Enable complex type relationships.
Schema evolution through careful changes, deprecation notices. Implement backwards compatibility, gradual updates. Features: version management, client compatibility. Maintain API stability.
Schema stitching/federation combines multiple schemas. Implement service composition, type merging. Features: distributed schemas, service integration. Enable microservices architecture.
Enum types define fixed set of values. Implement type restrictions, validation. Features: type safety, clear constraints. Limit possible field values.
Validation directives through custom implementations. Implement field validation, input constraints. Features: runtime validation, error handling. Ensure data integrity.
Input types define mutation arguments. Structure complex inputs, validate data. Features: type safety, nested objects. Essential for mutation parameters.
Type nullability through exclamation mark (!). Define required fields, optional fields. Features: type safety, error prevention. Control field requirements.
List types through square brackets notation. Handle array fields, collection types. Features: array validation, null handling. Manage collections of values.
Abstract types through interfaces, unions. Implement polymorphic schemas, type conditions. Features: type flexibility, shared fields. Enable complex type hierarchies.
Recursive types through self-referential definitions. Implement tree structures, nested data. Features: circular references, depth control. Handle hierarchical data.
Schema validation through tools, linting rules. Implement type checking, consistency rules. Features: schema testing, error detection. Ensure schema quality.
Type resolvers through resolver functions, field definitions. Implement data fetching, field computation. Features: data resolution, business logic. Handle field values.
Schema deprecation through @deprecated directive, documentation. Implement migration paths, client communication. Features: version management, backward compatibility. Handle API evolution.
Type design through proper naming, field organization. Implement clear interfaces, proper relationships. Features: schema clarity, maintainability. Follow GraphQL conventions.
Code generation through GraphQL tools, type generators. Generate TypeScript/Flow types. Features: type safety, development efficiency. Automate type definitions.
