GraphQL API style guide

This document outlines the style guide for GitLab's 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 (GitLab team members only: https://gitlab.com/gitlab-org/create-stage/issues/1) 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.

GraphiQL

GraphiQL is an interactive GraphQL API explorer where you can play around with existing queries. You can access it in any GitLab environment on https://<your-gitlab-site.com>/-/graphql-explorer. For example, the one for GitLab.com.

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: true
field :title, GraphQL::STRING_TYPE, null: true

# 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.

Nullable fields

GraphQL allows fields to be "nullable" or "non-nullable". The former means that null may be returned instead of a value of the specified type. In general, you should prefer using nullable fields to non-nullable ones, for the following reasons:

It's common for data to switch from required to not-required, and back again
Even when there is no prospect of a field becoming optional, it may not be available at query time
- For instance, the content of a blob may need to be looked up from Gitaly
- If the content is nullable, we can return a partial response, instead of failing the whole query
Changing from a non-nullable field to a nullable field is difficult with a versionless schema

Non-nullable fields should only be used when a field is required, very unlikely to become optional in the future, and very easy to calculate. An example would be id fields.

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 behavior, 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 information, 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.

Shortcut fields

Sometimes it can seem easy to implement a "shortcut field", having the resolver return the first of a collection if no parameters are passed. These "shortcut fields" are discouraged because they create maintenance overhead. They need to be kept in sync with their canonical field, and deprecated or modified if their canonical field changes. Use the functionality the framework provides unless there is a compelling reason to do otherwise.

For example, instead of latest_pipeline, use pipelines(last: 1).

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

permission_field: Will act the same as graphql-ruby's field method but setting a default description and type and making them non-nullable. These options can still be overridden by adding them as arguments.
ability_field: Expose an ability defined in our policies. This behaves the same way as permission_field and the same arguments can be overridden.
abilities: Allows exposing several abilities defined in our policies at once. The fields for these will all have be non-nullable booleans with a default description.

Feature flags

Developers can add feature flags to GraphQL fields in the following ways:

Add the feature_flag property to a field. This will allow the field to be hidden from the GraphQL schema when the flag is disabled.
Toggle the return value when resolving the field.

You can refer to these guidelines to decide which approach to use:

If your field is experimental, and its name or type is subject to change, use the feature_flag property.
If your field is stable and its definition will not change, even after the flag is removed, toggle the return value of the field instead. Note that all fields should be nullable anyway.

`feature_flag` property

The feature_flag property allows you to toggle the field's visibility within the GraphQL schema. This will remove the field from the schema when the flag is disabled.

A description is appended to the field indicating that it is behind a feature flag.

CAUTION: Caution: If a client queries for the field when the feature flag is disabled, the query will fail. Consider this when toggling the visibility of the feature on or off on production.

The feature_flag property does not allow the use of feature gates based on actors. This means that the feature flag cannot be toggled only for particular projects, groups, or users, but instead can only be toggled globally for everyone.

Example:

field :test_field, type: GraphQL::STRING_TYPE,
      null: true,
      description: 'Some test field',
      feature_flag: :my_feature_flag

Toggle the value of a field

This method of using feature flags for fields is to toggle the return value of the field. This can be done in the resolver, in the type, or even in a model method, depending on your preference and situation.

When applying a feature flag to toggle the value of a field, the description of the field must:

State that the value of the field can be toggled by a feature flag.
Name the feature flag.
State what the field will return when the feature flag is disabled (or enabled, if more appropriate).

Example:

field :foo, GraphQL::STRING_TYPE,
      null: true,
      description: 'Some test field. Will always return `null`' \
                   'if `my_feature_flag` feature flag is disabled'

def foo
  object.foo unless Feature.enabled?(:my_feature_flag, object)
end

Deprecating fields

GitLab's GraphQL API is versionless, which means we maintain backwards compatibility with older versions of the API with every change. Rather than removing a field, we need to deprecate the field instead. In future, GitLab may remove deprecated fields.

