Federated Subgraphs

Federated Subgraphs is an important part of building production-ready GraphQL systems. This lesson explains what federated subgraphs means, how it works, and how to apply it with practical examples you can reuse.

Federated Subgraphs Overview

Federated Subgraphs lets you structure GraphQL work so it stays readable, testable, and easy to scale. Instead of ad-hoc code, you follow a clear pattern that other developers can recognise immediately.

The key is to keep federated subgraphs focused and predictable. Start from the minimal example here, then layer in only the complexity your feature actually needs.

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

const typeDefs = gql`
  type User @key(fields: "id") {
    id: ID!
    name: String!
  }
`;

const schema = buildSubgraphSchema({ typeDefs, resolvers });

Federation composes multiple subgraph services into one unified GraphQL API.

Federated Subgraphs Example

const typeDefs = gql`
  type Query { hello: String! }
`;
const resolvers = { Query: { hello: () => 'world' } };
const server = new ApolloServer({ typeDefs, resolvers });

Start from a minimal Federated Subgraphs example and grow it only as needed.
Keep configuration explicit so Federated Subgraphs behaves the same in every environment.
Name things clearly so teammates understand your Federated Subgraphs at a glance.
Add tests around Federated Subgraphs early to lock in expected behaviour.

GraphQL Cheatsheet

Quick GraphQL reference related to federated subgraphs.

Concept	Example	Purpose
Schema	`type Query { user(id: ID!): User }`	Define the API shape
Resolver	`Query: { user: (_, { id }) => ... }`	Provide field data
Query	`query { user(id: 1) { name } }`	Read exactly what you need
Mutation	`mutation { createUser(input) { id } }`	Change data
Subscription	`subscription { postAdded { id } }`	Real-time updates
Context	`context: ({ req }) => ({ user })`	Auth and shared state
DataLoader	`loader.load(id)`	Batch to avoid N+1

How Federated Subgraphs Works in GraphQL

Federated Subgraphs fits into GraphQL's model of a single typed schema that clients query for exactly the data they need. The server resolves each requested field through resolver functions.

Federation composes multiple subgraph services into one unified GraphQL API.

The schema is the contract between client and server.
Resolvers fetch data field by field, including nested types.
Clients request only the fields they use, avoiding over-fetching.
Context carries auth and shared services into every resolver.

Practical Guidance for Federated Subgraphs

In production, federated subgraphs should be efficient and secure. Batch data access with DataLoader, guard resolvers with authorization, and limit query depth and complexity.

Concern	Recommendation
N+1 queries	Batch with DataLoader
Security	Auth in context, depth/complexity limits
Errors	Typed GraphQLError with extension codes
Performance	Cache and paginate large lists

Common Mistakes

Copying federated subgraphs snippets without understanding what each line does.
Skipping error handling and edge cases when wiring up federated subgraphs.
Leaving federated subgraphs untested, so regressions slip into production.
Over-engineering federated subgraphs before you actually need the extra flexibility.

Key Takeaways

Federated Subgraphs is a core part of working effectively with GraphQL.
Start small and keep federated subgraphs focused on a single responsibility.
Apply consistent patterns so federated subgraphs scales across your project.
Test and document federated subgraphs to keep it maintainable over time.

Pro Tip

When you get stuck on federated subgraphs, reduce it to the smallest reproducible example first — most GraphQL issues become obvious once the noise is gone.