Introduction to Apollo Federation

To get the most out of GraphQL, your organization should expose a single graph that provides a unified interface for querying any combination of your backing data sources. However, it can be challenging to represent an enterprise-scale graph with a single, monolithic GraphQL server.

To remedy this, you can use Apollo Federation to divide your graph's implementation across multiple back-end services (called subgraphs):

Unlike other distributed GraphQL architectures (such as schema stitching), Apollo Federation uses a declarative programming model that enables each subgraph to implement only the part of your composed supergraph that it's responsible for.

Apollo Federation also supports a free managed mode with Apollo Studio, which enables you to add, remove, and refactor your subgraphs without requiring any downtime for your production graph.

Architecture

An Apollo Federation architecture consists of:

• A collection of subgraphs (usually represented by different back-end services) that each define a distinct GraphQL schema
• A gateway that uses a supergraph schema (composed from all subgraph schemas) to execute queries across multiple subgraphs

Apollo Server provides libraries for acting both as a subgraph and as a gateway, but these components can be implemented in any language and framework.

Apollo Federation does not currently support GraphQL subscription operations.

The following presentation by Mandi Wise further describes the architecture of Apollo Federation and walks through implementing a federated graph:

Design principles

Incremental adoption

Like the rest of the Apollo platform, Apollo Federation can (and should) be adopted incrementally:

• If you currently use a monolithic GraphQL server, you can break its functionality out one service at a time.
• If you currently use a different federated architecture (such as schema stitching), you can add federation support to your existing services one at a time.

In both of these cases, all of your clients will continue to work throughout your incremental migration. In fact, clients have no way to distinguish between different graph implementations.

Separation of concerns

Apollo Federation encourages a design principle called separation of concerns. This enables different teams to work on different products and features within a single graph, without interfering with each other.

Limitations of type-based separation

When considering how to split a single GraphQL schema across multiple subgraphs, it seems straightforward to divide schemas up by type. For example, a users subgraph would define the entirety of a User type, the products subgraph would define a Product type, and so on:

Although this separation looks clean, it quickly causes issues. Specifically, a particular feature (or concern) usually spans multiple types.

Consider the recentPurchases field of the User type in the above schema. Even though this field is a member of the User type, a list of Products should probably be populated by the products subgraph, not the users subgraph.

By defining the recentPurchases field in the products subgraph instead:

• The subgraph that defines the field is also the subgraph that knows how to populate the field. The users subgraph might not even have access to the back-end data store that contains product data.
• The team that manages product data can contain all product-related logic in a single subgraph that they own unilaterally.

Concern-based separation

The following schema uses Apollo Federation to divide the same set of types and fields across the same three subgraphs:

The difference is that now, each subgraph defines the types and fields that it is capable of (and should be responsible for) populating from its back-end data store.

The result is the best of both worlds: an implementation that keeps all the code for a given feature in a single subgraph and separated from unrelated concerns, and a product-centric schema with rich types that reflects the natural way an application developer would want to consume the graph.

Federated schemas

A federated graph uses multiple "types" of GraphQL schemas:

• Subgraph schemas. Each of your subgraphs has a distinct schema that indicates which types and fields of your composed supergraph it's responsible for resolving.
• Supergraph schema. This schema is the result of performing composition on your collection of subgraph schemas. It combines all of the types and fields from your subgraph schemas, plus some federation-specific directives that tell your gateway which subgraphs are responsible for resolving which fields.
• API schema. This schema is like the supergraph schema, but it omits types, fields, and directives that are considered "machinery" and are not part of your public API (this includes federation-specific directives).
  • This is the schema that your gateway exposes to your GraphQL API's consumers, who don't need to know any internal implementation details about your graph.

Let's look at an example!

Subgraph schemas

Below, we define the schemas for three subgraphs in a basic e-commerce application. Each subgraph is implemented as a standalone back-end service:

extend type Query {
  me: User
}

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

extend type Query {
  topProducts(first: Int = 5): [Product]
}

type Product @key(fields: "upc") {
  upc: String!
  name: String!
  price: Int
}

type Review {
  body: String
  author: User @provides(fields: "username")
  product: Product
}

extend type User @key(fields: "id") {
  id: ID! @external
  username: String! @external
  reviews: [Review]
}

extend type Product @key(fields: "upc") {
  upc: String! @external
  reviews: [Review]
}

These subgraph schemas illustrate several important conventions of Apollo Federation:

• A subgraph can reference a type that's defined by another subgraph. For example, the Review type includes a product field of type Product, even though the Product type is defined in a different subgraph.

• A subgraph can also extend a type that's defined by another subgraph. For example, the reviews subgraph extends the User type by adding a reviews field to it.

• A subgraph must add the @key directive to an object type's definition in order for other subgraphs to be able to reference or extend that type. This directive makes an object type an entity.

Supergraph schema

To create our supergraph schema, we perform composition on our collection of subgraph schemas. With managed federation, Apollo performs composition automatically whenever one of your subgraphs registers an updated schema.

We can also perform composition manually with the Rover CLI:

rover supergraph compose --config ./supergraph.yaml

For an example configuration file, see the Rover docs.

This outputs the following supergraph schema that our gateway can use to route queries to the correct subgraphs:

As you can see, the supergraph schema includes a lot of federation-specific additions! These additions are used only by the gateway, and you'll never need to add them manually.

API schema

The gateway uses its supergraph schema to produce an API schema, which is what's exposed to clients as your actual GraphQL API. This schema cleanly and logically represents the combination of your subgraph schemas:

Gateway example

You provide a composed supergraph schema to the ApolloGateway constructor, like so:

const supergraphSdl = readFileSync('./supergraph.graphql').toString();
const gateway = new ApolloGateway({
  supergraphSdl});

const server = new ApolloServer({ gateway });
server.listen();

That’s it! With Apollo Federation, resolvers live in your subgraphs. The gateway serves only to plan and execute GraphQL operations across those subgraphs.

Now we can execute GraphQL operations against our gateway just as if it were implemented as a single, monolithic GraphQL server:

# A query that the gateway resolves by calling all three services
query GetCurrentUserReviews {
  me {
    username
    reviews {
      body
      product {
        name
        upc
      }
    }
  }
}

Managed federation

In addition to providing its supergraph schema on startup, Apollo Gateway can operate in managed federation mode, where Apollo Studio acts as the source of truth for each subgraph's schema.

This mode enables multiple teams working on a graph to coordinate when and how underlying subgraphs change. It's recommended for all federated graphs. For more information, read Managed federation overview.

Apollo Server libraries

Apollo Server supports Apollo Federation via two open-source extension libraries:

• @apollo/subgraph provides primitives that subgraphs use to make their individual GraphQL schemas composable.
• @apollo/gateway enables you to set up an instance of Apollo Server as a gateway that distributes incoming GraphQL operations across one or more subgraphs.

Ready to try out Apollo Federation? Jump into the Quickstart!