GraphQL Core Knowledge

Deep Knowledge: Use mcp__documentation__fetch_docs with technology: graphql for comprehensive documentation.

Schema Definition

type User {
  id: ID!
  name: String!
  email: String!
  posts: [Post!]!
  createdAt: DateTime!
}

type Post {
  id: ID!
  title: String!
  content: String
  author: User!
  published: Boolean!
}

type Query {
  user(id: ID!): User
  users(limit: Int, offset: Int): [User!]!
  post(id: ID!): Post
}

type Mutation {
  createUser(input: CreateUserInput!): User!
  updateUser(id: ID!, input: UpdateUserInput!): User!
  deleteUser(id: ID!): Boolean!
}

input CreateUserInput {
  name: String!
  email: String!
}

Resolvers

const resolvers = {
  Query: {
    user: (_, { id }, context) => {
      return context.db.users.findUnique({ where: { id } });
    },
    users: (_, { limit, offset }, context) => {
      return context.db.users.findMany({ take: limit, skip: offset });
    },
  },
  Mutation: {
    createUser: (_, { input }, context) => {
      return context.db.users.create({ data: input });
    },
  },
  User: {
    posts: (parent, _, context) => {
      return context.db.posts.findMany({ where: { authorId: parent.id } });
    },
  },
};

Queries

query GetUser($id: ID!) {
  user(id: $id) {
    id
    name
    email
    posts {
      title
      published
    }
  }
}

mutation CreateUser($input: CreateUserInput!) {
  createUser(input: $input) {
    id
    name
  }
}

When NOT to Use This Skill

• REST API design (use rest-api skill)
• OpenAPI/Swagger documentation (use openapi skill)
• tRPC type-safe APIs (use trpc skill)
• Generating GraphQL types from schema (use graphql-codegen skill)
• Simple CRUD operations where REST is sufficient

Best Practices

Anti-Patterns

Quick Troubleshooting

Production Readiness

Security Configuration

// Query depth limiting
import depthLimit from 'graphql-depth-limit';

const server = new ApolloServer({
  schema,
  validationRules: [depthLimit(10)], // Max 10 levels deep
});

// Query complexity limiting
import { createComplexityLimitRule } from 'graphql-validation-complexity';

const complexityLimitRule = createComplexityLimitRule(1000, {
  scalarCost: 1,
  objectCost: 10,
  listFactor: 10,
});

// Disable introspection in production
const server = new ApolloServer({
  introspection: process.env.NODE_ENV !== 'production',
  plugins: [
    process.env.NODE_ENV === 'production'
      ? ApolloServerPluginLandingPageDisabled()
      : ApolloServerPluginLandingPageLocalDefault(),
  ],
});

N+1 Query Prevention (DataLoader)

import DataLoader from 'dataloader';

// Create loader per request (in context)
function createLoaders(db: PrismaClient) {
  return {
    userLoader: new DataLoader<string, User>(async (ids) => {
      const users = await db.user.findMany({
        where: { id: { in: [...ids] } },
      });
      const userMap = new Map(users.map(u => [u.id, u]));
      return ids.map(id => userMap.get(id) || null);
    }),

    postsByUserLoader: new DataLoader<string, Post[]>(async (userIds) => {
      const posts = await db.post.findMany({
        where: { authorId: { in: [...userIds] } },
      });
      const postsByUser = new Map<string, Post[]>();
      posts.forEach(p => {
        const existing = postsByUser.get(p.authorId) || [];
        postsByUser.set(p.authorId, [...existing, p]);
      });
      return userIds.map(id => postsByUser.get(id) || []);
    }),
  };
}

// Use in resolvers
const resolvers = {
  User: {
    posts: (parent, _, context) => {
      return context.loaders.postsByUserLoader.load(parent.id);
    },
  },
};

