varsnotwars / graphql-n-plus-one-demo

Node.js graphql example of the N+1 problem, with solution using dataloader.

Graphql N+1 problem demonstration

TL;DR

Using graphql type resolvers result in the N+1 problem, that is, N extra database queries per 1 'parent' database query. This repo demonstrates this with a basic example, and one possible solution using dataloader.

Description

TODO

Setup

Installation

npm install

Database setup and seed

npm run db:migrate

npm run db:seed

Start server

npm start

Toggle usage of dataloader

Change shouldUseDataLoader in the config.js file to true or false, to use or not use the dataloader, respectively. nodemon will reload the server, and you will see a message in the console saying if the dataloader is being used.

Steps to observe N+1 problem

1. Send the following query from the graphql playground (http://localhost:4000/)

query	 {
  books {
    id
    title
    author {
      id
      name
    }
  }
}

2. While not using the dataloader, you will see 1 sql query executed to select all books, and N (25 in this example) sql queries executed to get all the authors.

3. Set shouldUseDataLoader in the config.js file to true and save the file, and resend the query in step 1. You will see 1 sql query executed to select all books, and 1 sql query executed to select all authors 🥳.

4. To see the an even more drastic example, send the following query with shouldUseDataLoader set to false:

query {
  authors {
    id
    name
    books {
      id
      title
    }
  }
}

5. Each author may have up to 5 books. Not a great deal of data, by any means, but in a scenario where every author had 5 books, 1 sql query would be executed to get all authors, and 125 sql queries would be executed to get all books ☠️.

Note

• built using node v12.13.1

• npx is used to run knex and nodemon so no global installation is required. (see: package.json)

• npm start script is configured for windows, to run on osx change the following:

  "set DEBUG=knex:query && npx nodemon src/index"

  to

  "DEBUG=knex:query npx nodemon src/index"