Agent Skills: GraphQL Expert

GraphQL Expert

Expert guidance for GraphQL API development, schema design, resolvers, subscriptions, and best practices for building type-safe, efficient APIs.

Core Concepts

Schema Design

• Type system and schema definition language (SDL)
• Object types, interfaces, unions, and enums
• Input types and custom scalars
• Schema stitching and federation
• Modular schema organization

Resolvers

• Resolver functions and data sources
• Context and info arguments
• Field-level resolvers
• Resolver chains and data loaders
• Error handling in resolvers

Queries and Mutations

• Query design and naming conventions
• Mutation patterns and best practices
• Input validation and sanitization
• Pagination strategies (cursor-based, offset)
• Filtering and sorting

Subscriptions

• Real-time updates with WebSocket
• Subscription resolvers
• PubSub patterns
• Subscription filtering
• Connection management

Performance

• N+1 query problem and DataLoader
• Query complexity analysis
• Depth limiting and query cost
• Caching strategies (field-level, full response)
• Batching and deduplication

GraphQL Federation

Federated Schema

// Users service
import { buildSubgraphSchema } from '@apollo/subgraph';

const typeDefs = gql`
  extend schema @link(url: "https://specs.apollo.dev/federation/v2.3")

  type User @key(fields: "id") {
    id: ID!
    email: String!
    name: String!
  }

  type Query {
    user(id: ID!): User
    users: [User!]!
  }
`;

const resolvers = {
  User: {
    __resolveReference: async (reference, { dataSources }) => {
      return dataSources.userAPI.getUserById(reference.id);
    },
  },
  Query: {
    user: (_, { id }, { dataSources }) => dataSources.userAPI.getUserById(id),
    users: (_, __, { dataSources }) => dataSources.userAPI.getUsers(),
  },
};

// Posts service
const typeDefs = gql`
  extend schema @link(url: "https://specs.apollo.dev/federation/v2.3")

  type Post @key(fields: "id") {
    id: ID!
    title: String!
    content: String!
    author: User!
  }

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

  type Query {
    post(id: ID!): Post
    posts: [Post!]!
  }
`;

const resolvers = {
  Post: {
    author: (post) => ({ __typename: 'User', id: post.authorId }),
  },
  User: {
    posts: (user, _, { dataSources }) =>
      dataSources.postAPI.getPostsByAuthorId(user.id),
  },
};

// Gateway
import { ApolloGateway, IntrospectAndCompose } from '@apollo/gateway';

const gateway = new ApolloGateway({
  supergraphSdl: new IntrospectAndCompose({
    subgraphs: [
      { name: 'users', url: 'http://localhost:4001/graphql' },
      { name: 'posts', url: 'http://localhost:4002/graphql' },
    ],
  }),
});

const server = new ApolloServer({ gateway });

Anti-Patterns to Avoid

❌ Exposing internal IDs: Use opaque IDs or UUIDs ❌ Overly nested queries: Limit query depth ❌ No pagination: Always paginate lists ❌ Resolving in mutations: Keep mutations focused ❌ Exposing database schema directly: Design API-first ❌ No DataLoader: Leads to N+1 queries ❌ Generic error messages: Provide actionable errors ❌ No versioning strategy: Plan for schema evolution

Testing

import { ApolloServer } from '@apollo/server';
import { describe, it, expect } from 'vitest';

describe('GraphQL Server', () => {
  it('should fetch user by id', async () => {
    const server = new ApolloServer({ typeDefs, resolvers });

    const response = await server.executeOperation({
      query: `
        query GetUser($id: ID!) {
          user(id: $id) {
            id
            name
            email
          }
        }
      `,
      variables: { id: '1' },
    });

    expect(response.body.kind).toBe('single');
    expect(response.body.singleResult.data?.user).toEqual({
      id: '1',
      name: 'Alice',
      email: 'alice@example.com',
    });
  });

  it('should create post', async () => {
    const response = await server.executeOperation({
      query: `
        mutation CreatePost($input: CreatePostInput!) {
          createPost(input: $input) {
            id
            title
          }
        }
      `,
      variables: {
        input: {
          title: 'Test Post',
          content: 'Content',
        },
      },
    });

    expect(response.body.singleResult.data?.createPost).toHaveProperty('id');
  });
});

Reference Documentation

Detailed material lives alongside this skill and is read on demand:

• Common Patterns — Relay Cursor Pagination, File Upload
• Best Practices — Schema Design, Performance Optimization, Security
• Modern GraphQL Development — Apollo Server 4, DataLoader for N+1 Prevention, GraphQL Codegen, Error Handling, Authentication & Authorization, Subscriptions with WebSocket, GraphQL Client (Apollo Client), Query Complexity & Depth Limiting

Resources

• Apollo Server: https://www.apollographql.com/docs/apollo-server/
• GraphQL Spec: https://spec.graphql.org/
• DataLoader: https://github.com/graphql/dataloader
• GraphQL Code Generator: https://the-guild.dev/graphql/codegen
• GraphQL Tools: https://the-guild.dev/graphql/tools