GraphQL API

How GitLab implements GraphQL

We use the graphql-ruby gem written by Robert Mosolgo.

All GraphQL queries are directed to a single endpoint (app/controllers/graphql_controller.rb#execute), which is exposed as an API endpoint at /api/graphql.

Deep Dive

In March 2019, Nick Thomas hosted a Deep Dive on GitLab's GraphQL API to share his domain specific knowledge with anyone who may work in this part of the code base in the future. You can find the recording on YouTube, and the slides on Google Slides and in PDF. Everything covered in this deep dive was accurate as of GitLab 11.9, and while specific details may have changed since then, it should still serve as a good introduction.

Authentication

Authentication happens through the GraphqlController, right now this uses the same authentication as the Rails application. So the session can be shared.

It is also possible to add a private_token to the querystring, or add a HTTP_PRIVATE_TOKEN header.

Types

We use a code-first schema, and we declare what type everything is in Ruby.

For example, app/graphql/types/issue_type.rb:

graphql_name 'Issue'

field :iid, GraphQL::ID_TYPE, null: false
field :title, GraphQL::STRING_TYPE, null: false

# we also have a method here that we've defined, that extends `field`
markdown_field :title_html, null: true
field :description, GraphQL::STRING_TYPE, null: true
markdown_field :description_html, null: true

We give each type a name (in this case Issue).

The iid, title and description are scalar GraphQL types. iid is a GraphQL::ID_TYPE, a special string type that signifies a unique ID. title and description are regular GraphQL::STRING_TYPE types.

When exposing a model through the GraphQL API, we do so by creating a new type in app/graphql/types. You can also declare custom GraphQL data types for scalar data types (e.g. TimeType).

When exposing properties in a type, make sure to keep the logic inside the definition as minimal as possible. Instead, consider moving any logic into a presenter:

class Types::MergeRequestType < BaseObject
  present_using MergeRequestPresenter

  name 'MergeRequest'
end

An existing presenter could be used, but it is also possible to create a new presenter specifically for GraphQL.

The presenter is initialized using the object resolved by a field, and the context.

Exposing Global IDs

When exposing an ID field on a type, we will by default try to expose a global ID by calling to_global_id on the resource being rendered.

To override this behaviour, you can implement an id method on the type for which you are exposing an ID. Please make sure that when exposing a GraphQL::ID_TYPE using a custom method that it is globally unique.

The records that are exposing a full_path as an ID_TYPE are one of these exceptions. Since the full path is a unique identifier for a Project or Namespace.

Connection Types

GraphQL uses cursor based pagination to expose collections of items. This provides the clients with a lot of flexibility while also allowing the backend to use different pagination models.

To expose a collection of resources we can use a connection type. This wraps the array with default pagination fields. For example a query for project-pipelines could look like this:

query($project_path: ID!) {
  project(fullPath: $project_path) {
    pipelines(first: 2) {
      pageInfo {
        hasNextPage
        hasPreviousPage
      }
      edges {
        cursor
        node {
          id
          status
        }
      }
    }
  }
}

This would return the first 2 pipelines of a project and related pagination info., ordered by descending ID. The returned data would look like this:

{
  "data": {
    "project": {
      "pipelines": {
        "pageInfo": {
          "hasNextPage": true,
          "hasPreviousPage": false
        },
        "edges": [
          {
            "cursor": "Nzc=",
            "node": {
              "id": "gid://gitlab/Pipeline/77",
              "status": "FAILED"
            }
          },
          {
            "cursor": "Njc=",
            "node": {
              "id": "gid://gitlab/Pipeline/67",
              "status": "FAILED"
            }
          }
        ]
      }
    }
  }
}

To get the next page, the cursor of the last known element could be passed:

query($project_path: ID!) {
  project(fullPath: $project_path) {
    pipelines(first: 2, after: "Njc=") {
      pageInfo {
        hasNextPage
        hasPreviousPage
      }
      edges {
        cursor
        node {
          id
          status
        }
      }
    }
  }
}

To ensure that we get consistent ordering, we will append an ordering on the primary key, in descending order. This is usually id, so basically we will add order(id: :desc) to the end of the relation. A primary key must be available on the underlying table.

