Federation

Federation offers a means of splitting your monolithic GraphQL server into independent microservices. It consists of two components: a gateway and one or more federated microservices. Each microservice holds part of the schema and the gateway merges the schemas into a single schema that can be consumed by the client.

To quote the Apollo docs, Federation is designed with these core principles:

• Building a graph should be declarative. With federation, you compose a graph declaratively from within your schema instead of writing imperative schema stitching code.
• Code should be separated by concern, not by types. Often no single team controls every aspect of an important type like a User or Product, so the definition of these types should be distributed across teams and codebases, rather than centralized.
• The graph should be simple for clients to consume. Together, federated services can form a complete, product-focused graph that accurately reflects how it’s being consumed on the client.
• It’s just GraphQL, using only spec-compliant features of the language. Any language, not just JavaScript, can implement federation.

warning Warning Federation currently does not support subscriptions.

In the following sections, we'll set up a demo application that consists of a gateway and two federated endpoints: Users service and Posts service.

Federation with Apollo

Start by installing the required dependencies:

$ npm install --save @apollo/subgraph

Schema first

The "User service" provides a simple schema. Note the @key directive: it instructs the Apollo query planner that a particular instance of User can be fetched if you specify its id. Also, note that we extend the Query type.

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

extend type Query {
  getUser(id: ID!): User
}

Resolver provides one additional method named resolveReference(). This method is triggered by the Apollo Gateway whenever a related resource requires a User instance. We'll see an example of this in the Posts service later. Please note that the method must be annotated with the @ResolveReference() decorator.

import { Args, Query, Resolver, ResolveReference } from '@nestjs/graphql';
import { UsersService } from './users.service';

@Resolver('User')
export class UsersResolver {
  constructor(private usersService: UsersService) {}

  @Query()
  getUser(@Args('id') id: string) {
    return this.usersService.findById(id);
  }

  @ResolveReference()
  resolveReference(reference: { __typename: string; id: string }) {
    return this.usersService.findById(reference.id);
  }
}

Finally, we hook everything up by registering the GraphQLModule passing the ApolloFederationDriver driver in the configuration object:

import {
  ApolloFederationDriver,
  ApolloFederationDriverConfig,
} from '@nestjs/apollo';
import { Module } from '@nestjs/common';
import { GraphQLModule } from '@nestjs/graphql';
import { UsersResolver } from './users.resolver';

@Module({
  imports: [
    GraphQLModule.forRoot<ApolloFederationDriverConfig>({
      driver: ApolloFederationDriver,
      typePaths: ['**/*.graphql'],
    }),
  ],
  providers: [UsersResolver],
})
export class AppModule {}

Code first

Start by adding some extra decorators to the User entity.

import { Directive, Field, ID, ObjectType } from '@nestjs/graphql';

@ObjectType()
@Directive('@key(fields: "id")')
export class User {
  @Field((type) => ID)
  id: number;

  @Field()
  name: string;
}

Resolver provides one additional method named resolveReference(). This method is triggered by the Apollo Gateway whenever a related resource requires a User instance. We'll see an example of this in the Posts service later. Please note that the method must be annotated with the @ResolveReference() decorator.

import { Args, Query, Resolver, ResolveReference } from '@nestjs/graphql';
import { User } from './user.entity';
import { UsersService } from './users.service';

@Resolver((of) => User)
export class UsersResolver {
  constructor(private usersService: UsersService) {}

  @Query((returns) => User)
  getUser(@Args('id') id: number): User {
    return this.usersService.findById(id);
  }

  @ResolveReference()
  resolveReference(reference: { __typename: string; id: number }): User {
    return this.usersService.findById(reference.id);
  }
}

Finally, we hook everything up by registering the GraphQLModule passing the ApolloFederationDriver driver in the configuration object:

import {
  ApolloFederationDriver,
  ApolloFederationDriverConfig,
} from '@nestjs/apollo';
import { Module } from '@nestjs/common';
import { UsersResolver } from './users.resolver';
import { UsersService } from './users.service'; // Not included in this example

@Module({
  imports: [
    GraphQLModule.forRoot<ApolloFederationDriverConfig>({
      driver: ApolloFederationDriver,
      autoSchemaFile: true,
    }),
  ],
  providers: [UsersResolver, UsersService],
})
export class AppModule {}

A working example is available here in code first mode and here in schema first mode.

