GraphQL SDL (Schema Definition Language)

 GraphQL SDL (Schema Definition Language) is a concise syntax for defining GraphQL schemas, including types, fields, queries, mutations, and subscriptions. It provides a declarative way to specify the capabilities of a GraphQL API, making it easy to understand, communicate, and evolve the API over time. Here's an overview of GraphQL SDL:

Basic Structure:

  • GraphQL SDL defines the schema of a GraphQL API using a set of type definitions.
  • Types represent the objects and data structures exposed by the API, including custom types, scalars, enums, interfaces, unions, queries, mutations, and subscriptions.

Type Definitions:

  • Types are defined using the type keyword followed by the type name and a set of fields enclosed in curly braces.
  • Fields represent the properties or attributes of a type and are defined with a name and a type (which can be a scalar type, custom type, or list type).

Scalar Types:

  • Scalar types represent primitive data types (e.g., String, Int, Float, Boolean, ID) that cannot be further decomposed.
  • Scalars are used to represent simple values like strings, numbers, and booleans in GraphQL queries and responses.

Custom Types:

  • Custom types are user-defined data structures representing complex objects or entities in the API.
  • Custom types can be composed of scalar types, other custom types, lists, and non-null types.

Enum Types:

  • Enum types define a set of possible values for a field, providing a way to represent a finite set of options.
  • Enum types are useful for fields with a predefined set of values, such as status codes or categorical data.

Interfaces and Unions:

  • Interfaces define a set of fields that must be implemented by object types that implement the interface.
  • Unions represent a type that can be one of several object types, providing a way to express polymorphic queries.

Directives:

  • Directives provide a way to annotate schema definitions and modify the behavior of GraphQL operations.
  • Directives can be used to conditionally include or exclude fields, apply transformations, or enforce validation rules.

Queries, Mutations, and Subscriptions:

  • Queries define the read operations that clients can perform to fetch data from the GraphQL API.
  • Mutations define the write operations that clients can perform to modify data in the GraphQL API.
  • Subscriptions define real-time event streams that clients can subscribe to receive updates from the GraphQL API.

Example SDL:

type Query {
  hello: String
  user(id: ID!): User
}

type User {
  id: ID!
  name: String
  age: Int
}

type Mutation {
  createUser(name: String!, age: Int): User
}

In this example:

  • The Query type defines a hello field that returns a string and a user field that takes an ID argument and returns a User object.
  • The User type defines fields for representing user data, including id, name, and age.
  • The Mutation type defines a createUser field for creating a new user with a given name and age.

GraphQL SDL provides a clear and concise syntax for defining GraphQL schemas, making it easy to design, communicate, and implement GraphQL APIs effectively.

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