github.com/mstephano/gqlgen-schemagen@v0.0.0-20230113041936-dd2cd4ea46aa/docs/content/recipes/federation.md

---
title: "Using Apollo federation gqlgen"
description: How federate many services into a single graph using Apollo
linkTitle: Apollo Federation
menu: { main: { parent: "recipes" } }
---

In this quick guide we are going to implement the example [Apollo Federation](https://www.apollographql.com/docs/apollo-server/federation/introduction/)
server in gqlgen. You can find the finished result in the [examples directory](https://github.com/mstephano/gqlgen-schemagen/tree/master/_examples/federation).

## Enable federation

Uncomment federation configuration in your `gqlgen.yml`

```yml
# Uncomment to enable federation
federation:
  filename: graph/federation.go
  package: graph
```

### Federation 2

If you are using Apollo's Federation 2 standard, your schema should automatically be upgraded so long as you include the required `@link` directive within your schema. If you want to force Federation 2 composition, the `federation` configuration supports a `version` flag to override that. For example:

```yml
federation:
  filename: graph/federation.go
  package: graph
  version: 2
```

## Create the federated servers

For each server to be federated we will create a new gqlgen project.

```bash
go run github.com/mstephano/gqlgen
```

Update the schema to reflect the federated example

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

extend type User @key(fields: "id") {
	id: ID! @external # External directive not required for key fields in federation v2
	reviews: [Review]
}

extend type Product @key(fields: "upc") {
	upc: String! @external # External directive not required for key fields in federation v2
	reviews: [Review]
}
```

and regenerate

```bash
go run github.com/mstephano/gqlgen
```

then implement the resolvers

```go
// These two methods are required for gqlgen to resolve the internal id-only wrapper structs.
// This boilerplate might be removed in a future version of gqlgen that can no-op id only nodes.
func (r *entityResolver) FindProductByUpc(ctx context.Context, upc string) (*model.Product, error) {
	return &model.Product{
		Upc: upc,
	}, nil
}

func (r *entityResolver) FindUserByID(ctx context.Context, id string) (*model.User, error) {
	return &model.User{
		ID: id,
	}, nil
}

// Here we implement the stitched part of this service, returning reviews for a product. Of course normally you would
// go back to the database, but we are just making some data up here.
func (r *productResolver) Reviews(ctx context.Context, obj *model.Product) ([]*model.Review, error) {
	switch obj.Upc {
	case "top-1":
		return []*model.Review{{
			Body: "A highly effective form of birth control.",
		}}, nil

	case "top-2":
		return []*model.Review{{
			Body: "Fedoras are one of the most fashionable hats around and can look great with a variety of outfits.",
		}}, nil

	case "top-3":
		return []*model.Review{{
			Body: "This is the last straw. Hat you will wear. 11/10",
		}}, nil

	}
	return nil, nil
}

func (r *userResolver) Reviews(ctx context.Context, obj *model.User) ([]*model.Review, error) {
	if obj.ID == "1234" {
		return []*model.Review{{
			Body: "Has an odd fascination with hats.",
		}}, nil
	}
	return nil, nil
}
```

> Note
>
> Repeat this step for each of the services in the apollo doc (accounts, products, reviews)

## Create the federation gateway

```bash
npm install --save @apollo/gateway apollo-server graphql
```

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

const gateway = new ApolloGateway({
	serviceList: [
		{ name: "accounts", url: "http://localhost:4001/query" },
		{ name: "products", url: "http://localhost:4002/query" },
		{ name: "reviews", url: "http://localhost:4003/query" },
	],
});

const server = new ApolloServer({
	gateway,

	subscriptions: false,
});

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

## Start all the services

In separate terminals:

```bash
go run accounts/server.go
go run products/server.go
go run reviews/server.go
node gateway/index.js
```

## Query the federated gateway

The examples from the apollo doc should all work, eg

```graphql
query {
	me {
		username
		reviews {
			body
			product {
				name
				upc
			}
		}
	}
}
```

should return

```json
{
	"data": {
		"me": {
			"username": "Me",
			"reviews": [
				{
					"body": "A highly effective form of birth control.",
					"product": {
						"name": "Trilby",
						"upc": "top-1"
					}
				},
				{
					"body": "Fedoras are one of the most fashionable hats around and can look great with a variety of outfits.",
					"product": {
						"name": "Trilby",
						"upc": "top-1"
					}
				}
			]
		}
	}
}
```