Federated example: Posts

Post service is supposed to serve aggregated posts through the getPosts query, but also extend our User type with the user.posts field.

Schema first

"Posts service" references the User type in its schema by marking it with the extend keyword. It also declares one additional property on the User type (posts). Note the @key directive used for matching instances of User, and the @external directive indicating that the id field is managed elsewhere.

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

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

extend type Query {
  getPosts: [Post]
}

In the following example, the PostsResolver provides the getUser() method that returns a reference containing __typename and some additional properties your application may need to resolve the reference, in this case id. __typename is used by the GraphQL Gateway to pinpoint the microservice responsible for the User type and retrieve the corresponding instance. The "Users service" described above will be requested upon execution of the resolveReference() method.

import { Query, Resolver, Parent, ResolveField } from '@nestjs/graphql';
import { PostsService } from './posts.service';
import { Post } from './posts.interfaces';

@Resolver('Post')
export class PostsResolver {
  constructor(private postsService: PostsService) {}

  @Query('getPosts')
  getPosts() {
    return this.postsService.findAll();
  }

  @ResolveField('user')
  getUser(@Parent() post: Post) {
    return { __typename: 'User', id: post.userId };
  }
}

Lastly, we must register the GraphQLModule, similarly to what we did in the "Users service" section.

import {
  ApolloFederationDriver,
  ApolloFederationDriverConfig,
} from '@nestjs/apollo';
import { Module } from '@nestjs/common';
import { GraphQLModule } from '@nestjs/graphql';
import { PostsResolver } from './posts.resolver';

@Module({
  imports: [
    GraphQLModule.forRoot<ApolloFederationDriverConfig>({
      driver: ApolloFederationDriver,
      typePaths: ['**/*.graphql'],
    }),
  ],
  providers: [PostsResolvers],
})
export class AppModule {}

Code first

First, we will have to declare a class representing the User entity. Although the entity itself lives in another service, we will be using it (extending its definition) here. Note the @extends and @external directives.

import { Directive, ObjectType, Field, ID } from '@nestjs/graphql';
import { Post } from './post.entity';

@ObjectType()
@Directive('@extends')
@Directive('@key(fields: "id")')
export class User {
  @Field((type) => ID)
  @Directive('@external')
  id: number;

  @Field((type) => [Post])
  posts?: Post[];
}

Now let's create the corresponding resolver for our extension on the User entity, as follows:

import { Parent, ResolveField, Resolver } from '@nestjs/graphql';
import { PostsService } from './posts.service';
import { Post } from './post.entity';
import { User } from './user.entity';

@Resolver((of) => User)
export class UsersResolver {
  constructor(private readonly postsService: PostsService) {}

  @ResolveField((of) => [Post])
  public posts(@Parent() user: User): Post[] {
    return this.postsService.forAuthor(user.id);
  }
}

We also have to define the Post entity class:

import { Directive, Field, ID, Int, ObjectType } from '@nestjs/graphql';
import { User } from './user.entity';

@ObjectType()
@Directive('@key(fields: "id")')
export class Post {
  @Field((type) => ID)
  id: number;

  @Field()
  title: string;

  @Field((type) => Int)
  authorId: number;

  @Field((type) => User)
  user?: User;
}

And its resolver:

import { Query, Args, ResolveField, Resolver, Parent } from '@nestjs/graphql';
import { PostsService } from './posts.service';
import { Post } from './post.entity';
import { User } from './user.entity';

@Resolver((of) => Post)
export class PostsResolver {
  constructor(private readonly postsService: PostsService) {}

  @Query((returns) => Post)
  findPost(@Args('id') id: number): Post {
    return this.postsService.findOne(id);
  }

  @Query((returns) => [Post])
  getPosts(): Post[] {
    return this.postsService.all();
  }

  @ResolveField((of) => User)
  user(@Parent() post: Post): any {
    return { __typename: 'User', id: post.authorId };
  }
}

And finally, tie it together in a module. Note the schema build options, where we specify that User is an orphaned (external) type.

import {
  ApolloFederationDriver,
  ApolloFederationDriverConfig,
} from '@nestjs/apollo';
import { Module } from '@nestjs/common';
import { User } from './user.entity';
import { PostsResolvers } from './posts.resolvers';
import { UsersResolvers } from './users.resolvers';
import { PostsService } from './posts.service'; // Not included in example

@Module({
  imports: [
    GraphQLModule.forRoot<ApolloFederationDriverConfig>({
      driver: ApolloFederationDriver,
      autoSchemaFile: true,
      buildSchemaOptions: {
        orphanedTypes: [User],
      },
    }),
  ],
  providers: [PostsResolver, UsersResolver, PostsService],
})
export class AppModule {}

A working example is available here for the code first mode and here for the schema first mode.

