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Subgraphs


This article describes how to create a subgraph for a federated 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:

index.js
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/subgraph package in our project:

npm install @apollo/subgraph

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:

index.js
const { ApolloServer, gql } = require('apollo-server');
const { buildSubgraphSchema } = require('@apollo/subgraph');

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:

index.js
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 subgraph schema

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

index.js
const server = new ApolloServer({
  schema: buildSubgraphSchema([{ 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 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):

index.js
const { ApolloServer, gql } = require('apollo-server');
const { buildSubgraphSchema } = require('@apollo/subgraph');

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: buildSubgraphSchema([{ typeDefs, resolvers }])
});

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

Securing your subgraphs

Because of the power and flexibility of the Query._entities and Query._service fields, 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:

Gateway
Users
subgraph
Products
subgraph
Reviews
subgraph
Web app
iOS app

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.

Subgraph-specific symbols

When you generate your subgraph schema, some federation-specific definitions are automatically added to it. In addition to directive definitions like @key, the most useful of these definitions for debugging are two fields of the Query type: _service and _entities:

type Query {
  # ...your field definitions...

  # Added automatically
  _service: _Service!
  _entities(representations: [_Any!]!): [_Entity]!
}

Query._service

This field returns a _Service object with one field of its own: sdl. You can query it like so:

query GetSubgraphSchema {
  _service {
    sdl
  }
}

The sdl field returns your subgraph's schema as an SDL string. This field has a couple of important differences from a standard introspection query that a tool like Apollo Sandbox uses:

  • Unlike introspection, the sdl field is not disabled by default in production environments (this is safe if you properly secure your subgraph).
  • Unlike introspection, the sdl field's returned string includes federation-specific directives like @key.

Whenever your gateway needs to fetch a subgraph's schema (this occurs only if your gateway uses the serviceList option), it uses this field instead of an introspection query so it can obtain federation-specific details.

Query._entities

Learn about entities if you haven't yet.

This field takes a list of entity representations and returns a list of corresponding entities.

Whenever one subgraph references another subgraph's entity, it uses an entity representation to do so. An entity representation is an object that includes only the entity's __typename and the fields in the entity's @key.

_entities(representations: [_Any!]!): [_Entity]!
  • The _Any type is a special scalar that enables you to provide entity representations of any valid shape.
  • The _Entity type is a generated union type that includes every entity defined in your subgraph's schema.

You can query this field like so, providing a value for the $representations variable as shown:

Query
query ($representations: [_Any!]!) {
  _entities(representations: $representations) {
    ... on User {
      id
      username
    }
  }
}
Variable
{
  "representations": [
    {
      "__typename": "User",
      "id": "5"
    }
  ]
}

Using in tests and debugging

If you're writing integration tests for your subgraph, you can test the return value of the _entities field for various entity representations that your other subgraphs use.

If you're developing your subgraph in your local environment, you can mock the return value of the _entities field for your other subgraphs so you don't have to connect those subgraphs to their respective data stores.

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 { buildSubgraphSchema } = require ('@apollo/subgraph')

// 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 = buildSubgraphSchema([{ 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.

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