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Backend GraphQL API guide

This document contains style and technical guidance for engineers implementing the backend of the GitLab GraphQL API.

Relation to REST API

See the GraphQL and REST APIs section.

Versioning

The GraphQL API is versionless.

Learning GraphQL at GitLab

Backend engineers who wish to learn GraphQL at GitLab should read this guide in conjunction with the guides for the GraphQL Ruby gem. Those guides teach you the features of the gem, and the information in it is generally not reproduced here.

To learn about the design and features of GraphQL itself read the guide on graphql.org which is an accessible but shortened version of information in the GraphQL spec.

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 the GitLab GraphQL API to share domain-specific knowledge with anyone who may work in this part of the codebase in the future. You can find the recording on YouTube, and the slides on Google Slides and in PDF. Specific details have changed since then, but it should still serve as a good introduction.

How GitLab implements GraphQL

We use the GraphQL Ruby gem written by Robert Mosolgo. In addition, we have a subscription to GraphQL Pro. For details see GraphQL Pro subscription.

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.

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.

Reviewing merge requests with GraphQL changes

The GraphQL framework has some specific gotchas to be aware of, and domain expertise is required to ensure they are satisfied.

If you are asked to review a merge request that modifies any GraphQL files or adds an endpoint, have a look at our GraphQL review guide.

Reading GraphQL logs

See the Reading GraphQL logs guide for tips on how to inspect logs of GraphQL requests and monitor the performance of your GraphQL queries.

Authentication

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

It's also possible to add a private_token to the query string, or add a HTTP_PRIVATE_TOKEN header.

Limits

Several limits apply to the GraphQL API and some of these can be overridden by developers.

Max page size

By default, connections can only return at most a maximum number of records defined in app/graphql/gitlab_schema.rb per page.

Developers can specify a custom max page size when defining a connection.

Max complexity

Complexity is explained on our client-facing API page.

Fields default to adding 1 to a query's complexity score, but developers can specify a custom complexity when defining a field.

The complexity score of a query can itself be queried for.

Request timeout

Requests time out at 30 seconds.

Limit maximum field call count

In some cases, you want to prevent the evaluation of a specific field on multiple parent nodes because it results in an N+1 query problem and there is no optimal solution. This should be considered an option of last resort, to be used only when methods such as lookahead to preload associations, or using batching have been considered.

For example:

# This usage is expected.
query {
  project {
    environments
  }
}

# This usage is NOT expected.
# It results in N+1 query problem. EnvironmentsResolver can't use GraphQL batch loader in favor of GraphQL pagination.
query {
  projects {
    nodes {
      environments
    }
  }
}

To prevent this, you can use the Gitlab::Graphql::Limit::FieldCallCount extension on the field:

# This allows maximum 1 call to the `environments` field. If the field is evaluated on more than one node,
# it raises an error.
field :environments do
        extension(::Gitlab::Graphql::Limit::FieldCallCount, limit: 1)
      end

or you can apply the extension in a resolver class:

module Resolvers
  class EnvironmentsResolver < BaseResolver
    extension(::Gitlab::Graphql::Limit::FieldCallCount, limit: 1)
    # ...
  end
end

When you add this limit, make sure that the affected field's description is also updated accordingly. For example,

field :environments,
      description: 'Environments of the project. This field can only be resolved for one project in any single request.'

Breaking changes

The GitLab GraphQL API is versionless which means developers must familiarize themselves with our Deprecation and Removal process.

Breaking changes are:

  • Removing or renaming a field, argument, enum value, or mutation.
  • Changing the type or type name of an argument. The type of an argument is declared by the client when using variables, and a change would cause a query using the old type name to be rejected by the API.
  • Changing the scalar type of a field or enum value where it results in a change to how the value serializes to JSON. For example, a change from a JSON String to a JSON Number, or a change to how a String is formatted. A change to another object type can be allowed so long as all scalar type fields of the object continue to serialize in the same way.
  • Raising the complexity of a field or complexity multipliers in a resolver.
  • Changing a field from being not nullable (null: false) to nullable (null: true), as discussed in Nullable fields.
  • Changing an argument from being optional (required: false) to being required (required: true).
  • Changing the max page size of a connection.
  • Lowering the global limits for query complexity and depth.
  • Anything else that can result in queries hitting a limit that previously was allowed.

See the deprecating schema items section for how to deprecate items.

Breaking change exemptions

See the GraphQL API breaking change exemptions documentation.

Global IDs

The GitLab GraphQL API uses Global IDs (i.e: "gid://gitlab/MyObject/123") and never database primary key IDs.

Global ID is a convention used for caching and fetching in client-side libraries.

See also:

We have a custom scalar type (Types::GlobalIDType) which should be used as the type of input and output arguments when the value is a GlobalID. The benefits of using this type instead of ID are:

  • it validates that the value is a GlobalID
  • it parses it into a GlobalID before passing it to user code
  • it can be parameterized on the type of the object (for example, GlobalIDType[Project]) which offers even better validation and security.

Consider using this type for all new arguments and result types. Remember that it is perfectly possible to parameterize this type with a concern or a supertype, if you want to accept a wider range of objects (such as GlobalIDType[Issuable] vs GlobalIDType[Issue]).

Optimizations

By default, GraphQL tends to introduce N+1 problems unless you actively try to minimize them.

For stability and scalability, you must ensure that our queries do not suffer from N+1 performance issues.

The following are a list of tools to help you to optimize your GraphQL code:

How to see N+1 problems in development

N+1 problems can be discovered during development of a feature by:

  • Tailing development.log while you execute GraphQL queries that return collections of data. Bullet may help.
  • Observing the performance bar if executing queries in the GitLab UI.
  • Adding a request spec that asserts there are no (or limited) N+1 problems with the feature.

Fields

Types

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

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

graphql_name 'Project'

field :full_path, GraphQL::Types::ID, null: true
field :name, GraphQL::Types::String, null: true

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

The full_path and name are of scalar GraphQL types. full_path is a GraphQL::Types::ID (see when to use GraphQL::Types::ID). name is a regular GraphQL::Types::String type. You can also declare custom GraphQL data types for scalar data types (for example TimeType).

When exposing a model through the GraphQL API, we do so by creating a new type in app/graphql/types.

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 straightforward to calculate. An example would be id fields.

A non-nullable GraphQL schema field is an object type followed by the exclamation point (bang) !. Here's an example from the gitlab_schema.graphql file:

  id: ProjectID!

