/api-graphql-mercurius

name: api-graphql-mercurius
description: GraphQL server for Fastify with Mercurius — loaders, subscriptions, federation, JIT compilation

GraphQL with Mercurius

> **Quick Guide:** Use Mercurius as a Fastify plugin for GraphQL APIs with built-in loader batching (solves N+1), JIT query compilation, subscriptions via WebSocket, and federation support. Register with `app.register(mercurius, { schema, resolvers, loaders })`. Loaders are Mercurius's killer feature: define them per-type to batch field resolution automatically. Use `jit: 1` to enable query compilation for production performance.

---

<critical_requirements>

CRITICAL: Before Using This Skill

**(You MUST define loaders for any field that fetches related data — loaders solve the N+1 problem automatically through batching)**

**(You MUST use named constants for all numeric values — JIT thresholds, query depth limits, port numbers)**

**(You MUST return an array from loaders matching the exact length and order of the `queries` parameter)**

**(You MUST use `fastify.graphql.pubsub.publish()` inside mutations to trigger subscriptions — not external pubsub directly)**

</critical_requirements>

---

**Auto-detection:** Mercurius, mercurius, app.graphql, fastify.graphql, mercurius loaders, mercurius subscription, pubsub.publish, pubsub.subscribe, @mercuriusjs/federation, @mercuriusjs/gateway, mercurius-codegen, MercuriusContext, graphql-jit, withFilter, preParsing, preValidation, preExecution, onResolution

**When to use:**

• Building GraphQL APIs on Fastify (Mercurius is Fastify-native)
• Need automatic batching/caching for N+1 query prevention (loader system)
• Want JIT query compilation for production performance
• Building federated GraphQL services with `@mercuriusjs/federation`
• Need real-time subscriptions via WebSocket with built-in pubsub
• Want GraphQL lifecycle hooks (preParsing, preValidation, preExecution, onResolution)

**When NOT to use:**

• Not using Fastify (Mercurius is Fastify-only)
• Need a framework-agnostic GraphQL server
• Building a standalone schema-first design tool (use the schema library directly)
• Simple REST endpoints without GraphQL requirements

**Key patterns covered:**

• Plugin registration with schema, resolvers, and loaders
• Loader system for batched data fetching (the core differentiator)
• JIT compilation configuration for production performance
• Subscriptions with built-in pubsub and `withFilter`
• Federation services and gateway composition
• TypeScript context typing with `MercuriusContext` augmentation
• GraphQL lifecycle hooks for cross-cutting concerns

---

**Detailed Resources:**

• [examples/core.md](examples/core.md) - Registration, resolvers, loaders, context, error handling, testing
• [examples/subscriptions.md](examples/subscriptions.md) - Pubsub, subscription resolvers, withFilter, WebSocket config
• [examples/federation.md](examples/federation.md) - Federated services, gateway, \_\_resolveReference as loader
• [reference.md](reference.md) - Decision frameworks, hook lifecycle, plugin options, anti-patterns

---

<philosophy>

Philosophy

**Fastify-native GraphQL.** Mercurius is not a standalone server bolted onto Fastify — it is a Fastify plugin that deeply integrates with Fastify's lifecycle, encapsulation model, and plugin system. This means your GraphQL API inherits Fastify's performance characteristics and plugin architecture naturally.

**Loaders over DataLoader.** Instead of requiring a separate DataLoader library, Mercurius provides a built-in loader system. Loaders are defined per-type/per-field and receive batched queries automatically. This is simpler than manually instantiating DataLoader instances per-request and is the primary mechanism for solving the N+1 problem.

**JIT for production.** Mercurius uses graphql-jit to compile frequently-executed queries into optimized V8 functions. After a configurable threshold of executions, subsequent runs of the same query bypass the GraphQL execution engine entirely — delivering significant performance gains for repeated queries.

**Federation as a plugin.** Federation support is split into separate packages (`@mercuriusjs/federation` for services, `@mercuriusjs/gateway` for composition), keeping the core library lean for non-federated use cases.

</philosophy>

---

<patterns>

Core Patterns

Pattern 1: Plugin Registration

Register Mercurius as a Fastify plugin with schema (SDL string), resolvers, and optional loaders.

import Fastify from "fastify";
import mercurius from "mercurius";

const JIT_THRESHOLD = 1;
const SERVER_PORT = 3000;

const app = Fastify({ logger: true });

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

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

  type Post {
    id: ID!
    title: String!
  }
`;

app.register(mercurius, {
  schema,
  resolvers,
  loaders,
  jit: JIT_THRESHOLD,
  graphiql: process.env.NODE_ENV !== "production",
});

**Why good:** JIT threshold as named constant, GraphiQL disabled in production, loaders passed at registration level alongside resolvers

> Full registration with context, error handling, and all options: [examples/core.md](examples/core.md)

---

Pattern 2: Loaders (N+1 Prevention)

Loaders are Mercurius's primary mechanism for batch data fetching. Define them per-type per-field. Each loader receives an array of `queries` (batched requests) and must return an array of results in the same order.

const loaders = {
  User: {
    async posts(
      queries: Array<{ obj: User; params: Record<string, unknown> }>,
      context: MercuriusContext,
    ) {
      const userIds = queries.map(({ obj }) => obj.id);
      const allPosts = await fetchPostsByUserIds(userIds);
      // Return array matching queries order
      return queries.map(({ obj }) =>
        allPosts.filter((post) => post.authorId === obj.id),
      );
    },
  },
};

**Why good:** Single bulk