Exposing permissions for a type

To expose permissions the current user has on a resource, you can call the expose_permissions passing in a separate type representing the permissions for the resource.

For example:

module Types
  class MergeRequestType < BaseObject
    expose_permissions Types::MergeRequestPermissionsType
  end
end

The permission type inherits from BasePermissionType which includes some helper methods, that allow exposing permissions as non-nullable booleans:

class MergeRequestPermissionsType < BasePermissionType
  present_using MergeRequestPresenter

  graphql_name 'MergeRequestPermissions'

  abilities :admin_merge_request, :update_merge_request, :create_note

  ability_field :resolve_note,
                description: 'Indicates the user can resolve discussions on the merge request'
  permission_field :push_to_source_branch, method: :can_push_to_source_branch?
end

Enums

GitLab GraphQL enums are defined in app/graphql/types. When defining new enums, the following rules apply:

For example:

module Types
  class TrafficLightStateEnum < BaseEnum
    graphql_name 'TrafficLightState'
    description 'State of a traffic light'

    value 'RED', description: 'Drivers must stop'
    value 'YELLOW', description: 'Drivers must stop when it is safe to'
    value 'GREEN', description: 'Drivers can start or keep driving'
  end
end

If the enum will be used for a class property in Ruby that is not an uppercase string, you can provide a value: option that will adapt the uppercase value.

In the following example:

module Types
  class EpicStateEnum < BaseEnum
    graphql_name 'EpicState'
    description 'State of a GitLab epic'

    value 'OPENED', value: 'opened', description: 'An open Epic'
    value 'CLOSED', value: 'closed', description: 'An closed Epic'
  end
end

Descriptions

All fields and arguments must have descriptions.

A description of a field or argument is given using the description: keyword. For example:

field :id, GraphQL::ID_TYPE, description: 'ID of the resource'

Descriptions of fields and arguments are viewable to users through:

Description styleguide

To ensure consistency, the following should be followed whenever adding or updating descriptions:

Example:

field :id, GraphQL::ID_TYPE, description: 'ID of the Issue'
field :confidential, GraphQL::BOOLEAN_TYPE, description: 'Indicates the issue is confidential'
field :closed_at, Types::TimeType, description: 'Timestamp of when the issue was closed'

Authorization

Authorizations can be applied to both types and fields using the same abilities as in the Rails app.

If the:

Also see authorizing resources in a mutation.

TIP: Tip: Try to load only what the currently authenticated user is allowed to view with our existing finders first, without relying on authorization to filter the records. This minimizes database queries and unnecessary authorization checks of the loaded records.

Type authorization

Authorize a type by passing an ability to the authorize method. All fields with the same type will be authorized by checking that the currently authenticated user has the required ability.

For example, the following authorization ensures that the currently authenticated user can only see projects that they have the read_project ability for (so long as the project is returned in a field that uses Types::ProjectType):

module Types
  class ProjectType < BaseObject
    authorize :read_project
  end
end

You can also authorize against multiple abilities, in which case all of the ability checks must pass.

For example, the following authorization ensures that the currently authenticated user must have read_project and another_ability abilities to see a project:

module Types
  class ProjectType < BaseObject
    authorize [:read_project, :another_ability]
  end
end

Field authorization

Fields can be authorized with the authorize option.

For example, the following authorization ensures that the currently authenticated user must have the owner_access ability to see the project:

module Types
  class MyType < BaseObject
    field :project, Types::ProjectType, null: true, resolver: Resolvers::ProjectResolver, authorize: :owner_access
  end
end

Fields can also be authorized against multiple abilities, in which case all of ability checks must pass. Note: This requires explicitly passing a block to field:

module Types
  class MyType < BaseObject
    field :project, Types::ProjectType, null: true, resolver: Resolvers::ProjectResolver do
      authorize [:owner_access, :another_ability]
    end
  end
end

NOTE: Note: If the field's type already has a particular authorization then there is no need to add that same authorization to the field.

