GRAPHQL Development

GraphQL Development

Comprehensive guidance on building production-quality GraphQL APIs, covering schema design, resolver implementation, the N+1 problem with DataLoader, subscriptions, Apollo Federation, caching strategies, security, and client-side patterns.

When to Use This Skill

Use this skill for:

• Designing GraphQL schemas (types, interfaces, unions, enums)
• Implementing resolvers with batching and caching
• Preventing N+1 query problems with DataLoader
• Adding real-time features with subscriptions
• Implementing pagination (Relay cursor, offset-based)
• Securing GraphQL APIs (authentication, authorization, query complexity)
• Setting up Apollo Federation for microservices
• Optimizing client-side data fetching (Apollo Client, urql)

Trigger phrases: "GraphQL", "schema", "resolver", "DataLoader", "subscription", "mutation", "query complexity", "N+1", "federation", "Apollo", "SDL", "GraphQL API"

What This Skill Does

Provides production-ready GraphQL patterns including:

• Schema Design: Types, interfaces, unions, enums, input types, custom scalars
• Resolvers: Field-level resolution, context injection, error handling
• Performance: DataLoader for batching, query complexity analysis, persisted queries
• Real-Time: Subscriptions via WebSocket, pub/sub patterns
• Federation: Apollo Federation gateway, subgraph design, entity resolution
• Security: Authentication, field-level authorization, depth limiting, rate limiting
• Clients: Apollo Client, urql, cache normalization

Instructions

Step 1: Design the Schema

Schema Design Principles:

1. Design for the client's needs, not the database schema
2. Use nullable fields by default; make non-null only when guaranteed
3. Prefer specific types over generic ones
4. Use interfaces for shared fields, unions for polymorphic returns
5. Suffix input types with "Input", payloads with "Payload"
6. Always return the modified object in mutation payloads

Complete Schema Example (SDL):

# ============================================
# Custom Scalars
# ============================================
scalar DateTime
scalar EmailAddress
scalar URL

# ============================================
# Enums
# ============================================
enum OrderStatus {
  PENDING
  CONFIRMED
  SHIPPED
  DELIVERED
  CANCELLED
}

enum SortDirection {
  ASC
  DESC
}

# ============================================
# Interfaces
# ============================================
interface Node {
  id: ID!
}

interface Timestamped {
  createdAt: DateTime!
  updatedAt: DateTime!
}

# ============================================
# Types
# ============================================
type User implements Node & Timestamped {
  id: ID!
  email: EmailAddress!
  name: String!
  avatar: URL
  orders(first: Int, after: String, status: OrderStatus): OrderConnection!
  createdAt: DateTime!
  updatedAt: DateTime!
}

type Product implements Node & Timestamped {
  id: ID!
  name: String!
  description: String
  price: Float!
  category: Category!
  images: [URL!]!
  inStock: Boolean!
  reviews(first: Int, after: String): ReviewConnection!
  averageRating: Float
  createdAt: DateTime!
  updatedAt: DateTime!
}

type Order implements Node & Timestamped {
  id: ID!
  user: User!
  items: [OrderItem!]!
  status: OrderStatus!
  total: Float!
  shippingAddress: Address!
  createdAt: DateTime!
  updatedAt: DateTime!
}

type OrderItem {
  product: Product!
  quantity: Int!
  unitPrice: Float!
  lineTotal: Float!
}

type Address {
  street: String!
  city: String!
  state: String
  postalCode: String!
  country: String!
}

type Category implements Node {
  id: ID!
  name: String!
  slug: String!
  products(first: Int, after: String): ProductConnection!
}

type Review implements Node & Timestamped {
  id: ID!
  author: User!
  product: Product!
  rating: Int!
  comment: String
  createdAt: DateTime!
  updatedAt: DateTime!
}