Here's an example of a non-nullable GraphQL array:


  errors: [String!]!

Further reading:

Exposing Global IDs

In keeping with the GitLab use of Global IDs, always convert database primary key IDs into Global IDs when you expose them.

All fields named id are converted automatically into the object's Global ID.

Fields that are not named id need to be manually converted. We can do this using Gitlab::GlobalID.build, or by calling #to_global_id on an object that has mixed in the GlobalID::Identification module.

Using an example from Types::Notes::DiscussionType:

field :reply_id, Types::GlobalIDType[Discussion]

def reply_id
  Gitlab::GlobalId.build(object, id: object.reply_id)
end

When to use GraphQL::Types::ID

When we use GraphQL::Types::ID the field becomes a GraphQL ID type, which is serialized as a JSON string. However, ID has a special significance for clients. The GraphQL spec says:

The ID scalar type represents a unique identifier, often used to refetch an object or as the key for a cache.

The GraphQL spec does not clarify what the scope should be for an ID's uniqueness. At GitLab we have decided that an ID must be at least unique by type name. Type name is the graphql_name of one our of Types:: classes, for example Project, or Issue.

Following this:

  • Project.fullPath should be an ID because there will be no other Project with that fullPath across the API, and the field is also an identifier.
  • Issue.iid should not be an ID because there can be many Issue types that have the same iid across the API. Treating it as an ID would be problematic if the client has a cache of Issues from different projects.
  • Project.id normally would qualify to be an ID because there can only be one Project with that ID value - except we use Global ID types instead of ID types for database ID values so we would type it as a Global ID instead.

This is summarized in the following table:

Field purpose Use GraphQL::Types::ID?
Full path {check-circle} Yes
Database ID {dotted-circle} No
IID {dotted-circle} No

markdown_field

markdown_field is a helper method that wraps field and should always be used for fields that return rendered Markdown.

This helper renders a model's Markdown field using the existing MarkupHelper with the context of the GraphQL query available to the helper.

Having the context available to the helper is needed for redacting links to resources that the current user is not allowed to see.

Because rendering the HTML can cause queries, the complexity of a these fields is raised by 5 above the default.

The Markdown field helper can be used as follows:

markdown_field :note_html, null: false

This would generate a field that renders the Markdown field note of the model. This could be overridden by adding the method: argument.

markdown_field :body_html, null: false, method: :note

The field is given this description by default:

The GitLab Flavored Markdown rendering of note

This can be overridden by passing a description: argument.

Connection types

NOTE: For specifics on implementation, see Pagination implementation.

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 append an ordering on the primary key, in descending order. The primary key is usually id, so we 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 straightforward 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).

Page size limit

By default, the API returns at most a maximum number of records defined in app/graphql/gitlab_schema.rb per page in a connection and this is also the default number of records returned per page if no limiting arguments (first: or last:) are provided by a client.

The max_page_size argument can be used to specify a different page size limit for a connection.

WARNING: It's better to change the frontend client, or product requirements, to not need large amounts of records per page than it is to raise the max_page_size, as the default is set to ensure the GraphQL API remains performant.

For example:

field :tags,
  Types::ContainerRepositoryTagType.connection_type,
  null: true,
  description: 'Tags of the container repository',
  max_page_size: 20

Field complexity

The GitLab GraphQL API uses a complexity score to limit performing overly complex queries. Complexity is described in our client documentation on the topic.

Complexity limits are defined in app/graphql/gitlab_schema.rb.

By default, fields add 1 to a query's complexity score. This can be overridden by providing a custom complexity value for a field.

Developers should specify higher complexity for fields that cause more work to be performed by the server to return data. Fields that represent data that can be returned with little-to-no work, for example in most cases; id or title, can be given a complexity of 0.

calls_gitaly

Fields that have the potential to perform a Gitaly call when resolving must be marked as such by passing calls_gitaly: true to field when defining it.

For example:

field :blob, type: Types::Snippets::BlobType,
      description: 'Snippet blob',
      null: false,
      calls_gitaly: true

This increments the complexity score of the field by 1.

If a resolver calls Gitaly, it can be annotated with BaseResolver.calls_gitaly!. This passes calls_gitaly: true to any field that uses this resolver.

For example:

class BranchResolver < BaseResolver
  type ::Types::BranchType, null: true
  calls_gitaly!

  argument name: ::GraphQL::Types::String, required: true

  def resolve(name:)
    object.branch(name)
  end
end

Then when we use it, any field that uses BranchResolver has the correct value for calls_gitaly:.

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
  graphql_name 'MergeRequestPermissions'

  present_using MergeRequestPresenter

  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: Acts 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 must all be non-nullable booleans with a default description.

Feature flags

You can implement feature flags in GraphQL to toggle:

  • The return value of a field.
  • The behavior of an argument or mutation.

This can be done in a resolver, in the type, or even in a model method, depending on your preference and situation.

NOTE: It's recommended that you also mark the item as an experiment while it is behind a feature flag. This signals to consumers of the public GraphQL API that the field is not meant to be used yet. You can also change or remove experimental items at any time without needing to deprecate them. When the flag is removed, "release" the schema item by removing its experiment property to make it public.

Descriptions for feature-flagged items

When using a feature flag to toggle the value or behavior of a schema item, the description of the item must:

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

Examples of using feature flags

Feature-flagged field

A field value is toggled based on the feature flag state. A common use is to return null if the feature flag is disabled:

field :foo, GraphQL::Types::String, null: true,
      experiment: { milestone: '10.0' },
      description: 'Some test field. Returns `null`' \
                   'if `my_feature_flag` feature flag is disabled.'

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

Feature-flagged argument

An argument can be ignored, or have its value changed, based on the feature flag state. A common use is to ignore the argument when a feature flag is disabled:

argument :foo, type: GraphQL::Types::String, required: false,
         experiment: { milestone: '10.0' },
         description: 'Some test argument. Is ignored if ' \
                      '`my_feature_flag` feature flag is disabled.'

def resolve(args)
  args.delete(:foo) unless Feature.enabled?(:my_feature_flag, object)
  # ...
end

Feature-flagged mutation

A mutation that cannot be performed due to a feature flag state is handled as a non-recoverable mutation error. The error is returned at the top level:

description 'Mutates an object. Does not mutate the object if ' \
            '`my_feature_flag` feature flag is disabled.'

def resolve(id: )
  object = authorized_find!(id: id)

  raise_resource_not_available_error! '`my_feature_flag` feature flag is disabled.' \
    if Feature.disabled?(:my_feature_flag, object)
  # ...
end

Deprecating schema items

The GitLab GraphQL API is versionless, which means we maintain backwards compatibility with older versions of the API with every change.

