DataLoader (GraphQL)

DataLoader is a generic utility to be used as part of your application’s data fetching layer to provide a consistent API over various backends and reduce requests to those backends via batching and caching.

Consider the following GraphQL query

{
  users {
    id
    name
    posts {
      id
      title
    }
  }
}

For each user, the resolver might query the database to fetch their posts.

• If there are 100 users, this can result in 1 query to fetch users and then 100 additional queries to fetch posts for each user (hence “N+1”).

DataLoader solves the N+1 problem by batching these individual queries into a single query and caching results to prevent redundant queries.

DataLoader is typically used in the context of resolvers in GraphQL. Here’s how it operates:

• Batching: When a resolver calls the DataLoader, it doesn’t immediately execute a database query. Instead, it queues the request. Then, before the response is returned to the client, all the queued requests are bundled into a single query and executed together.

For instance, if multiple users’ posts need to be fetched, instead of making multiple calls like:

SELECT * FROM posts WHERE user_id = 1;
SELECT * FROM posts WHERE user_id = 2;

DataLoader will group them:

SELECT * FROM posts WHERE user_id IN (1, 2);