Type and Field authorizations together

Authorizations are cumulative, so where authorizations are defined on a field, and also on the field's type, then the currently authenticated user would need to pass all ability checks.

In the following simplified example the currently authenticated user would need both first_permission and second_permission abilities in order to see the author of the issue.

class UserType
  authorize :first_permission
end

class IssueType
  field :author, UserType, authorize: :second_permission
end

Resolvers

We define how the application serves the response using resolvers stored in the app/graphql/resolvers directory. The resolver provides the actual implementation logic for retrieving the objects in question.

To find objects to display in a field, we can add resolvers to app/graphql/resolvers.

Arguments can be defined within the resolver, those arguments will be made available to the fields using the resolver. When exposing a model that had an internal ID (iid), prefer using that in combination with the namespace path as arguments in a resolver over a database ID. Othewise use a globally unique ID.

We already have a FullPathLoader that can be included in other resolvers to quickly find Projects and Namespaces which will have a lot of dependant objects.

To limit the amount of queries performed, we can use BatchLoader.

Mutations

Mutations are used to change any stored values, or to trigger actions. In the same way a GET-request should not modify data, we cannot modify data in a regular GraphQL-query. We can however in a mutation.

To find objects for a mutation, arguments need to be specified. As with resolvers, prefer using internal ID or, if needed, a global ID rather than the database ID.

Fields

In the most common situations, a mutation would return 2 fields:

By inheriting any new mutations from Mutations::BaseMutation the errors field is automatically added. A clientMutationId field is also added, this can be used by the client to identify the result of a single mutation when multiple are performed within a single request.

Building Mutations

Mutations live in app/graphql/mutations ideally grouped per resources they are mutating, similar to our services. They should inherit Mutations::BaseMutation. The fields defined on the mutation will be returned as the result of the mutation.

Always provide a consistent GraphQL-name to the mutation, this name is used to generate the input types and the field the mutation is mounted on. The name should look like <Resource being modified><Mutation class name>, for example the Mutations::MergeRequests::SetWip mutation has GraphQL name MergeRequestSetWip.

Arguments required by the mutation can be defined as arguments required for a field. These will be wrapped up in an input type for the mutation. For example, the Mutations::MergeRequests::SetWip with GraphQL-name MergeRequestSetWip defines these arguments:

argument :project_path, GraphQL::ID_TYPE,
         required: true,
         description: "The project the merge request to mutate is in"

argument :iid, GraphQL::STRING_TYPE,
         required: true,
         description: "The iid of the merge request to mutate"

argument :wip,
         GraphQL::BOOLEAN_TYPE,
         required: false,
         description: <<~DESC
                      Whether or not to set the merge request as a WIP.
                      If not passed, the value will be toggled.
                      DESC

This would automatically generate an input type called MergeRequestSetWipInput with the 3 arguments we specified and the clientMutationId.

These arguments are then passed to the resolve method of a mutation as keyword arguments. From here, we can call the service that will modify the resource.

The resolve method should then return a hash with the same field names as defined on the mutation and an errors array. For example, the Mutations::MergeRequests::SetWip defines a merge_request field:

field :merge_request,
      Types::MergeRequestType,
      null: true,
      description: "The merge request after mutation"

This means that the hash returned from resolve in this mutation should look like this:

{
  # The merge request modified, this will be wrapped in the type
  # defined on the field
  merge_request: merge_request,
  # An array if strings if the mutation failed after authorization
  errors: merge_request.errors.full_messages
}

To make the mutation available it should be defined on the mutation type that lives in graphql/types/mutation_types. The mount_mutation helper method will define a field based on the GraphQL-name of the mutation:

module Types
  class MutationType < BaseObject
    include Gitlab::Graphql::MountMutation

    graphql_name "Mutation"

    mount_mutation Mutations::MergeRequests::SetWip
  end
end

Will generate a field called mergeRequestSetWip that Mutations::MergeRequests::SetWip to be resolved.

