apollo-federation

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This gem extends the GraphQL Ruby gem to add support for creating an Apollo Federation schema.

Installation

Add this line to your application's Gemfile:

gem 'apollo-federation'

And then execute:

$ bundle

Or install it yourself as:

$ gem install apollo-federation

Getting Started

Include the ApolloFederation::Field module in your base field class:

require 'apollo-federation'

class BaseField < GraphQL::Schema::Field
  include ApolloFederation::Field
end

Include the ApolloFederation::Object module in your base object class:

class BaseObject < GraphQL::Schema::Object
  include ApolloFederation::Object

  field_class BaseField
end

Include the ApolloFederation::Interface module in your base interface module:

module BaseInterface
  include GraphQL::Schema::Interface
  include ApolloFederation::Interface

  field_class BaseField
end

Finally, include the ApolloFederation::Schema module in your schema:

class MySchema < GraphQL::Schema
  include ApolloFederation::Schema
end

Example

The example folder contains a Ruby implementation of Apollo's federation-demo. To run it locally, install the Ruby dependencies:

$ bundle

Install the Node dependencies:

$ yarn

Start all of the services:

$ yarn start-services

Start the gateway:

$ yarn start-gateway

This will start up the gateway and serve it at http://localhost:5000.

Usage

The API is designed to mimic the API of Apollo's federation library. It's best to read and understand the way federation works, in general, before attempting to use this library.

Extending a type

Apollo documentation

Call extend_type within your class definition:

class User < BaseObject
  extend_type
end

The @key directive

Apollo documentation

Call key within your class definition:

class User < BaseObject
  key fields: :id
end

Compound keys are also supported:

class User < BaseObject
  key fields: [:id, { organization: :id }]
end

See field set syntax for more details on the format of the fields option.

The @external directive

Apollo documentation

Pass the external: true option to your field definition:

class User < BaseObject
  field :id, ID, null: false, external: true
end

The @requires directive

Apollo documentation

Pass the requires: option to your field definition:

class Product < BaseObject
  field :price, Int, null: true, external: true
  field :weight, Int, null: true, external: true
  field :shipping_estimate, Int, null: true, requires: { fields: [:price, :weight] }
end

See field set syntax for more details on the format of the fields option.

The @provides directive

Apollo documentation

Pass the provides: option to your field definition:

class Review < BaseObject
  field :author, 'User', null: true, provides: { fields: :username }
end

See field set syntax for more details on the format of the fields option.

Field set syntax

Field sets can be either strings encoded with the Apollo Field Set syntax or arrays, hashes and snake case symbols that follow the graphql-ruby conventions:

# Equivalent to the "organizationId" field set
:organization_id

# Equivalent to the "price weight" field set
[:price, :weight]

# Equivalent to the "id organization { id }" field set
[:id, { organization: :id }]

Reference resolvers

Apollo documentation

Define a resolve_reference class method on your object. The method will be passed the reference from another service and the context for the query.

class User < BaseObject
  def self.resolve_reference(reference, context)
    USERS.find { |user| user[:id] == reference[:id] }
  end
end

Tracing

To support federated tracing:

  1. Add use ApolloFederation::Tracing to your schema class.
  2. Change your controller to add tracing_enabled: true to the execution context based on the presence of the "include trace" header: ruby def execute # ... context = { tracing_enabled: ApolloFederation::Tracing.should_add_traces(headers) } # ... end

Exporting the Federated SDL

When using tools like rover for schema validation, etc., add a Rake task that prints the Federated SDL to a file:

namespace :graphql do
  namespace :federation do
    task :dump do
      File.write("schema.graphql", MySchema.federation_sdl)
    end
  end
end

Example validation check with Rover and Apollo Studio:

bin/rake graphql:federation:dump
rover subgraph check [email protected] --name mysubgraph --schema schema.graphql

Testing the federated schema

This library does not include any testing helpers currently. A federated service receives subgraph queries from the Apollo Gateway via the _entities field and that can be tested in a request spec.

With Apollo Gateway setup to hit your service locally or by using existing query logs, you can retrieve the generated _entities queries.

For example, if you have a blog service that exposes posts by a given author, the query received by the service might look like this.

query($representations: [_Any!]!) {
  _entities(representations: $representations) {
    ... on BlogPost {
      id
      title
      body
    }
  }
}

Where $representations is an array of entity references from the gateway.

{
  "representations": [
    {
      "__typename": "BlogPost",
      "id": 1
    },
    {
      "__typename": "BlogPost",
      "id": 2
    }
  ]
}

Using RSpec as an example, a request spec for this query.

it "resolves the blog post entities" do
  blog_post = BlogPost.create!(attributes)

  query = <<~GRAPHQL
    query($representations: [_Any!]!) {
      _entities(representations: $representations) {
        ... on BlogPost {
          id
          title
          body
        }
      }
    }
  GRAPHQL

  variables = { representations: [{ __typename: "BlogPost", id: blog_post.id }] }

  result = Schema.execute(query, variables: variables)

  expect(result.dig("data", "_entities", 0, "id")).to eq(blog_post.id)
end

See discussion at #74 and an internal spec that resolves _entities for more details.

Known Issues and Limitations

  • For GraphQL older than 1.12, the interpreter runtime has to be used.
  • Does not add directives to the output of Schema.to_definition. Since graphql-ruby doesn't natively support schema directives, the directives will only be visible to the Apollo Gateway through the Query._service field (see the Apollo Federation specification) or via Schema#federation_sdl as explained above.

Maintainers