Apollo Federation Guide

Apollo Federation allows you to combine multiple GraphQL APIs into one. This can be extremely useful when working with a service oriented architecture.

Strawberry supports Apollo Federation out of the box, that means that you can create services using Strawberry and federate them via Apollo Gateway.

Federated schema example

Let’s look at an example on how to implement Apollo Federation using Strawberry. Let's assume we have an application with two services that each expose a GraphQL API:

1. books: a service to manage all the books we have
2. reviews: a service to manage book reviews

Books service

Our book service might look something like this:

@strawberry.federation.type(keys=["id"])
class Book:
    id: strawberry.ID
    title: str

def get_all_books() -> List[Book]:
    return [Book(id=1, title="The Dark Tower")]

@strawberry.type
class Query:
    all_books: List[Book] = strawberry.field(resolver=get_all_books)

schema = strawberry.federation.Schema(query=Query)

We defined two types: Book and Query, where Query has only one field that allows us to fetch all the books.

Notice that the Book type is used the strawberry.federation.type decorator, as opposed to the normal strawberry.type, this new decorator extends the base one and allows us to define federation-specific attributes on the type.

Here, we are telling the federation system that the Book's id field is its uniquely-identifying key.

Reviews service

Now, let’s take a look at our review service: we want to define a type for a review but also extend the Book type to have a list of reviews.

@strawberry.type
class Review:
    id: int
    body: str

def get_reviews(root: "Book") -> List[Review]:
    return [
      Review(id=id_, body=f"A review for {root.id}")
      for id_ in range(root.reviews_count)
    ]

@strawberry.federation.type(extend=True, keys=["id"])
class Book:
    id: strawberry.ID = strawberry.federation.field(external=True)
    reviews_count: int
    reviews: List[Review] = strawberry.field(resolver=get_reviews)

    @classmethod
    def resolve_reference(cls, id: strawberry.ID):
        # here we could fetch the book from the database
        # or even from an API
        return Book(id=id, reviews_count=3)

Now things are looking more interesting; the Review type is a GraphQL type that holds the contents of the review.

We've also been able to extend the Book type by using again strawberry.federation.type, this time passing extend=True as an argument. This is important because we need to tell federation that we are extending a type that already exists, not creating a new one.

We have also declared three fields on Book, one of which is id which is marked as external with strawberry.federation.field(external=True). This tells federation that this field is not available in this service, and that it comes from another service.

The other fields are reviews (the list of Reviews for this book) and reviews_count (the number of reviews for this book).

Finally, we also have a class method, resolve_reference, that allows us to instantiate types when they are referred to by other services. The resolve_reference method is called when a GraphQL operation references an entity across multiple services. For example, when making this query:

{
  # query defined in the books service
  books {
    title
    # field defined in the reviews service
    reviews {
      body
    }
  }
}

resolve_reference is called with the id of the book for each book returned by the books service. Recall that above we defined the id field as the key for the Book type. In this example we are creating an instance of Book with the requested id and a fixed number of reviews.

If we were to add more fields to Book that were stored in a database, this would be where we could perform queries for these fields' values.

Now we need to do is to define a Query type, even if our service only has one type that is not used directly in any GraphQL query. This is because the GraphQL spec mandates that a GraphQL server defines a Query type, even if it ends up being empty/unused. Finally we also need to let Strawberry know about our Book and Review types. Since they are not reachable from the Query field itself, Strawberry won't be able to find them by default.

@strawberry.type
class Query:
    _service: Optional[str]

schema = strawberry.federation.Schema(query=Query, types=[Book, Review])

The gateway

Now we have our services up and running, we need to configure a gateway to consume our services. Apollo Gateway is the official gateway server for Apollo Federation. Here's an example on how to configure the gateway:

const { ApolloServer } = require("apollo-server");
const { ApolloGateway } = require("@apollo/gateway");

const gateway = new ApolloGateway({
  serviceList: [
    { name: "books", url: "http://localhost:8000" },
    { name: "reviews", url: "http://localhost:8080" },
  ],
});

const server = new ApolloServer({ gateway });

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

When running this example you'll be able to run query like the following:

{
  books {
    id
    reviewsCount
    reviews {
      body
    }
  }
}

We have provided a full example that you can run and tweak to play with Strawberry and Federation. The repo is available here: https://github.com/strawberry-graphql/federation-demo