# ============================================
# Relay-Style Pagination
# ============================================
type PageInfo {
  hasNextPage: Boolean!
  hasPreviousPage: Boolean!
  startCursor: String
  endCursor: String
}

type ProductConnection {
  edges: [ProductEdge!]!
  pageInfo: PageInfo!
  totalCount: Int!
}

type ProductEdge {
  node: Product!
  cursor: String!
}

type OrderConnection {
  edges: [OrderEdge!]!
  pageInfo: PageInfo!
  totalCount: Int!
}

type OrderEdge {
  node: Order!
  cursor: String!
}

type ReviewConnection {
  edges: [ReviewEdge!]!
  pageInfo: PageInfo!
  totalCount: Int!
}

type ReviewEdge {
  node: Review!
  cursor: String!
}

# ============================================
# Inputs
# ============================================
input CreateOrderInput {
  items: [OrderItemInput!]!
  shippingAddress: AddressInput!
}

input OrderItemInput {
  productId: ID!
  quantity: Int!
}

input AddressInput {
  street: String!
  city: String!
  state: String
  postalCode: String!
  country: String!
}

input ProductFilterInput {
  categoryId: ID
  minPrice: Float
  maxPrice: Float
  inStock: Boolean
  search: String
}

input ProductSortInput {
  field: ProductSortField!
  direction: SortDirection!
}

enum ProductSortField {
  NAME
  PRICE
  CREATED_AT
  RATING
}

# ============================================
# Mutation Payloads (always return the object)
# ============================================
type CreateOrderPayload {
  order: Order
  errors: [UserError!]!
}

type UserError {
  field: String
  message: String!
  code: String!
}

# ============================================
# Queries, Mutations, Subscriptions
# ============================================
type Query {
  # Single resource (nullable for 404)
  user(id: ID!): User
  product(id: ID!): Product
  order(id: ID!): Order

  # Collections with filtering, sorting, pagination
  products(
    filter: ProductFilterInput
    sort: ProductSortInput
    first: Int
    after: String
  ): ProductConnection!

  # Viewer pattern (current authenticated user)
  viewer: User
}

type Mutation {
  createOrder(input: CreateOrderInput!): CreateOrderPayload!
  cancelOrder(id: ID!): CreateOrderPayload!
  addReview(productId: ID!, rating: Int!, comment: String): Review!
}

type Subscription {
  orderStatusChanged(orderId: ID!): Order!
  newReview(productId: ID!): Review!
}

Step 2: Implement Resolvers (Node.js)

Resolver Structure with Context:

// resolvers/index.js
const { GraphQLDateTime } = require("graphql-scalars");

const resolvers = {
  DateTime: GraphQLDateTime,

  Query: {
    viewer: (_parent, _args, context) => {
      // context.user is set by auth middleware
      if (!context.user) return null;
      return context.dataSources.users.getById(context.user.id);
    },

    product: (_parent, { id }, context) => {
      return context.dataSources.products.getById(id);
    },

    products: (_parent, { filter, sort, first = 20, after }, context) => {
      return context.dataSources.products.getConnection({
        filter,
        sort,
        first: Math.min(first, 100), // Cap page size
        after,
      });
    },
  },

  Mutation: {
    createOrder: async (_parent, { input }, context) => {
      if (!context.user) {
        return {
          order: null,
          errors: [{ message: "Authentication required", code: "UNAUTHENTICATED" }],
        };
      }

      try {
        const order = await context.dataSources.orders.create(context.user.id, input);
        // Publish event for subscriptions
        context.pubsub.publish(`ORDER_STATUS_${order.id}`, { orderStatusChanged: order });
        return { order, errors: [] };
      } catch (error) {
        return {
          order: null,
          errors: [{ message: error.message, code: "VALIDATION_ERROR", field: error.field }],
        };
      }
    },
  },

  // Field-level resolvers
  User: {
    orders: (user, { first = 10, after, status }, context) => {
      return context.dataSources.orders.getByUser(user.id, { first, after, status });
    },
  },

  Product: {
    category: (product, _args, context) => {
      // Uses DataLoader for batching (prevents N+1)
      return context.dataSources.categories.getById(product.categoryId);
    },
    reviews: (product, { first = 10, after }, context) => {
      return context.dataSources.reviews.getByProduct(product.id, { first, after });
    },
    averageRating: (product, _args, context) => {
      return context.dataSources.reviews.getAverageRating(product.id);
    },
  },

