GraphQL SDL

SDL sits at the heart of schema definition language in GraphQL. This guide walks through the concept step by step, with examples, a cheatsheet, and common mistakes to avoid.

SDL Overview

SDL lets you structure GraphQL work so it stays readable, testable, and easy to scale. Instead of ad-hoc code, you follow a clear pattern that other developers can recognise immediately.

The key is to keep sdl focused and predictable. Start from the minimal example here, then layer in only the complexity your feature actually needs.

import { gql } from 'graphql-tag';

const typeDefs = gql`
  type User {
    id: ID!
    name: String!
    posts: [Post!]!
  }

  type Post {
    id: ID!
    title: String!
    author: User!
  }

  type Query {
    users: [User!]!
    user(id: ID!): User
  }
`;

The schema (SDL) defines the types, fields, and entry points clients can query.

SDL Example

const typeDefs = gql`
  type Query { hello: String! }
`;
const resolvers = { Query: { hello: () => 'world' } };
const server = new ApolloServer({ typeDefs, resolvers });

Start from a minimal SDL example and grow it only as needed.
Keep configuration explicit so SDL behaves the same in every environment.
Name things clearly so teammates understand your SDL at a glance.
Add tests around SDL early to lock in expected behaviour.

GraphQL Cheatsheet

Quick GraphQL reference related to sdl.

Concept	Example	Purpose
Schema	`type Query { user(id: ID!): User }`	Define the API shape
Resolver	`Query: { user: (_, { id }) => ... }`	Provide field data
Query	`query { user(id: 1) { name } }`	Read exactly what you need
Mutation	`mutation { createUser(input) { id } }`	Change data
Subscription	`subscription { postAdded { id } }`	Real-time updates
Context	`context: ({ req }) => ({ user })`	Auth and shared state
DataLoader	`loader.load(id)`	Batch to avoid N+1

How SDL Works in GraphQL

SDL fits into GraphQL's model of a single typed schema that clients query for exactly the data they need. The server resolves each requested field through resolver functions.

The schema (SDL) defines the types, fields, and entry points clients can query.

The schema is the contract between client and server.
Resolvers fetch data field by field, including nested types.
Clients request only the fields they use, avoiding over-fetching.
Context carries auth and shared services into every resolver.

Practical Guidance for SDL

In production, sdl should be efficient and secure. Batch data access with DataLoader, guard resolvers with authorization, and limit query depth and complexity.

Concern	Recommendation
N+1 queries	Batch with DataLoader
Security	Auth in context, depth/complexity limits
Errors	Typed GraphQLError with extension codes
Performance	Cache and paginate large lists

Common Mistakes

Skipping error handling and edge cases when wiring up sdl.
Leaving sdl untested, so regressions slip into production.
Over-engineering sdl before you actually need the extra flexibility.
Ignoring documentation, which makes sdl hard for the next developer to change.

Key Takeaways

SDL is a core part of working effectively with GraphQL.
Start small and keep sdl focused on a single responsibility.
Apply consistent patterns so sdl scales across your project.
Test and document sdl to keep it maintainable over time.

Pro Tip

When you get stuck on sdl, reduce it to the smallest reproducible example first — most GraphQL issues become obvious once the noise is gone.