Federated example: Gateway

Start by installing the required dependency:

$ npm install --save @apollo/gateway

The gateway requires a list of endpoints to be specified and it will auto-discover the corresponding schemas. Therefore the implementation of the gateway service will remain the same for both code and schema first approaches.

import { IntrospectAndCompose } from '@apollo/gateway';
import { ApolloGatewayDriver, ApolloGatewayDriverConfig } from '@nestjs/apollo';
import { Module } from '@nestjs/common';
import { GraphQLModule } from '@nestjs/graphql';

@Module({
  imports: [
    GraphQLModule.forRoot<ApolloGatewayDriverConfig>({
      driver: ApolloGatewayDriver,
      server: {
        // ... Apollo server options
        cors: true,
      },
      gateway: {
        supergraphSdl: new IntrospectAndCompose({
          subgraphs: [
            { name: 'users', url: 'http://user-service/graphql' },
            { name: 'posts', url: 'http://post-service/graphql' },
          ],
        }),
      },
    }),
  ],
})
export class AppModule {}

A working example is available here for the code first mode and here for the schema first mode.

Federation with Mercurius

Start by installing the required dependencies:

$ npm install --save @apollo/subgraph @nestjs/mercurius

info Note The @apollo/subgraph package is required to build a subgraph schema (buildSubgraphSchema, printSubgraphSchema functions).

Schema first

The "User service" provides a simple schema. Note the @key directive: it instructs the Mercurius query planner that a particular instance of User can be fetched if you specify its id. Also, note that we extend the Query type.

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

extend type Query {
  getUser(id: ID!): User
}

Resolver provides one additional method named resolveReference(). This method is triggered by the Mercurius Gateway whenever a related resource requires a User instance. We'll see an example of this in the Posts service later. Please note that the method must be annotated with the @ResolveReference() decorator.

import { Args, Query, Resolver, ResolveReference } from '@nestjs/graphql';
import { UsersService } from './users.service';

@Resolver('User')
export class UsersResolver {
  constructor(private usersService: UsersService) {}

  @Query()
  getUser(@Args('id') id: string) {
    return this.usersService.findById(id);
  }

  @ResolveReference()
  resolveReference(reference: { __typename: string; id: string }) {
    return this.usersService.findById(reference.id);
  }
}

Finally, we hook everything up by registering the GraphQLModule passing the MercuriusFederationDriver driver in the configuration object:

import {
  MercuriusFederationDriver,
  MercuriusFederationDriverConfig,
} from '@nestjs/mercurius';
import { Module } from '@nestjs/common';
import { GraphQLModule } from '@nestjs/graphql';
import { UsersResolver } from './users.resolver';

@Module({
  imports: [
    GraphQLModule.forRoot<MercuriusFederationDriverConfig>({
      driver: MercuriusFederationDriver,
      typePaths: ['**/*.graphql'],
      federationMetadata: true,
    }),
  ],
  providers: [UsersResolver],
})
export class AppModule {}

Code first

Start by adding some extra decorators to the User entity.

import { Directive, Field, ID, ObjectType } from '@nestjs/graphql';

@ObjectType()
@Directive('@key(fields: "id")')
export class User {
  @Field((type) => ID)
  id: number;

  @Field()
  name: string;
}

Resolver provides one additional method named resolveReference(). This method is triggered by the Mercurius Gateway whenever a related resource requires a User instance. We'll see an example of this in the Posts service later. Please note that the method must be annotated with the @ResolveReference() decorator.

import { Args, Query, Resolver, ResolveReference } from '@nestjs/graphql';
import { User } from './user.entity';
import { UsersService } from './users.service';

@Resolver((of) => User)
export class UsersResolver {
  constructor(private usersService: UsersService) {}

  @Query((returns) => User)
  getUser(@Args('id') id: number): User {
    return this.usersService.findById(id);
  }

  @ResolveReference()
  resolveReference(reference: { __typename: string; id: number }): User {
    return this.usersService.findById(reference.id);
  }
}