Fields are deprecated using the deprecated property. The value of the property is a Hash of:

reason - Reason for the deprecation.
milestone - Milestone that the field was deprecated.

Example:

field :token, GraphQL::STRING_TYPE, null: true,
      deprecated: { reason: 'Login via token has been removed', milestone: '10.0' },
      description: 'Token for login'

The original description: of the field should be maintained, and should not be updated to mention the deprecation.

Deprecation reason style guide

Where the reason for deprecation is due to the field being replaced with another field, the reason must be:

Use `otherFieldName`

Example:

field :designs, ::Types::DesignManagement::DesignCollectionType, null: true,
      deprecated: { reason: 'Use `designCollection`', milestone: '10.0' },
      description: 'The designs associated with this issue',

If the field is not being replaced by another field, a descriptive deprecation reason should be given.

Enums

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

Values must be uppercase.
Class names must end with the string Enum.
The graphql_name must not contain the string Enum.

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:

GraphQL inputs of OPENED will be converted to 'opened'.
Ruby values of 'opened' will be converted to "OPENED" in GraphQL responses.

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:

The GraphiQL explorer.
The static GraphQL API reference.

Description style guide

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

Mention the name of the resource in the description. Example: 'Labels of the issue' (issue being the resource).
Use "{x} of the {y}" where possible. Example: 'Title of the issue'. Do not start descriptions with The.
Descriptions of GraphQL::BOOLEAN_TYPE fields should answer the question: "What does this field do?". Example: 'Indicates project has a Git repository'.
Always include the word "timestamp" when describing an argument or field of type Types::TimeType. This lets the reader know that the format of the property will be Time, rather than just Date.
No . at end of strings.

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:

Currently authenticated user fails the authorization, the authorized resource will be returned as null.
Resource is part of a collection, the collection will be filtered to exclude the objects that the user's authorization checks failed against.

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. Otherwise 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 dependent 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:

The resource being modified
A list of errors explaining why the action could not be performed. If the mutation succeeded, this list would be empty.

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.

Validating arguments

For validations of single arguments, use the prepare option as normal.

Sometimes a mutation or resolver may accept a number of optional arguments, but still want to validate that at least one of the optional arguments were given. In this situation, consider using the #ready? method within your mutation or resolver to provide the validation. The #ready? method will be called before any work is done within the #resolve method.

Example:

def ready?(**args)
  if args.values_at(:body, :position).compact.blank?
    raise Gitlab::Graphql::Errors::ArgumentError,
          'body or position arguments are required'
  end

  # Always remember to call `#super`
  super(args)
end

In the future this may be able to be done using InputUnions if this RFC is merged.

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:

Converts Ruby's Time and DateTime objects into standardized ISO-8601 formatted strings, when used as the type for our GraphQL fields.
Converts ISO-8601 formatted time strings into Ruby Time objects, when used as the type for our GraphQL arguments.

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: true, 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 parameters, 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 Gitlab
  module Graphql
    module Present
      #... some code above...

      def self.use(schema_definition)
        schema_definition.instrument(:field, ::Gitlab::Graphql::Present::Instrumentation.new)
      end
    end
  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 documentation.

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.

GraphQL API style guide

How GitLab implements GraphQL

Deep Dive

GraphiQL

Authentication

Types

Nullable fields

Exposing Global IDs

Connection Types

Shortcut fields

Exposing permissions for a type

Feature flags

feature_flag property

Toggle the value of a field

Deprecating fields

Deprecation reason style guide

Enums

Descriptions

Description style guide

Authorization

Type authorization

Field authorization

Type and Field authorizations together

Resolvers

Mutations

Fields

Building Mutations

Authorizing resources

Validating arguments

GitLab's custom scalars

Types::TimeType

Testing

Notes about Query flow and GraphQL infrastructure

Query limits

Documentation and Schema

`feature_flag` property

`Types::TimeType`