Schema Design Requirements
│
├─ Single service (monolith)?
│  └─ Schema-first design with single schema
│
├─ Multiple microservices?
│  ├─ Services owned by different teams?
│  │  └─ Apollo Federation
│  └─ Services owned by same team?
│     └─ Schema stitching (simpler)
│
├─ Existing REST APIs to wrap?
│  └─ GraphQL wrapper layer
│
└─ Need backward compatibility?
   └─ Hybrid REST + GraphQL

Resolver Implementation
│
├─ Field resolves to single related entity?
│  └─ DataLoader with batching
│
├─ Field resolves to list of related entities?
│  ├─ List size always small (<10)?
│  │  └─ Direct query acceptable
│  └─ List size unbounded?
│     └─ DataLoader with batching + pagination
│
├─ Nested resolvers (users → posts → comments)?
│  └─ Multi-level DataLoaders
│
└─ Aggregations or counts?
   └─ Separate DataLoader for counts

// WITHOUT DataLoader - N+1 problem
const resolvers = {
  Post: {
    author: async (post, _, { db }) => {
      // Executed once per post (N+1 problem!)
      return db.User.findByPk(post.userId);
    }
  }
};
// Query for 100 posts triggers 101 DB queries

import DataLoader from 'dataloader';

// Create loader per request (important!)
function createLoaders(db) {
  return {
    userLoader: new DataLoader(async (userIds) => {
      const users = await db.User.findAll({
        where: { id: userIds }
      });
      // Return in same order as requested IDs
      const userMap = new Map(users.map(u => [u.id, u]));
      return userIds.map(id => userMap.get(id));
    })
  };
}

// Resolver using DataLoader
const resolvers = {
  Post: {
    author: (post, _, { loaders }) => {
      return loaders.userLoader.load(post.userId);
    }
  }
};
// Same query now triggers 2 queries total!

type Query {
  users(first: Int, after: String, last: Int, before: String): UserConnection!
}

type UserConnection {
  edges: [UserEdge!]!
  pageInfo: PageInfo!
  totalCount: Int!
}

type UserEdge {
  node: User!
  cursor: String!
}

type PageInfo {
  hasNextPage: Boolean!
  hasPreviousPage: Boolean!
  startCursor: String
  endCursor: String
}

type Mutation {
  createUser(input: CreateUserInput!): CreateUserPayload!
}

type CreateUserPayload {
  user: User
  errors: [UserError!]!
}

type UserError {
  field: String
  message: String!
  code: ErrorCode!
}

enum ErrorCode {
  VALIDATION_ERROR
  NOT_FOUND
  UNAUTHORIZED
  CONFLICT
}