Field-Level Authorization

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

const isAuthenticated = rule()((parent, args, context) => {
  return context.user !== null;
});

const isAdmin = rule()((parent, args, context) => {
  return context.user?.role === 'ADMIN';
});

const isOwner = rule()((parent, args, context) => {
  return parent.authorId === context.user?.id;
});

const permissions = shield({
  Query: {
    users: isAuthenticated,
    user: isAuthenticated,
  },
  Mutation: {
    deleteUser: and(isAuthenticated, or(isAdmin, isOwner)),
    updateUser: and(isAuthenticated, or(isAdmin, isOwner)),
  },
  User: {
    email: or(isAdmin, isOwner), // Only owner or admin can see email
  },
});

const server = new ApolloServer({
  schema: applyMiddleware(schema, permissions),
});

Rate Limiting

import { rateLimitDirective } from 'graphql-rate-limit-directive';

const { rateLimitDirectiveTypeDefs, rateLimitDirectiveTransformer } =
  rateLimitDirective();

const typeDefs = gql`
  ${rateLimitDirectiveTypeDefs}

  type Query {
    users: [User!]! @rateLimit(limit: 100, duration: 60)
  }

  type Mutation {
    createUser(input: CreateUserInput!): User!
      @rateLimit(limit: 10, duration: 60)
  }
`;

Error Handling

// Custom error formatting
const server = new ApolloServer({
  formatError: (formattedError, error) => {
    // Log original error
    logger.error(error);

    // Don't leak internal errors
    if (formattedError.extensions?.code === 'INTERNAL_SERVER_ERROR') {
      return {
        message: 'Internal server error',
        extensions: {
          code: 'INTERNAL_SERVER_ERROR',
        },
      };
    }

    // Remove stack trace in production
    if (process.env.NODE_ENV === 'production') {
      delete formattedError.extensions?.stacktrace;
    }

    return formattedError;
  },
});

Monitoring Metrics

Pagination (Relay-style)

type Query {
  users(first: Int, after: String, last: Int, before: String): UserConnection!
}

type UserConnection {
  edges: [UserEdge!]!
  pageInfo: PageInfo!
  totalCount: Int!
}

type UserEdge {
  cursor: String!
  node: User!
}

type PageInfo {
  hasNextPage: Boolean!
  hasPreviousPage: Boolean!
  startCursor: String
  endCursor: String
}

Request Logging

const server = new ApolloServer({
  plugins: [
    {
      async requestDidStart(requestContext) {
        const start = Date.now();

        return {
          async willSendResponse(ctx) {
            logger.info({
              operationName: ctx.request.operationName,
              query: ctx.request.query,
              variables: ctx.request.variables,
              duration: Date.now() - start,
              errors: ctx.errors?.length || 0,
            });
          },
        };
      },
    },
  ],
});

Checklist

• Query depth limiting
• Query complexity limiting
• Introspection disabled in production
• DataLoader for N+1 prevention
• Field-level authorization
• Rate limiting on mutations
• Custom error formatting
• Relay-style pagination
• Request logging with timing
• Input validation
• Persisted queries (optional)
• APQ (Automatic Persisted Queries) enabled

Code Generation

GraphQL Codegen generates TypeScript types and hooks from your GraphQL schema and operations.

Quick Setup

npm install -D @graphql-codegen/cli @graphql-codegen/client-preset

// codegen.ts
import { CodegenConfig } from '@graphql-codegen/cli';

const config: CodegenConfig = {
  schema: 'http://localhost:4000/graphql',
  documents: ['src/**/*.graphql', 'src/**/*.tsx'],
  generates: {
    './src/gql/': {
      preset: 'client',
      plugins: [],
    },
  },
};

export default config;

Generated Usage

import { graphql } from '@/gql';
import { useQuery } from '@tanstack/react-query';

const UserQuery = graphql(`
  query GetUser($id: ID!) {
    user(id: $id) {
      id
      name
      email
    }
  }
`);

function UserProfile({ id }: { id: string }) {
  const { data } = useQuery({
    queryKey: ['user', id],
    queryFn: () => request(endpoint, UserQuery, { id }),
  });

  return <div>{data?.user?.name}</div>;
}

Related Skills

Reference Documentation

• DataLoader
• Authentication
• Code Generation