Rather than removing fields, arguments, enum values, or mutations, they must be deprecated instead.

The deprecated parts of the schema can then be removed in a future release in accordance with the GitLab deprecation process.

To deprecate a schema item in GraphQL:

  1. Create a deprecation issue for the item.
  2. Mark the item as deprecated in the schema.

See also:

Create a deprecation issue

Every GraphQL deprecation should have a deprecation issue created using the Deprecations issue template to track its deprecation and removal.

Apply these two labels to the deprecation issue:

  • ~GraphQL
  • ~deprecation

Mark the item as deprecated

Fields, arguments, enum values, and mutations 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::Types::String, null: true,
      deprecated: { reason: 'Login via token has been removed', milestone: '10.0' },
      description: 'Token for login.'

The original description of the things being deprecated should be maintained, and should not be updated to mention the deprecation. Instead, the reason is appended to the description.

Deprecation reason style guide

Where the reason for deprecation is due to the field, argument, or enum value being replaced, the reason must indicate the replacement. For example, the following is a reason for a replaced field:

Use `otherFieldName`

Examples:

field :designs, ::Types::DesignManagement::DesignCollectionType, null: true,
      deprecated: { reason: 'Use `designCollection`', milestone: '10.0' },
      description: 'The designs associated with this issue.',
module Types
  class TodoStateEnum < BaseEnum
    value 'pending', deprecated: { reason: 'Use PENDING', milestone: '10.0' }
    value 'done', deprecated: { reason: 'Use DONE', milestone: '10.0' }
    value 'PENDING', value: 'pending'
    value 'DONE', value: 'done'
  end
end

If the field, argument, or enum value being deprecated is not being replaced, a descriptive deprecation reason should be given.

Deprecate Global IDs

We use the rails/globalid gem to generate and parse Global IDs, so as such they are coupled to model names. When we rename a model, its Global ID changes.

If the Global ID is used as an argument type anywhere in the schema, then the Global ID change would typically constitute a breaking change.

To continue to support clients using the old Global ID argument, we add a deprecation to Gitlab::GlobalId::Deprecations.

NOTE: If the Global ID is only exposed as a field then we do not need to deprecate it. We consider the change to the way a Global ID is expressed in a field to be backwards-compatible. We expect that clients don't parse these values: they are meant to be treated as opaque tokens, and any structure in them is incidental and not to be relied on.

Example scenario:

This example scenario is based on this merge request.

A model named PrometheusService is to be renamed Integrations::Prometheus. The old model name is used to create a Global ID type that is used as an argument for a mutation:

# Mutations::UpdatePrometheus:

argument :id, Types::GlobalIDType[::PrometheusService],
              required: true,
              description: "The ID of the integration to mutate."

Clients call the mutation by passing a Global ID string that looks like "gid://gitlab/PrometheusService/1", named as PrometheusServiceID, as the input.id argument:

mutation updatePrometheus($id: PrometheusServiceID!, $active: Boolean!) {
  prometheusIntegrationUpdate(input: { id: $id, active: $active }) {
    errors
    integration {
      active
    }
  }
}

We rename the model to Integrations::Prometheus, and then update the codebase with the new name. When we come to update the mutation, we pass the renamed model to Types::GlobalIDType[]:

# Mutations::UpdatePrometheus:

argument :id, Types::GlobalIDType[::Integrations::Prometheus],
              required: true,
              description: "The ID of the integration to mutate."

This would cause a breaking change to the mutation, as the API now rejects clients who pass an id argument as "gid://gitlab/PrometheusService/1", or that specify the argument type as PrometheusServiceID in the query signature.

To allow clients to continue to interact with the mutation unchanged, edit the DEPRECATIONS constant in Gitlab::GlobalId::Deprecations and add a new Deprecation to the array:

DEPRECATIONS = [
  Gitlab::Graphql::DeprecationsBase::NameDeprecation.new(old_name: 'PrometheusService', new_name: 'Integrations::Prometheus', milestone: '14.0')
].freeze

Then follow our regular deprecation process. To later remove support for the former argument style, remove the Deprecation:

DEPRECATIONS = [].freeze

During the deprecation period, the API accepts either of these formats for the argument value:

  • "gid://gitlab/PrometheusService/1"
  • "gid://gitlab/Integrations::Prometheus/1"

The API also accepts these types in the query signature for the argument:

  • PrometheusServiceID
  • IntegrationsPrometheusID

NOTE: Although queries that use the old type (PrometheusServiceID in this example) are considered valid and executable by the API, validator tools consider them to be invalid. They are considered invalid because we are deprecating using a bespoke method outside of the @deprecated directive, so validators are not aware of the support.

The documentation mentions that the old Global ID style is now deprecated.

Mark schema items as experiments

You can mark GraphQL schema items (fields, arguments, enum values, and mutations) as experiments.

An item marked as an experiment is exempt from the deprecation process and can be removed at any time without notice. Mark an item as an experiment when it is subject to change and not ready for public use.

NOTE: Only mark new items as an experiment. Never mark existing items as an experiment because they're already public.

To mark a schema item as an experiment, use the experiment: keyword. You must provide the milestone: that introduced the experimental item.

For example:

field :token, GraphQL::Types::String, null: true,
      experiment: { milestone: '10.0' },
      description: 'Token for login.'

Similarly, you can also mark an entire mutation as an experiment by updating where the mutation is mounted in app/graphql/types/mutation_type.rb:

mount_mutation Mutations::Ci::JobArtifact::BulkDestroy, experiment: { milestone: '15.10' }

Experimental GraphQL items is a custom GitLab feature that leverages GraphQL deprecations. An experimental item appears as deprecated in the GraphQL schema. Like all deprecated schema items, you can test an experimental field in the interactive GraphQL explorer (GraphiQL). However, be aware that the GraphiQL autocomplete editor doesn't suggest deprecated fields.

The item shows as experiment in our generated GraphQL documentation and its GraphQL schema description.

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 is used for a class property in Ruby that is not an uppercase string, you can provide a value: option that adapts the uppercase value.

In the following example:

  • GraphQL inputs of OPENED are converted to 'opened'.
  • Ruby values of 'opened' are 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: 'A closed Epic.'
  end
end

Enum values can be deprecated using the deprecated keyword.

