GraphQL API Architecture & Enterprise Federation: 2026 Interview Guide

import DataLoader from 'dataloader';

// Create factory to ensure fresh cache per HTTP request
export function createOrderLoader(db: Database) {
  return new DataLoader<string, Order[]>(async (userIds) => {
    // Single query replaces N individual queries
    const orders = await db.query(
      'SELECT * FROM orders WHERE user_id = ANY($1)',
      [userIds]
    );
    
    // Group orders by userId and map to original key array order
    const orderMap = new Map<string, Order[]>();
    orders.forEach(order => {
      const list = orderMap.get(order.userId) || [];
      list.push(order);
      orderMap.set(order.userId, list);
    });
    
    return userIds.map(id => orderMap.get(id) || []);
  });
}

# Users Subgraph (Accounts Team)
type User @key(fields: "id") {
  id: ID!
  username: String!
  email: String!
}

# Reviews Subgraph (Content Team)
type Review {
  id: ID!
  rating: Int!
  comment: String!
  author: User!
}

extend type User @key(fields: "id") {
  id: ID! @external
  reviews: [Review!]!
}