Authorizing resources

To authorize resources inside a mutation, we first provide the required abilities on the mutation like this:

module Mutations
  module MergeRequests
    class SetWip < Base
      graphql_name 'MergeRequestSetWip'

      authorize :update_merge_request
    end
  end
end

We can then call authorize! in the resolve method, passing in the resource we want to validate the abilities for.

Alternatively, we can add a find_object method that will load the object on the mutation. This would allow you to use the authorized_find! helper method.

When a user is not allowed to perform the action, or an object is not found, we should raise a Gitlab::Graphql::Errors::ResourceNotAvailable error. Which will be correctly rendered to the clients.

GitLab's custom scalars

Types::TimeType

Types::TimeType must be used as the type for all fields and arguments that deal with Ruby Time and DateTime objects.

The type is a custom scalar that:

This allows our GraphQL API to have a standardized way that it presents time and handles time inputs.

Example:

field :created_at, Types::TimeType, null: false, description: 'Timestamp of when the issue was created'

Testing

full stack tests for a graphql query or mutation live in spec/requests/api/graphql.

When adding a query, the a working graphql query shared example can be used to test if the query renders valid results.

Using the GraphqlHelpers#all_graphql_fields_for-helper, a query including all available fields can be constructed. This makes it easy to add a test rendering all possible fields for a query.

To test GraphQL mutation requests, GraphqlHelpers provides 2 helpers: graphql_mutation which takes the name of the mutation, and a hash with the input for the mutation. This will return a struct with a mutation query, and prepared variables.

This struct can then be passed to the post_graphql_mutation helper, that will post the request with the correct params, like a GraphQL client would do.

To access the response of a mutation, the graphql_mutation_response helper is available.

Using these helpers, we can build specs like this:

let(:mutation) do
  graphql_mutation(
    :merge_request_set_wip,
    project_path: 'gitlab-org/gitlab-foss',
    iid: '1',
    wip: true
  )
end

it 'returns a successful response' do
   post_graphql_mutation(mutation, current_user: user)

   expect(response).to have_gitlab_http_status(:success)
   expect(graphql_mutation_response(:merge_request_set_wip)['errors']).to be_empty
end

Notes about Query flow and GraphQL infrastructure

GitLab's GraphQL infrastructure can be found in lib/gitlab/graphql.

Instrumentation is functionality that wraps around a query being executed. It is implemented as a module that uses the Instrumentation class.

Example: Present

module Present
  #... some code above...

  def self.use(schema_definition)
    schema_definition.instrument(:field, Instrumentation.new)
  end
end

A Query Analyzer contains a series of callbacks to validate queries before they are executed. Each field can pass through the analyzer, and the final value is also available to you.

Multiplex queries enable multiple queries to be sent in a single request. This reduces the number of requests sent to the server. (there are custom Multiplex Query Analyzers and Multiplex Instrumentation provided by graphql-ruby).

Query limits

Queries and mutations are limited by depth, complexity, and recursion to protect server resources from overly ambitious or malicious queries. These values can be set as defaults and overridden in specific queries as needed. The complexity values can be set per object as well, and the final query complexity is evaluated based on how many objects are being returned. This is useful for objects that are expensive (e.g. requiring Gitaly calls).

For example, a conditional complexity method in a resolver:

def self.resolver_complexity(args, child_complexity:)
  complexity = super
  complexity += 2 if args[:labelName]

  complexity
end

More about complexity: graphql-ruby docs

Documentation and Schema

Our schema is located at app/graphql/gitlab_schema.rb. See the schema reference for details.

This generated GraphQL documentation needs to be updated when the schema changes. For information on generating GraphQL documentation and schema files, see updating the schema documentation.