Defining GraphQL enums dynamically from Rails enums

If your GraphQL enum is backed by a Rails enum, then consider using the Rails enum to dynamically define the GraphQL enum values. Doing so binds the GraphQL enum values to the Rails enum definition, so if values are ever added to the Rails enum then the GraphQL enum automatically reflects the change.

Example:

module Types
  class IssuableSeverityEnum < BaseEnum
    graphql_name 'IssuableSeverity'
    description 'Incident severity'

    ::IssuableSeverity.severities.each_key do |severity|
      value severity.upcase, value: severity, description: "#{severity.titleize} severity."
    end
  end
end

JSON

When data to be returned by GraphQL is stored as JSON, we should continue to use GraphQL types whenever possible. Avoid using the GraphQL::Types::JSON type unless the JSON data returned is truly unstructured.

If the structure of the JSON data varies, but is one of a set of known possible structures, use a union. An example of the use of a union for this purpose is !30129.

Field names can be mapped to hash data keys using the hash_key: keyword if needed.

For example, given the following JSON data:

{
  "title": "My chart",
  "data": [
    { "x": 0, "y": 1 },
    { "x": 1, "y": 1 },
    { "x": 2, "y": 2 }
  ]
}

We can use GraphQL types like this:

module Types
  class ChartType < BaseObject
    field :title, GraphQL::Types::String, null: true, description: 'Title of the chart.'
    field :data, [Types::ChartDatumType], null: true, description: 'Data of the chart.'
  end
end

module Types
  class ChartDatumType < BaseObject
    field :x, GraphQL::Types::Int, null: true, description: 'X-axis value of the chart datum.'
    field :y, GraphQL::Types::Int, null: true, description: 'Y-axis value of the chart datum.'
  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::Types::ID, description: 'ID of the issue.'
field :confidential, GraphQL::Types::Boolean, description: 'Indicates the issue is confidential.'
field :closed_at, Types::TimeType, description: 'Timestamp of when the issue was closed.'

You can view descriptions of fields and arguments in:

Description style guide

Language and punctuation

To describe fields and arguments, use {x} of the {y} where possible, where {x} is the item you're describing, and {y} is the resource it applies to. For example:

ID of the issue.
Author of the epics.

For arguments that sort or search, start with the appropriate verb. To indicate the specified values, for conciseness, you can use this instead of the given or the specified. For example:

Sort issues by this criteria.

Do not start descriptions with The or A, for consistency and conciseness.

End all descriptions with a period (.).

Booleans

For a boolean field (GraphQL::Types::Boolean), start with a verb that describes what it does. For example:

Indicates the issue is confidential.

If necessary, provide the default. For example:

Sets the issue to confidential. Default is false.

Sort enums

Enums for sorting should have the description 'Values for sorting {x}.'. For example:

Values for sorting container repositories.

Types::TimeType field description

For Types::TimeType GraphQL fields, include the word timestamp. This lets the reader know that the format of the property is Time, rather than just Date.

For example:

field :closed_at, Types::TimeType, description: 'Timestamp of when the issue was closed.'

copy_field_description helper

Sometimes we want to ensure that two descriptions are always identical. For example, to keep a type field description the same as a mutation argument when they both represent the same property.

Instead of supplying a description, we can use the copy_field_description helper, passing it the type, and field name to copy the description of.

Example:

argument :title, GraphQL::Types::String,
          required: false,
          description: copy_field_description(Types::MergeRequestType, :title)

Documentation references

Sometimes we want to refer to external URLs in our descriptions. To make this easier, and provide proper markup in the generated reference documentation, we provide a see property on fields. For example:

field :genus,
      type: GraphQL::Types::String,
      null: true,
      description: 'A taxonomic genus.'
      see: { 'Wikipedia page on genera' => 'https://wikipedia.org/wiki/Genus' }

This renders in our documentation as:

A taxonomic genus. See: [Wikipedia page on genera](https://wikipedia.org/wiki/Genus)

Multiple documentation references can be provided. The syntax for this property is a HashMap where the keys are textual descriptions, and the values are URLs.

Subscription tier badges

If a field or argument is available to higher subscription tiers than the other fields, add the availability details inline.

For example:

description: 'Full path of a custom template. Premium and Ultimate only.'

Authorization

See: GraphQL Authorization

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 in the resolver in the same way as in a mutation. See the Arguments section.

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

Writing resolvers

Our code should aim to be thin declarative wrappers around finders and services. You can repeat lists of arguments, or extract them to concerns. Composition is preferred over inheritance in most cases. Treat resolvers like controllers: resolvers should be a DSL that compose other application abstractions.

For example:

class PostResolver < BaseResolver
  type Post.connection_type, null: true
  authorize :read_blog
  description 'Blog posts, optionally filtered by name'

  argument :name, [::GraphQL::Types::String], required: false, as: :slug

  alias_method :blog, :object

  def resolve(**args)
    PostFinder.new(blog, current_user, args).execute
  end
end

While you can use the same resolver class in two different places, such as in two different fields where the same object is exposed, you should never re-use resolver objects directly. Resolvers have a complex lifecycle, with authorization, readiness and resolution orchestrated by the framework, and at each stage lazy values can be returned to take advantage of batching opportunities. Never instantiate a resolver or a mutation in application code.

Instead, the units of code reuse are much the same as in the rest of the application:

  • Finders in queries to look up data.
  • Services in mutations to apply operations.
  • Loaders (batch-aware finders) specific to queries.

There is never any reason to use batching in a mutation. Mutations are executed in series, so there are no batching opportunities. All values are evaluated eagerly as soon as they are requested, so batching is unnecessary overhead. If you are writing:

  • A Mutation, feel free to lookup objects directly.
  • A Resolver or methods on a BaseObject, then you want to allow for batching.

Error handling

Resolvers may raise errors, which are converted to top-level errors as appropriate. All anticipated errors should be caught and transformed to an appropriate GraphQL error (see Gitlab::Graphql::Errors). Any uncaught errors are suppressed and the client receives the message Internal service error.

The one special case is permission errors. In the REST API we return 404 Not Found for any resources that the user does not have permission to access. The equivalent behavior in GraphQL is for us to return null for all absent or unauthorized resources. Query resolvers should not raise errors for unauthorized resources.

The rationale for this is that clients must not be able to distinguish between the absence of a record and the presence of one they do not have access to. To do so is a security vulnerability, because it leaks information we want to keep hidden.