Finally, we hook everything up by registering the GraphQLModule passing the MercuriusFederationDriver driver in the configuration object:

import {
  MercuriusFederationDriver,
  MercuriusFederationDriverConfig,
} from '@nestjs/mercurius';
import { Module } from '@nestjs/common';
import { UsersResolver } from './users.resolver';
import { UsersService } from './users.service'; // Not included in this example

@Module({
  imports: [
    GraphQLModule.forRoot<MercuriusFederationDriverConfig>({
      driver: MercuriusFederationDriver,
      autoSchemaFile: true,
      federationMetadata: true,
    }),
  ],
  providers: [UsersResolver, UsersService],
})
export class AppModule {}

Federated example: Posts

Post service is supposed to serve aggregated posts through the getPosts query, but also extend our User type with the user.posts field.

Schema first

"Posts service" references the User type in its schema by marking it with the extend keyword. It also declares one additional property on the User type (posts). Note the @key directive used for matching instances of User, and the @external directive indicating that the id field is managed elsewhere.

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

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

extend type Query {
  getPosts: [Post]
}

In the following example, the PostsResolver provides the getUser() method that returns a reference containing __typename and some additional properties your application may need to resolve the reference, in this case id. __typename is used by the GraphQL Gateway to pinpoint the microservice responsible for the User type and retrieve the corresponding instance. The "Users service" described above will be requested upon execution of the resolveReference() method.

import { Query, Resolver, Parent, ResolveField } from '@nestjs/graphql';
import { PostsService } from './posts.service';
import { Post } from './posts.interfaces';

@Resolver('Post')
export class PostsResolver {
  constructor(private postsService: PostsService) {}

  @Query('getPosts')
  getPosts() {
    return this.postsService.findAll();
  }

  @ResolveField('user')
  getUser(@Parent() post: Post) {
    return { __typename: 'User', id: post.userId };
  }
}

Lastly, we must register the GraphQLModule, similarly to what we did in the "Users service" section.

import {
  MercuriusFederationDriver,
  MercuriusFederationDriverConfig,
} from '@nestjs/mercurius';
import { Module } from '@nestjs/common';
import { GraphQLModule } from '@nestjs/graphql';
import { PostsResolver } from './posts.resolver';

@Module({
  imports: [
    GraphQLModule.forRoot<MercuriusFederationDriverConfig>({
      driver: MercuriusFederationDriver,
      federationMetadata: true,
      typePaths: ['**/*.graphql'],
    }),
  ],
  providers: [PostsResolvers],
})
export class AppModule {}

Code first

First, we will have to declare a class representing the User entity. Although the entity itself lives in another service, we will be using it (extending its definition) here. Note the @extends and @external directives.

import { Directive, ObjectType, Field, ID } from '@nestjs/graphql';
import { Post } from './post.entity';

@ObjectType()
@Directive('@extends')
@Directive('@key(fields: "id")')
export class User {
  @Field((type) => ID)
  @Directive('@external')
  id: number;

  @Field((type) => [Post])
  posts?: Post[];
}

Now let's create the corresponding resolver for our extension on the User entity, as follows:

import { Parent, ResolveField, Resolver } from '@nestjs/graphql';
import { PostsService } from './posts.service';
import { Post } from './post.entity';
import { User } from './user.entity';

@Resolver((of) => User)
export class UsersResolver {
  constructor(private readonly postsService: PostsService) {}

  @ResolveField((of) => [Post])
  public posts(@Parent() user: User): Post[] {
    return this.postsService.forAuthor(user.id);
  }
}

We also have to define the Post entity class:

import { Directive, Field, ID, Int, ObjectType } from '@nestjs/graphql';
import { User } from './user.entity';

@ObjectType()
@Directive('@key(fields: "id")')
export class Post {
  @Field((type) => ID)
  id: number;

  @Field()
  title: string;

  @Field((type) => Int)
  authorId: number;

  @Field((type) => User)
  user?: User;
}

And its resolver:

import { Query, Args, ResolveField, Resolver, Parent } from '@nestjs/graphql';
import { PostsService } from './posts.service';
import { Post } from './post.entity';
import { User } from './user.entity';

@Resolver((of) => Post)
export class PostsResolver {
  constructor(private readonly postsService: PostsService) {}

  @Query((returns) => Post)
  findPost(@Args('id') id: number): Post {
    return this.postsService.findOne(id);
  }

  @Query((returns) => [Post])
  getPosts(): Post[] {
    return this.postsService.all();
  }

