graphql

GraphQL

GraphQL is a query language for APIs and a runtime for fulfilling those queries with your existing data. It gives clients the power to ask for exactly what they need and nothing more.

When to Use

• Mobile Apps: Minimize bandwidth by fetching only needed fields.
• Complex Systems: Fetching related data (User + Orders + Products) in a single request.
• Rapid Iteration: Frontend can change data requirements without Backend changes.

Quick Start

# The Schema
type User {
  id: ID!
  name: String!
  orders: [Order]
}

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

// The Query (Client)
query {
  user(id: "123") {
    name
    orders {
      total
      status
    }
  }
}

Core Concepts

Schema First

The schema (.graphql) is the contract. Teams agree on the schema before writing code.

Resolvers

Functions that fetch the data for a specific field in the schema.

Strong Typing

Every field has a specific type (Int, String, Object). Validation happens automatically.

Common Patterns

n+1 Problem

Fetching a list of users and then firing a separate DB query for each user's address.

• Solution: DataLoader. Batches requests into a single query (WHERE id IN (...)).

Federation

Splitting a single GraphQL graph across multiple services (Microservices). Apollo Federation is the standard.

Best Practices

Do:

• Use Fragments on the client to reuse query logic.
• Limit Query Depth to prevent DoS attacks (e.g., user { friends { friends { friends ... } } }).
• Use Cursor-based Pagination for infinite scrolling lists.

Don't:

• Don't simply wrap a REST API 1:1. Redesign for the Graph.
• Don't utilize it for simple binary file uploads (use Signed URLs + REST/S3 for that).

Troubleshooting

Error	Cause	Solution
Cannot query field	Typo or field restricted.	Check Schema and Introspection.
N+1 Performance	Slow response on lists.	Implement DataLoader.
Caching	Hard to cache via HTTP.	Use Normalized Caching in Client (Apollo Client/Urql).