In most cases you don't need to worry about this - this is handled correctly by the resolver field authorization we declare with the authorize DSL calls. If you need to do something more custom however, remember, if you encounter an object the current_user does not have access to when resolving a field, then the entire field should resolve to null.

Deriving resolvers

(including BaseResolver.single and BaseResolver.last)

For some use cases, we can derive resolvers from others. The main use case for this is one resolver to find all items, and another to find one specific one. For this, we supply convenience methods:

  • BaseResolver.single, which constructs a new resolver that selects the first item.
  • BaseResolver.last, which constructs a resolver that selects the last item.

The correct singular type is inferred from the collection type, so we don't have to define the type here.

Before you make use of these methods, consider if it would be simpler to either:

  • Write another resolver that defines its own arguments.
  • Write a concern that abstracts out the query.

Using BaseResolver.single too freely is an anti-pattern. It can lead to non-sensical fields, such as a Project.mergeRequest field that just returns the first MR if no arguments are given. Whenever we derive a single resolver from a collection resolver, it must have more restrictive arguments.

To make this possible, use the when_single block to customize the single resolver. Every when_single block must:

  • Define (or re-define) at least one argument.
  • Make optional filters required.

For example, we can do this by redefining an existing optional argument, changing its type and making it required:

class JobsResolver < BaseResolver
  type JobType.connection_type, null: true
  authorize :read_pipeline

  argument :name, [::GraphQL::Types::String], required: false

  when_single do
    argument :name, ::GraphQL::Types::String, required: true
  end

  def resolve(**args)
    JobsFinder.new(pipeline, current_user, args.compact).execute
  end

Here we have a resolver for getting pipeline jobs. The name argument is optional when getting a list, but required when getting a single job.

If there are multiple arguments, and neither can be made required, we can use the block to add a ready condition:

class JobsResolver < BaseResolver
  alias_method :pipeline, :object

  type JobType.connection_type, null: true
  authorize :read_pipeline

  argument :name, [::GraphQL::Types::String], required: false
  argument :id, [::Types::GlobalIDType[::Job]],
           required: false,
           prepare: ->(ids, ctx) { ids.map(&:model_id) }

  when_single do
    argument :name, ::GraphQL::Types::String, required: false
    argument :id, ::Types::GlobalIDType[::Job],
             required: false
             prepare: ->(id, ctx) { id.model_id }

    def ready?(**args)
      raise ::Gitlab::Graphql::Errors::ArgumentError, 'Only one argument may be provided' unless args.size == 1
    end
  end

  def resolve(**args)
    JobsFinder.new(pipeline, current_user, args.compact).execute
  end

Then we can use these resolver on fields:

# In PipelineType

field :jobs, resolver: JobsResolver, description: 'All jobs.'
field :job, resolver: JobsResolver.single, description: 'A single job.'

Optimizing Resolvers

Look-Ahead

The full query is known in advance during execution, which means we can make use of lookahead to optimize our queries, and batch load associations we know we need. Consider adding lookahead support in your resolvers to avoid N+1 performance issues.

To enable support for common lookahead use-cases (pre-loading associations when child fields are requested), you can include LooksAhead. For example:

# Assuming a model `MyThing` with attributes `[child_attribute, other_attribute, nested]`,
# where nested has an attribute named `included_attribute`.
class MyThingResolver < BaseResolver
  include LooksAhead

  # Rather than defining `resolve(**args)`, we implement: `resolve_with_lookahead(**args)`
  def resolve_with_lookahead(**args)
    apply_lookahead(MyThingFinder.new(current_user).execute)
  end

  # We list things that should always be preloaded:
  # For example, if child_attribute is always needed (during authorization
  # perhaps), then we can include it here.
  def unconditional_includes
    [:child_attribute]
  end

  # We list things that should be included if a certain field is selected:
  def preloads
    {
        field_one: [:other_attribute],
        field_two: [{ nested: [:included_attribute] }]
    }
  end
end

By default, fields defined in #preloads are preloaded if that field is selected in the query. Occasionally, finer control may be needed to avoid preloading too much or incorrect content.

Extending the above example, we might want to preload a different association if certain fields are requested together. This can be done by overriding #filtered_preloads:

class MyThingResolver < BaseResolver
  # ...

  def filtered_preloads
    return [:alternate_attribute] if lookahead.selects?(:field_one) && lookahead.selects?(:field_two)

    super
  end
end

The LooksAhead concern also provides basic support for preloading associations based on nested GraphQL field definitions. The WorkItemsResolver is a good example for this. nested_preloads is another method you can define to return a hash, but unlike the preloads method, the value for each hash key is another hash and not the list of associations to preload. So in the previous example, you could override nested_preloads like this:

class MyThingResolver < BaseResolver
  # ...

  def nested_preloads
    {
      root_field: {
        nested_field1: :association_to_preload,
        nested_field2: [:association1, :association2]
      }
    }
  end
end

For an example of real world use, see ResolvesMergeRequests.

before_connection_authorization

A before_connection_authorization hook can help resolvers eliminate N+1 problems that originate from type authorization permission checks.

The before_connection_authorization method receives the resolved nodes and the current user. In the block, use ActiveRecord::Associations::Preloader or a Preloaders:: class to preload data for the type authorization check.

Example:

class LabelsResolver < BaseResolver
  before_connection_authorization do |labels, current_user|
    Preloaders::LabelsPreloader.new(labels, current_user).preload_all
  end
end

BatchLoading

See GraphQL BatchLoader.

Correct use of Resolver#ready?

Resolvers have two public API methods as part of the framework: #ready?(**args) and #resolve(**args). We can use #ready? to perform set-up or early-return without invoking #resolve.

Good reasons to use #ready? include:

  • Returning Relation.none if we know before-hand that no results are possible.
  • Performing setup such as initializing instance variables (although consider lazily initialized methods for this).

Implementations of Resolver#ready?(**args) should return (Boolean, early_return_data) as follows:

def ready?(**args)
  [false, 'have this instead']
end

For this reason, whenever you call a resolver (mainly in tests because framework abstractions Resolvers should not be considered re-usable, finders are to be preferred), remember to call the ready? method and check the boolean flag before calling resolve! An example can be seen in our GraphqlHelpers.

For validating arguments, validators are preferred over using #ready?.

