GraphQL API Development for Web Application

Developing GraphQL API for Web Application

GraphQL is a query language for APIs and a runtime for executing them. The client requests exactly the fields it needs and gets precisely those—no more, no less. This solves two REST problems: over-fetching (extra data) and under-fetching (multiple requests for one screen).

Core Concepts

Schema-first: API is defined through types:

type Article {
  id: ID!
  title: String!
  body: String!
  author: User!
  tags: [Tag!]!
  createdAt: DateTime!
}

type Query {
  article(id: ID!): Article
  articles(filter: ArticleFilter, page: Int, limit: Int): ArticleConnection!
}

type Mutation {
  createArticle(input: CreateArticleInput!): Article!
  updateArticle(id: ID!, input: UpdateArticleInput!): Article!
}

type Subscription {
  articleUpdated(id: ID!): Article!
}

Queries and Fragments

# Client requests only needed fields
query ArticlePage($id: ID!) {
  article(id: $id) {
    title
    body
    author {
      name
      avatar
    }
    tags { name, slug }
  }
}

# Reusable fragments
fragment ArticleCard on Article {
  id, title, slug
  author { name }
  createdAt
}

query ArticleList {
  articles(limit: 10) {
    nodes { ...ArticleCard }
    pageInfo { hasNextPage, endCursor }
  }
}

N+1 Problem and DataLoader

GraphQL's main technical issue—N+1 queries. For a list of 20 articles with author field: 1 + 20 = 21 SQL queries.

Solution—DataLoader (Facebook, available for all languages):

const userLoader = new DataLoader(async (userIds: readonly string[]) => {
  const users = await db.user.findMany({
    where: { id: { in: [...userIds] } }
  });
  return userIds.map(id => users.find(u => u.id === id));
});

// In resolver
const articleResolver = {
  author: (article, _, { loaders }) => loaders.user.load(article.authorId),
};
// Now: 1 query for articles + 1 batch query for all authors

Implementation (Node.js + Apollo Server + Prisma)

import { ApolloServer } from '@apollo/server';
import { makeExecutableSchema } from '@graphql-tools/schema';

const typeDefs = gql`...`;

const resolvers = {
  Query: {
    article: async (_, { id }, { db }) =>
      db.article.findUnique({ where: { id } }),

    articles: async (_, { filter, page = 1, limit = 20 }, { db }) =>
      db.article.findMany({
        where: filter ? { status: filter.status } : undefined,
        skip: (page - 1) * limit,
        take: limit,
      }),
  },

  Mutation: {
    createArticle: async (_, { input }, { db, user }) => {
      if (!user) throw new GraphQLError('Unauthorized', {
        extensions: { code: 'UNAUTHENTICATED' }
      });
      return db.article.create({ data: { ...input, authorId: user.id } });
    },
  },
};

const server = new ApolloServer({ schema: makeExecutableSchema({ typeDefs, resolvers }) });

Authorization at Resolver Level

import { shield, rule, and } from 'graphql-shield';

const isAuthenticated = rule()((_, __, ctx) => !!ctx.user);
const isOwner = rule()(async (_, { id }, ctx) => {
  const article = await ctx.db.article.findUnique({ where: { id } });
  return article.authorId === ctx.user?.id;
});

const permissions = shield({
  Mutation: {
    createArticle: isAuthenticated,
    updateArticle: and(isAuthenticated, isOwner),
  },
});

Subscriptions

subscription CommentAdded($articleId: ID!) {
  commentAdded(articleId: $articleId) {
    id, body, author { name }
  }
}

Implementation via WebSocket (graphql-ws) + Redis Pub/Sub for scaling across instances.