Apollo Federation 2 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 2 out of the box, that means that you can create services using Strawberry and federate them via Apollo Gateway or Apollo Router.

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

Our folder structure will look something like this:

my-app/
├─ books/
│  ├─ app.py
├─ reviews/
│  ├─ app.py

Books service

Let's create the books service, copy the following inside books/app.py

from typing import List

import strawberry


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


def get_all_books() -> List[Book]:
    return [Book(id=strawberry.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, enable_federation_2=True)

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 using 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.

Copy the following inside reviews/app.py:

from typing import List

import strawberry


@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(keys=["id"])
class Book:
    id: strawberry.ID
    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)


@strawberry.type
class Query:
    _hi: str = strawberry.field(resolver=lambda: "Hello World!")


schema = strawberry.federation.Schema(
    query=Query, types=[Book, Review], enable_federation_2=True
)

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

But we also have a Book which has 3 fields, id, reviews_count and reviews.

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.

We also defined a Query type that has a single field, _hi, which returns a string. This is required 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.

@strawberry.federation.type(keys=["id"])
class Book:
    id: strawberry.ID
    reviews: List[Review] = strawberry.field(resolver=get_reviews)

Let's run our services

Before starting Apollo Router to compose our schemas we need to run the services.

In two terminal windows, run the following commands:

cd books
strawberry server --port 3500 app

cd reviews
strawberry server --port 3000 app

Apollo Router

Now we have our services up and running, we need to configure a gateway to consume our services. Apollo provides a router that can be used for this.

Before continuing we'll need to install Apollo Router by following their installation guide and we'll need to install Apollo's CLI to compose the schema.

Create a file called supergraph.yaml with the following contents:

federation_version: 2
subgraphs:
  reviews:
    routing_url: http://localhost:3000
    schema:
      subgraph_url: http://localhost:3000

  books:
    routing_url: http://localhost:3500
    schema:
      subgraph_url: http://localhost:3500

This file will be used by rover to compose the schema, which can be done with the following command:

# Creates prod-schema.graphql or overwrites if it already exists
rover supergraph compose --config ./supergraph.yaml > supergraph-schema.graphql

Now that we have the composed schema, we can start the router.

./router --supergraph supergraph-schema.graphql

Now that router is running we can go to http://localhost:4000 and try to run the following query:

{
  allBooks {
    id
    reviewsCount
    reviews {
      body
    }
  }
}

if everything went well we should get the following result:

{
  "data": {
    "allBooks": [
      {
        "id": "1",
        "reviewsCount": 3,
        "reviews": [
          {
            "body": "A review for 1"
          },
          {
            "body": "A review for 1"
          },
          {
            "body": "A review for 1"
          }
        ]
      }
    ]
  }
}

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

Additional resources

Apollo Federation Quickstart