Negated arguments

Negated filters can filter some resources (for example, find all issues that have the bug label, but don't have the bug2 label assigned). The not argument is the preferred syntax to pass negated arguments:

issues(labelName: "bug", not: {labelName: "bug2"}) {
  nodes {
    id
    title
  }
}

You can use the negated helper from Gitlab::Graphql::NegatableArguments in your type or resolver. For example:

extend ::Gitlab::Graphql::NegatableArguments

negated do
  argument :labels, [GraphQL::STRING_TYPE],
            required: false,
            as: :label_name,
            description: 'Array of label names. All resolved merge requests will not have these labels.'
end

Metadata

When using resolvers, they can and should serve as the SSoT for field metadata. All field options (apart from the field name) can be declared on the resolver. These include:

  • type (required - all resolvers must include a type annotation)
  • extras
  • description
  • Gitaly annotations (with calls_gitaly!)

Example:

module Resolvers
  MyResolver < BaseResolver
    type Types::MyType, null: true
    extras [:lookahead]
    description 'Retrieve a single MyType'
    calls_gitaly!
  end
end

Pass a parent object into a child Presenter

Sometimes you need to access the resolved query parent in a child context to compute fields. Usually the parent is only available in the Resolver class as parent.

To find the parent object in your Presenter class:

  1. Add the parent object to the GraphQL context from your resolver's resolve method:

      def resolve(**args)
    context[:parent_object] = parent
  end

  2. Declare that your resolver or fields require the parent field context. For example:

      # in ChildType
  field :computed_field, SomeType, null: true,
        method: :my_computing_method,
        extras: [:parent], # Necessary
        description: 'My field description.'

  field :resolver_field, resolver: SomeTypeResolver

  # In SomeTypeResolver

  extras [:parent]
  type SomeType, null: true
  description 'My field description.'

  3. Declare your field's method in your Presenter class and have it accept the parent keyword argument. This argument contains the parent GraphQL context, so you have to access the parent object with parent[:parent_object] or whatever key you used in your Resolver:

      # in ChildPresenter
  def my_computing_method(parent:)
    # do something with `parent[:parent_object]` here
  end

  # In SomeTypeResolver

  def resolve(parent:)
    # ...
  end

For an example of real-world use, check this MR that added scopedPath and scopedUrl to IterationPresenter

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.

Building Mutations

Mutations are stored 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 are returned as the result of the mutation.

Update mutation granularity

The service-oriented architecture in GitLab means that most mutations call a Create, Delete, or Update service, for example UpdateMergeRequestService. For Update mutations, you might want to only update one aspect of an object, and thus only need a fine-grained mutation, for example MergeRequest::SetDraft.

It's acceptable to have both fine-grained mutations and coarse-grained mutations, but be aware that too many fine-grained mutations can lead to organizational challenges in maintainability, code comprehensibility, and testing. Each mutation requires a new class, which can lead to technical debt. It also means the schema becomes very big, which can make it difficult for users to navigate our schema. As each new mutation also needs tests (including slower request integration tests), adding mutations slows down the test suite.

To minimize changes:

  • Use existing mutations, such as MergeRequest::Update, when available.
  • Expose existing services as a coarse-grained mutation.

When a fine-grained mutation might be more appropriate:

  • Modifying a property that requires specific permissions or other specialized logic.
  • Exposing a state-machine-like transition (locking issues, merging MRs, closing epics, etc).
  • Accepting nested properties (where we accept properties for a child object).
  • The semantics of the mutation can be expressed clearly and concisely.

See issue #233063 for further context.

Naming conventions

Each mutation must define a graphql_name, which is the name of the mutation in the GraphQL schema.

Example:

class UserUpdateMutation < BaseMutation
  graphql_name 'UserUpdate'
end

Due to changes in the 1.13 version of the graphql-ruby gem, graphql_name should be the first line of the class to ensure that type names are generated correctly. The Graphql::GraphqlNamePosition cop enforces this. See issue #27536 for further context.

Our GraphQL mutation names are historically inconsistent, but new mutation names should follow the convention '{Resource}{Action}' or '{Resource}{Action}{Attribute}'.

Mutations that create new resources should use the verb Create.

Example:

  • CommitCreate

Mutations that update data should use:

  • The verb Update.
  • A domain-specific verb like Set, Add, or Toggle if more appropriate.

Examples:

  • EpicTreeReorder
  • IssueSetWeight
  • IssueUpdate
  • TodoMarkDone

Mutations that remove data should use:

  • The verb Delete rather than Destroy.
  • A domain-specific verb like Remove if more appropriate.

Examples:

  • AwardEmojiRemove
  • NoteDelete

If you need advice for mutation naming, canvass the Slack #graphql channel for feedback.

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 in a single request.

The resolve method

Similar to writing resolvers, the resolve method of a mutation should aim to be a thin declarative wrapper around a service.

The resolve method receives the mutation's arguments as keyword arguments. From here, we can call the service that modifies the resource.

The resolve method should then return a hash with the same field names as defined on the mutation including an errors array. For example, the Mutations::MergeRequests::SetDraft 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 of strings if the mutation failed after authorization.
  # The `errors_on_object` helper collects `errors.full_messages`
  errors: errors_on_object(merge_request)
}

Mounting the mutation

To make the mutation available it must be defined on the mutation type that is stored in graphql/types/mutation_type. The mount_mutation helper method defines a field based on the GraphQL-name of the mutation:

module Types
  class MutationType < BaseObject
    graphql_name 'Mutation'

    include Gitlab::Graphql::MountMutation

    mount_mutation Mutations::MergeRequests::SetDraft
  end
end

Generates a field called mergeRequestSetDraft that Mutations::MergeRequests::SetDraft 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 SetDraft < Base
      graphql_name 'MergeRequestSetDraft'

      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 loads 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 by calling raise_resource_not_available_error! from in the resolve method.

Errors in mutations

We encourage following the practice of errors as data for mutations, which distinguishes errors by who they are relevant to, defined by who can deal with them.

Key points:

  • All mutation responses have an errors field. This should be populated on failure, and may be populated on success.
  • Consider who needs to see the error: the user or the developer.
  • Clients should always request the errors field when performing mutations.
  • Errors may be reported to users either at $root.errors (top-level error) or at $root.data.mutationName.errors (mutation errors). The location depends on what kind of error this is, and what information it holds.
  • Mutation fields must have null: true

Consider an example mutation doTheThing that returns a response with two fields: errors: [String], and thing: ThingType. The specific nature of the thing itself is irrelevant to these examples, as we are considering the errors.

The three states a mutation response can be in are:

Success

In the happy path, errors may be returned, along with the anticipated payload, but if everything was successful, then errors should be an empty array, because there are no problems we need to inform the user of.