  Order: {
    user: (order, _args, context) => {
      return context.dataSources.users.getById(order.userId);
    },
    items: (order, _args, context) => {
      return context.dataSources.orderItems.getByOrder(order.id);
    },
  },

  OrderItem: {
    product: (item, _args, context) => {
      return context.dataSources.products.getById(item.productId);
    },
  },

  Subscription: {
    orderStatusChanged: {
      subscribe: (_parent, { orderId }, context) => {
        if (!context.user) throw new Error("Authentication required");
        return context.pubsub.asyncIterator(`ORDER_STATUS_${orderId}`);
      },
    },
    newReview: {
      subscribe: (_parent, { productId }, context) => {
        return context.pubsub.asyncIterator(`NEW_REVIEW_${productId}`);
      },
    },
  },
};

Step 3: Solve the N+1 Problem with DataLoader

DataLoader Setup:

// dataloaders.js
const DataLoader = require("dataloader");

function createLoaders(db) {
  return {
    // Batch: given [id1, id2, id3], run ONE query instead of three
    userById: new DataLoader(async (ids) => {
      const users = await db.query(
        "SELECT * FROM users WHERE id = ANY($1)",
        [ids]
      );
      // IMPORTANT: return results in the same order as the input IDs
      const userMap = new Map(users.map((u) => [u.id, u]));
      return ids.map((id) => userMap.get(id) || null);
    }),

    productById: new DataLoader(async (ids) => {
      const products = await db.query(
        "SELECT * FROM products WHERE id = ANY($1)",
        [ids]
      );
      const map = new Map(products.map((p) => [p.id, p]));
      return ids.map((id) => map.get(id) || null);
    }),

    categoryById: new DataLoader(async (ids) => {
      const categories = await db.query(
        "SELECT * FROM categories WHERE id = ANY($1)",
        [ids]
      );
      const map = new Map(categories.map((c) => [c.id, c]));
      return ids.map((id) => map.get(id) || null);
    }),

    // One-to-many DataLoader (orders by user)
    ordersByUserId: new DataLoader(async (userIds) => {
      const orders = await db.query(
        "SELECT * FROM orders WHERE user_id = ANY($1) ORDER BY created_at DESC",
        [userIds]
      );
      const grouped = new Map();
      for (const order of orders) {
        const list = grouped.get(order.user_id) || [];
        list.push(order);
        grouped.set(order.user_id, list);
      }
      return userIds.map((id) => grouped.get(id) || []);
    }),
  };
}

// Create loaders per-request (they cache within a single request)
// In Apollo Server:
const server = new ApolloServer({
  typeDefs,
  resolvers,
  context: ({ req }) => ({
    user: req.user,
    dataSources: createLoaders(db), // Fresh loaders per request
  }),
});

Python DataLoader (Strawberry + aiodataloader):

from aiodataloader import DataLoader
from typing import list, Optional

class UserLoader(DataLoader):
    async def batch_load_fn(self, user_ids: list[str]) -> list[Optional[dict]]:
        """Load multiple users in a single query."""
        users = await db.fetch_all(
            "SELECT * FROM users WHERE id = ANY(:ids)",
            {"ids": user_ids},
        )
        user_map = {u["id"]: u for u in users}
        return [user_map.get(uid) for uid in user_ids]

class ProductLoader(DataLoader):
    async def batch_load_fn(self, product_ids: list[str]) -> list[Optional[dict]]:
        products = await db.fetch_all(
            "SELECT * FROM products WHERE id = ANY(:ids)",
            {"ids": product_ids},
        )
        product_map = {p["id"]: p for p in products}
        return [product_map.get(pid) for pid in product_ids]

# Usage in resolvers
async def resolve_user(order, info):
    return await info.context["loaders"].user.load(order.user_id)

Step 4: Implement Subscriptions

WebSocket Subscription Server (Node.js):

const { createServer } = require("http");
const { WebSocketServer } = require("ws");
const { useServer } = require("graphql-ws/lib/use/ws");
const { ApolloServer } = require("@apollo/server");
const { expressMiddleware } = require("@apollo/server/express4");
const { makeExecutableSchema } = require("@graphql-tools/schema");
const { PubSub } = require("graphql-subscriptions");
const express = require("express");

const pubsub = new PubSub();
const schema = makeExecutableSchema({ typeDefs, resolvers });

const app = express();
const httpServer = createServer(app);

