Subgraphs

Implementing services are now known as subgraphs.

If you find an outdated use of "implementing service," please submit article feedback or a pull request using the links on the right.

This article describes how to create a subgraph (formerly known as an implementing service) for a federated data graph using Apollo Server.

Defining a subgraph

To be part of a supergraph, a subgraph must conform to the Apollo Federation specification, which exposes the subgraph's capabilities to the gateway, as well as to tools like Apollo Studio.

Converting an existing monolithic graph into a single subgraph is a convenient first step in building a federated supergraph. To start, here's a non-federated Apollo Server setup:

const { ApolloServer, gql } = require('apollo-server');

const typeDefs = gql`
  type Query {
    me: User
  }

  type User {
    id: ID!
    username: String
  }
`;

const resolvers = {
  Query: {
    me() {
      return { id: "1", username: "@ava" }
    }
  }
};

const server = new ApolloServer({
  typeDefs,
  resolvers,
});

server.listen(4001).then(({ url }) => {
    console.log(`🚀 Server ready at ${url}`);
});

This should look familiar if you've set up Apollo Server before. If it doesn't, we recommend you familiarize yourself with the basics before jumping into federation.

Now, let's convert this to a subgraph. The first step is to install the @apollo/federation package in our project:

npm install @apollo/federation

Defining an entity

As part of our federated architecture, we want other subgraphs to be able to extend the User type this subgraph defines. To enable this, we add the @key directive to the User type's definition to designate it as an entity:

const { ApolloServer, gql } = require('apollo-server');
const { buildFederatedSchema } = require('@apollo/federation');

const typeDefs = gql`
  type Query {
    me: User
  }

  type User @key(fields: "id") {
    id: ID!
    username: String
  }
`;

The @key directive tells other subgraphs which field(s) of the User type to use to uniquely identify a particular instance. In this case, subgraphs should use the single field id.

Next, we add a reference resolver for the User type. A reference resolver tells the gateway how to fetch an entity by its @key fields:

const resolvers = {
  Query: {
    me() {
      return { id: "1", username: "@ava" }
    }
  },
  User: {
    __resolveReference(user, { fetchUserById }){
      return fetchUserById(user.id)
    }
  }
};

(This example requires defining the fetchUserById function to obtain the appropriate User from our backing data store.)

Learn more about entities

Generating a federated schema

Finally, we use the buildFederatedSchema function from the @apollo/federation package to augment our schema definition with federation support. We provide the result of this function to the ApolloServer constructor:

const server = new ApolloServer({
  schema: buildFederatedSchema([{ typeDefs, resolvers }])
});

server.listen(4001).then(({ url }) => {
    console.log(`🚀 Server ready at ${url}`);
});

The server is now ready to act as a subgraph in a federated data graph!

Combined example

Here are the snippets above combined (again, note that for this sample to be complete, you must define the fetchUserById function for your data source):

const { ApolloServer, gql } = require('apollo-server');
const { buildFederatedSchema } = require('@apollo/federation');

const typeDefs = gql`
  type Query {
    me: User
  }

  type User @key(fields: "id") {
    id: ID!
    username: String
  }
`;

const resolvers = {
  Query: {
    me() {
      return { id: "1", username: "@ava" }
    }
  },
  User: {
    __resolveReference(user, { fetchUserById }){
      return fetchUserById(user.id)
    }
  }
}

const server = new ApolloServer({
  schema: buildFederatedSchema([{ typeDefs, resolvers }])
});

server.listen(4001).then(({ url }) => {
    console.log(`🚀 Server ready at ${url}`);
});

Defining custom directives

The method for defining custom directives differs slightly in Apollo Federation.

Without Apollo Federation, you provide your directive definitions to the constructor of ApolloServer in the schemaDirectives argument, like so:

With Apollo Federation, you instead call SchemaDirectiveVisitor.visitSchemaDirectives, passing in your schema and your directives, before you provide your schema to the constructor of ApolloServer:

const { ApolloServer, gql, SchemaDirectiveVisitor } = require('apollo-server');
const { buildFederatedSchema } = require ('@apollo/federation')

// typeDefs and resolvers defined here

class DeprecatedDirective extends SchemaDirectiveVisitor {
  public visitFieldDefinition(field: GraphQLField<any, any>) {
    field.isDeprecated = true;
    field.deprecationReason = this.args.reason;
  }
}

const directives = {
  deprecated: DeprecatedDirective
};
let schema = buildFederatedSchema([{ typeDefs, resolvers }]);

SchemaDirectiveVisitor.visitSchemaDirectives(schema, directives);

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

Also make sure to read about the gateway's support for custom directives.

Securing subgraphs

Because of the power and flexibility of Apollo Federation's _entities field, your subgraphs should not be directly accessible by clients. Instead, only your gateway should have access to your subgraphs. Clients then communicate with the gateway:

Make sure to implement any necessary firewall rules, access control lists, or other measures to ensure that individual subgraphs can be accessed only via the gateway.