{
  data: {
    doTheThing: {
      errors: [] // if successful, this array will generally be empty.
      thing: { .. }
    }
  }
}

Failure (relevant to the user)

An error that affects the user occurred. We refer to these as mutation errors.

In a create mutation there is typically no thing to return.

In an update mutation we return the current true state of thing. Developers may need to call #reset on the thing instance to ensure this happens.

{
  data: {
    doTheThing: {
      errors: ["you cannot touch the thing"],
      thing: { .. }
    }
  }
}

Examples of this include:

  • Model validation errors: the user may need to change the inputs.
  • Permission errors: the user needs to know they cannot do this, they may need to request permission or sign in.
  • Problems with the application state that prevent the user's action (for example, merge conflicts or a locked resource).

Ideally, we should prevent the user from getting this far, but if they do, they need to be told what is wrong, so they understand the reason for the failure and what they can do to achieve their intent. For example, they might only need to retry the request.

It is possible to return recoverable errors alongside mutation data. For example, if a user uploads 10 files and 3 of them fail and the rest succeed, the errors for the failures can be made available to the user, alongside the information about the successes.

Failure (irrelevant to the user)

One or more non-recoverable errors can be returned at the top level. These are things over which the user has little to no control, and should mainly be system or programming problems, that a developer needs to know about. In this case there is no data:

{
  errors: [
    {"message": "argument error: expected an integer, got null"},
  ]
}

This results from raising an error during the mutation. In our implementation, the messages of argument errors and validation errors are returned to the client, and all other StandardError instances are caught, logged and presented to the client with the message set to "Internal server error". See GraphqlController for details.

These represent programming errors, such as:

  • A GraphQL syntax error, where an Int was passed instead of a String, or a required argument was not present.
  • Errors in our schema, such as being unable to provide a value for a non-nullable field.
  • System errors: for example, a Git storage exception, or database unavailability.

The user should not be able to cause such errors in regular usage. This category of errors should be treated as internal, and not shown to the user in specific detail.

We need to inform the user when the mutation fails, but we do not need to tell them why, because they cannot have caused it, and nothing they can do fixes it, although we may offer to retry the mutation.

Categorizing errors

When we write mutations, we need to be conscious about which of these two categories an error state falls into (and communicate about this with frontend developers to verify our assumptions). This means distinguishing the needs of the user from the needs of the client.

Never catch an error unless the user needs to know about it.

If the user does need to know about it, communicate with frontend developers to make sure the error information we are passing back is relevant and serves a purpose.

See also the frontend GraphQL guide.

Aliasing and deprecating mutations

The #mount_aliased_mutation helper allows us to alias a mutation as another name in MutationType.

For example, to alias a mutation called FooMutation as BarMutation:

mount_aliased_mutation 'BarMutation', Mutations::FooMutation

This allows us to rename a mutation and continue to support the old name, when coupled with the deprecated argument.

Example:

mount_aliased_mutation 'UpdateFoo',
                        Mutations::Foo::Update,
                        deprecated: { reason: 'Use fooUpdate', milestone: '13.2' }

Deprecated mutations should be added to Types::DeprecatedMutations and tested for in the unit test of Types::MutationType. The merge request !34798 can be referred to as an example of this, including the method of testing deprecated aliased mutations.

Deprecating EE mutations

EE mutations should follow the same process. For an example of the merge request process, read merge request !42588.

Subscriptions

We use subscriptions to push updates to clients. We use the Action Cable implementation to deliver the messages over websockets.

When a client subscribes to a subscription, we store their query in-memory in Puma workers. Then when the subscription is triggered, the Puma workers execute the stored GraphQL queries and push the results to the clients.

NOTE: We cannot test subscriptions using GraphiQL, because they require an Action Cable client, which GraphiQL does not support at the moment.

Building subscriptions

All fields under Types::SubscriptionType are subscriptions that clients can subscribe to. These fields require a subscription class, which is a descendant of Subscriptions::BaseSubscription and is stored under app/graphql/subscriptions.

The arguments required to subscribe and the fields that are returned are defined in the subscription class. Multiple fields can share the same subscription class if they have the same arguments and return the same fields.

This class runs during the initial subscription request and subsequent updates. You can read more about this in the GraphQL Ruby guides.

Authorization

You should implement the #authorized? method of the subscription class so that the initial subscription and subsequent updates are authorized.

When a user is not authorized, you should call the unauthorized! helper so that execution is halted and the user is unsubscribed. Returning false results in redaction of the response, but we leak information that some updates are happening. This leakage is due to a bug in the GraphQL gem.

Triggering subscriptions

Define a method under the GraphqlTriggers module to trigger a subscription. Do not call GitlabSchema.subscriptions.trigger directly in application code so that we have a single source of truth and we do not trigger a subscription with different arguments and objects.

Pagination implementation

For more information, see GraphQL pagination.

Arguments

Arguments for a resolver or mutation are defined using argument.

Example:

argument :my_arg, GraphQL::Types::String,
         required: true,
         description: "A description of the argument."

Nullability

Arguments can be marked as required: true which means the value must be present and not null. If a required argument's value can be null, use the required: :nullable declaration.

Example:

argument :due_date,
         Types::TimeType,
         required: :nullable,
         description: 'The desired due date for the issue. Due date is removed if null.'

In the above example, the due_date argument must be given, but unlike the GraphQL spec, the value can be null. This allows 'unsetting' the due date in a single mutation rather than creating a new mutation for removing the due date.

{ due_date: null } # => OK
{ due_date: "2025-01-10" } # => OK
{  } # => invalid (not given)

Nullability and required: false

If an argument is marked required: false the client is permitted to send null as a value. Often this is undesirable.

If an argument is optional but null is not an allowed value, use validation to ensure that passing null returns an error:

argument :name, GraphQL::Types::String,
         required: false,
         validates: { allow_null: false }

Alternatively, if you wish to allow null when it is not an allowed value, you can replace it with a default value:

argument :name, GraphQL::Types::String,
         required: false,
         default_value: "No Name Provided",
         replace_null_with_default: true

See Validation, Nullability and Default Values for more details.

Mutually exclusive arguments

Arguments can be marked as mutually exclusive, ensuring that they are not provided at the same time. When more than one of the listed arguments are given, a top-level error will be added.

Example:

argument :user_id, GraphQL::Types::String, required: false
argument :username, GraphQL::Types::String, required: false

validates mutually_exclusive: [:user_id, :username]

When exactly one argument is required, you can use the exactly_one_of validator.