// WebSocket server for subscriptions
const wsServer = new WebSocketServer({
  server: httpServer,
  path: "/graphql",
});

const serverCleanup = useServer(
  {
    schema,
    context: async (ctx) => {
      // Authenticate WebSocket connections
      const token = ctx.connectionParams?.authToken;
      const user = token ? await verifyToken(token) : null;
      return { user, pubsub };
    },
    onConnect: async (ctx) => {
      console.log("Client connected for subscriptions");
    },
    onDisconnect: () => {
      console.log("Client disconnected");
    },
  },
  wsServer
);

const server = new ApolloServer({
  schema,
  plugins: [
    {
      async serverWillStart() {
        return {
          async drainServer() {
            await serverCleanup.dispose();
          },
        };
      },
    },
  ],
});

await server.start();
app.use("/graphql", expressMiddleware(server));
httpServer.listen(4000);

Step 5: Secure the API

Authentication and Authorization:

// Auth directive implementation
const { mapSchema, getDirective, MapperKind } = require("@graphql-tools/utils");
const { defaultFieldResolver } = require("graphql");

function authDirectiveTransformer(schema) {
  return mapSchema(schema, {
    [MapperKind.OBJECT_FIELD]: (fieldConfig) => {
      const authDirective = getDirective(schema, fieldConfig, "auth")?.[0];
      if (!authDirective) return fieldConfig;

      const { requires: role } = authDirective;
      const originalResolve = fieldConfig.resolve || defaultFieldResolver;

      fieldConfig.resolve = async function (source, args, context, info) {
        if (!context.user) {
          throw new Error("Authentication required");
        }
        if (role && !context.user.roles.includes(role)) {
          throw new Error(`Role '${role}' required`);
        }
        return originalResolve(source, args, context, info);
      };

      return fieldConfig;
    },
  });
}

// Schema directive usage:
// directive @auth(requires: Role = USER) on FIELD_DEFINITION
// type Query {
//   viewer: User @auth
//   adminStats: Stats @auth(requires: ADMIN)
// }

Query Complexity and Depth Limiting:

const { createComplexityLimitRule } = require("graphql-validation-complexity");
const depthLimit = require("graphql-depth-limit");

const server = new ApolloServer({
  schema,
  validationRules: [
    depthLimit(10), // Max query depth of 10
    createComplexityLimitRule(1000, {
      // Max complexity score of 1000
      scalarCost: 1,
      objectCost: 2,
      listFactor: 10,
      formatErrorMessage: (cost) =>
        `Query complexity ${cost} exceeds maximum of 1000`,
    }),
  ],
});

Persisted Queries (Automatic):

const { ApolloServer } = require("@apollo/server");
const {
  ApolloServerPluginPersistedQueries,
} = require("@apollo/server/plugin/persistedQueries");
const { KeyvAdapter } = require("@apollo/utils.keyvadapter");
const Keyv = require("keyv");

const server = new ApolloServer({
  schema,
  plugins: [
    ApolloServerPluginPersistedQueries({
      cache: new KeyvAdapter(new Keyv("redis://localhost:6379")),
      ttl: 86400, // 24 hours
    }),
  ],
});

Step 6: Set Up Apollo Federation

Subgraph Definition (Products Service):

# products/schema.graphql
extend schema @link(url: "https://specs.apollo.dev/federation/v2.0", import: ["@key", "@shareable"])

type Product @key(fields: "id") {
  id: ID!
  name: String!
  price: Float!
  category: Category!
  inStock: Boolean!
}

type Category @key(fields: "id") {
  id: ID!
  name: String!
  products(first: Int, after: String): ProductConnection!
}

type Query {
  product(id: ID!): Product
  products(filter: ProductFilterInput, first: Int, after: String): ProductConnection!
}

Subgraph Definition (Orders Service):

# orders/schema.graphql
extend schema @link(url: "https://specs.apollo.dev/federation/v2.0", import: ["@key", "@external"])

type Order @key(fields: "id") {
  id: ID!
  user: User!
  items: [OrderItem!]!
  status: OrderStatus!
  total: Float!
}

