Data Loader

ELI5 — The Vibe Check

DataLoader batches and caches database calls in GraphQL. Without it, fetching 100 users with their posts makes 101 database queries (the N+1 problem). DataLoader collects all the IDs, makes ONE query for all of them, and distributes the results. It's carpooling for database queries.

Real Talk

DataLoader is a utility for batching and caching database requests, primarily used to solve the N+1 problem in GraphQL resolvers. It collects individual load requests within a single tick of the event loop, batches them into a single database query, and distributes results back to the callers. Each DataLoader instance caches results per-request.

Show Me The Code

const userLoader = new DataLoader(async (ids) => {
  const users = await db.users.findByIds(ids);
  return ids.map(id => users.find(u => u.id === id));
});

// Instead of N queries, makes 1 batched query
const user1 = await userLoader.load('id-1');
const user2 = await userLoader.load('id-2');

When You'll Hear This

"Add a DataLoader to batch the user lookups — we're making 500 queries per request." / "DataLoader reduced our GraphQL query from 200ms to 15ms by batching."