  @ResolveField((of) => User)
  user(@Parent() post: Post): any {
    return { __typename: 'User', id: post.authorId };
  }
}

And finally, tie it together in a module. Note the schema build options, where we specify that User is an orphaned (external) type.

import {
  MercuriusFederationDriver,
  MercuriusFederationDriverConfig,
} from '@nestjs/mercurius';
import { Module } from '@nestjs/common';
import { User } from './user.entity';
import { PostsResolvers } from './posts.resolvers';
import { UsersResolvers } from './users.resolvers';
import { PostsService } from './posts.service'; // Not included in example

@Module({
  imports: [
    GraphQLModule.forRoot<MercuriusFederationDriverConfig>({
      driver: MercuriusFederationDriver,
      autoSchemaFile: true,
      federationMetadata: true,
      buildSchemaOptions: {
        orphanedTypes: [User],
      },
    }),
  ],
  providers: [PostsResolver, UsersResolver, PostsService],
})
export class AppModule {}

Federated example: Gateway

The gateway requires a list of endpoints to be specified and it will auto-discover the corresponding schemas. Therefore the implementation of the gateway service will remain the same for both code and schema first approaches.

import {
  MercuriusGatewayDriver,
  MercuriusGatewayDriverConfig,
} from '@nestjs/mercurius';
import { Module } from '@nestjs/common';
import { GraphQLModule } from '@nestjs/graphql';

@Module({
  imports: [
    GraphQLModule.forRoot<MercuriusGatewayDriverConfig>({
      driver: MercuriusGatewayDriver,
      gateway: {
        services: [
          { name: 'users', url: 'http://user-service/graphql' },
          { name: 'posts', url: 'http://post-service/graphql' },
        ],
      },
    }),
  ],
})
export class AppModule {}

Federation 2

To quote the Apollo docs, Federation 2 improves developer experience from the original Apollo Federation (called Federation 1 in this doc), which is backward compatible with most original supergraphs.

warning Warning Mercurius doesn't fully support Federation 2. You can see the list of libraries that support Federation 2 here.

In the following sections, we'll upgrade the previous example to Federation 2.

Federated example: Users

One change in Federation 2 is that entities have no originating subgraph, so we don't need to extend Query anymore. For more detail please refer to the entities topic in Apollo Federation 2 docs.

Schema first

We can simply remove extend keyword from the schema.

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

type Query {
  getUser(id: ID!): User
}

Code first

To use Federation 2, we need to specify the federation version in autoSchemaFile option.

import {
  ApolloFederationDriver,
  ApolloFederationDriverConfig,
} from '@nestjs/apollo';
import { Module } from '@nestjs/common';
import { UsersResolver } from './users.resolver';
import { UsersService } from './users.service'; // Not included in this example

@Module({
  imports: [
    GraphQLModule.forRoot<ApolloFederationDriverConfig>({
      driver: ApolloFederationDriver,
      autoSchemaFile: {
        federation: 2,
      },
    }),
  ],
  providers: [UsersResolver, UsersService],
})
export class AppModule {}

Federated example: Posts

With the same reason as above, we don't need to extend User and Query anymore.

Schema first

We can simply remove extend and external directives from the schema

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

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

type Query {
  getPosts: [Post]
}

Code first

Since we don't extend User entity anymore, we can simply remove extends and external directives from User.

import { Directive, ObjectType, Field, ID } from '@nestjs/graphql';
import { Post } from './post.entity';

@ObjectType()
@Directive('@key(fields: "id")')
export class User {
  @Field((type) => ID)
  id: number;

  @Field((type) => [Post])
  posts?: Post[];
}

Also, similarly to the User service, we need to specify in the GraphQLModule to use Federation 2.

import {
  ApolloFederationDriver,
  ApolloFederationDriverConfig,
} from '@nestjs/apollo';
import { Module } from '@nestjs/common';
import { User } from './user.entity';
import { PostsResolvers } from './posts.resolvers';
import { UsersResolvers } from './users.resolvers';
import { PostsService } from './posts.service'; // Not included in example

@Module({
  imports: [
    GraphQLModule.forRoot<ApolloFederationDriverConfig>({
      driver: ApolloFederationDriver,
      autoSchemaFile: {
        federation: 2,
      },
      buildSchemaOptions: {
        orphanedTypes: [User],
      },
    }),
  ],
  providers: [PostsResolver, UsersResolver, PostsService],
})
export class AppModule {}