# Reference to User type defined in users subgraph
type User @key(fields: "id") {
  id: ID! @external
  orders(first: Int, after: String): OrderConnection!
}

type OrderItem {
  product: Product!
  quantity: Int!
  unitPrice: Float!
}

# Reference to Product type defined in products subgraph
type Product @key(fields: "id") {
  id: ID! @external
}

Apollo Router Configuration:

# router.yaml (Apollo Router - the federation gateway)
supergraph:
  listen: 0.0.0.0:4000

subgraphs:
  products:
    routing_url: http://products-service:4001/graphql
  orders:
    routing_url: http://orders-service:4002/graphql
  users:
    routing_url: http://users-service:4003/graphql

traffic_shaping:
  all:
    timeout: 30s
  subgraphs:
    products:
      timeout: 10s

telemetry:
  instrumentation:
    spans:
      mode: spec_compliant
  exporters:
    tracing:
      otlp:
        endpoint: http://otel-collector:4317

Best Practices

• Design schemas for clients, not for the database; think in terms of UI components
• Use DataLoader for every relationship resolver to prevent N+1 queries
• Return mutation payloads (not bare types) so errors can be communicated in-band
• Implement cursor-based pagination (Relay spec) for stable, efficient paging
• Set query depth and complexity limits to prevent abuse and denial-of-service
• Use persisted queries in production to reduce payload size and prevent arbitrary query injection
• Create fresh DataLoader instances per request; their cache is request-scoped
• Keep resolvers thin; delegate business logic to service/data layers
• Use subscriptions only for data the client is actively viewing; not for background sync
• Version schemas additively; deprecate fields with @deprecated instead of removing them

Common Patterns

Pattern 1: Relay Cursor Pagination Implementation

function buildConnection(rows, hasMore, getCursor) {
  const edges = rows.map((row) => ({
    node: row,
    cursor: getCursor(row),
  }));

  return {
    edges,
    pageInfo: {
      hasNextPage: hasMore,
      hasPreviousPage: false, // Simplified; track if "before" was used
      startCursor: edges[0]?.cursor || null,
      endCursor: edges[edges.length - 1]?.cursor || null,
    },
    totalCount: null, // Compute separately if needed (can be expensive)
  };
}

Pattern 2: Error Union Pattern

union CreateOrderResult = Order | ValidationError | InsufficientStockError

type ValidationError {
  field: String!
  message: String!
}

type InsufficientStockError {
  productId: ID!
  requested: Int!
  available: Int!
}

type Mutation {
  createOrder(input: CreateOrderInput!): CreateOrderResult!
}

Pattern 3: Viewer Pattern for Auth Context

type Query {
  # Viewer is the authenticated user; null if not logged in
  viewer: User
}

type User {
  # Private fields only accessible to the viewer
  email: EmailAddress!
  orders: OrderConnection!
  # Sensitive operations as mutations on the User type
  cart: Cart!
}

Quality Checklist

• Schema follows naming conventions (PascalCase types, camelCase fields)
• All relationship fields use DataLoader (no N+1 queries)
• Mutations return payload types with an errors field
• Pagination uses Relay cursor specification
• Query depth limit configured (recommended: 10)
• Query complexity limit configured (recommended: 1000)
• Authentication/authorization applied to sensitive fields
• Custom scalars used for emails, dates, URLs
• Deprecated fields annotated with @deprecated(reason: "...")
• Subscriptions authenticate on connection
• Error handling distinguishes user errors from system errors
• Schema documented with descriptions on types and fields

Related Skills

• api-documentation - Documenting GraphQL schemas
• performance-testing - Load testing GraphQL endpoints
• async-patterns - Subscription and real-time patterns
• security-review - GraphQL security assessment

Version: 1.0.0 Last Updated: March 2026

Iterative Refinement Strategy

This skill is optimized for an iterative approach:

1. Execute: Perform the core steps defined above.
2. Review: Critically analyze the output (coverage, quality, completeness).
3. Refine: If targets aren't met, repeat the specific implementation steps with improved context.
4. Loop: Continue until the definition of done is satisfied.