Example:

argument :group_path, GraphQL::Types::String, required: false
argument :project_path, GraphQL::Types::String, required: false

validates exactly_one_of: [:group_path, :project_path]

Keywords

Each GraphQL argument defined is passed to the #resolve method of a mutation as keyword arguments.

Example:

def resolve(my_arg:)
  # Perform mutation ...
end

Input Types

graphql-ruby wraps up arguments into an input type.

For example, the mergeRequestSetDraft mutation defines these arguments (some through inheritance):

argument :project_path, GraphQL::Types::ID,
         required: true,
         description: "Project the merge request belongs to."

argument :iid, GraphQL::Types::String,
         required: true,
         description: "IID of the merge request."

argument :draft,
         GraphQL::Types::Boolean,
         required: false,
         description: <<~DESC
           Whether or not to set the merge request as a draft.
         DESC

These arguments automatically generate an input type called MergeRequestSetDraftInput with the 3 arguments we specified and the clientMutationId.

Object identifier arguments

Arguments that identify an object should be:

Full path object identifier arguments

Historically we have been inconsistent with the naming of full path arguments, but prefer to name the argument:

  • project_path for a project full path
  • group_path for a group full path
  • namespace_path for a namespace full path

Using an example from the ciJobTokenScopeRemoveProject mutation:

argument :project_path, GraphQL::Types::ID,
         required: true,
         description: 'Project the CI job token scope belongs to.'

IID object identifier arguments

Use the iid of an object in combination with its parent project_path or group_path. For example:

argument :project_path, GraphQL::Types::ID,
         required: true,
         description: 'Project the issue belongs to.'

argument :iid, GraphQL::Types::String,
         required: true,
         description: 'IID of the issue.'

Global ID object identifier arguments

Using an example from the discussionToggleResolve mutation:

argument :id, Types::GlobalIDType[Discussion],
         required: true,
         description: 'Global ID of the discussion.'

See also Deprecate Global IDs.

Sort arguments

Sort arguments should use an enum type whenever possible to describe the set of available sorting values.

The enum can inherit from Types::SortEnum to inherit some common values.

The enum values should follow the format {PROPERTY}_{DIRECTION}. For example:

TITLE_ASC

Also see the description style guide for sort enums.

Example from ContainerRepositoriesResolver:

# Types::ContainerRepositorySortEnum:
module Types
  class ContainerRepositorySortEnum < SortEnum
    graphql_name 'ContainerRepositorySort'
    description 'Values for sorting container repositories'

    value 'NAME_ASC', 'Name by ascending order.', value: :name_asc
    value 'NAME_DESC', 'Name by descending order.', value: :name_desc
  end
end

# Resolvers::ContainerRepositoriesResolver:
argument :sort, Types::ContainerRepositorySortEnum,
          description: 'Sort container repositories by this criteria.',
          required: false,
          default_value: :created_desc

GitLab 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.'

Global ID scalars

All of our Global IDs are custom scalars. They are dynamically created from the abstract scalar class Types::GlobalIDType.

Testing

For testing mutations and resolvers, consider the unit of test a full GraphQL request, not a call to a resolver. This allows us to avoid tight coupling to the framework because such coupling makes upgrades to dependencies much more difficult.

You should:

  • Prefer request specs (either using the full API endpoint or going through GitlabSchema.execute) to unit specs for resolvers and mutations.
  • Prefer GraphqlHelpers#execute_query and GraphqlHelpers#run_with_clean_state to GraphqlHelpers#resolve and GraphqlHelpers#resolve_field.

For example:

# Good:
gql_query = %q(some query text...)
post_graphql(gql_query, current_user: current_user)
# or:
GitlabSchema.execute(gql_query, context: { current_user: current_user })

# Deprecated: avoid
resolve(described_class, obj: project, ctx: { current_user: current_user })

Writing unit tests (deprecated)

WARNING: Avoid writing unit tests if the same thing can be tested with a full GraphQL request.

Before creating unit tests, review the following examples:

Writing integration tests

Integration tests check the full stack for a GraphQL query or mutation and are stored in spec/requests/api/graphql.

For speed, consider calling GitlabSchema.execute directly, or making use of smaller test schemas that only contain the types under test.

However, full request integration tests that check if data is returned verify the following additional items:

  • The mutation is actually queryable in the schema (was mounted in MutationType).
  • The data returned by a resolver or mutation correctly matches the return types of the fields and resolves without errors.
  • The arguments coerce correctly on input, and the fields serialize correctly on output.

Integration tests can also verify the following items, because they invoke the full stack:

  • An argument or scalar's validations apply correctly.
  • Logic in a resolver or mutation's #ready? method applies correctly.
  • An argument's default_value applies correctly.
  • Objects resolve successfully, and there are no N+1 issues.

When adding a query, you can use the a working graphql query that returns data and a working graphql query that returns no data shared examples to test if the query renders valid results.

You can construct a query including all available fields using the GraphqlHelpers#all_graphql_fields_for helper. This makes it more straightforward to add a test rendering all possible fields for a query.

If you're adding a field to a query that supports pagination and sorting, visit Testing for details.

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

You can then pass this struct to the post_graphql_mutation helper, that posts the request with the correct parameters, like a GraphQL client would do.

To access the response of a mutation, you can use the graphql_mutation_response helper.

Using these helpers, you 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

Testing tips and tricks

  • Become familiar with the methods in the GraphqlHelpers support module. Many of these methods make writing GraphQL tests easier.

  • Use traversal helpers like GraphqlHelpers#graphql_data_at and GraphqlHelpers#graphql_dig_at to access result fields. For example:

    result = GitlabSchema.execute(query)

mr_iid = graphql_dig_at(result.to_h, :data, :project, :merge_request, :iid)

  • Use GraphqlHelpers#a_graphql_entity_for to match against results. For example:

    post_graphql(some_query)

# checks that it is a hash containing { id => global_id_of(issue) }
expect(graphql_data_at(:project, :issues, :nodes))
  .to contain_exactly(a_graphql_entity_for(issue))

# Additional fields can be passed, either as names of methods, or with values
expect(graphql_data_at(:project, :issues, :nodes))
  .to contain_exactly(a_graphql_entity_for(issue, :iid, :title, created_at: some_time))

  • Use GraphqlHelpers#empty_schema to create an empty schema, rather than creating one by hand. For example:

    # good
let(:schema) { empty_schema }

# bad
let(:query_type) { GraphQL::ObjectType.new }
let(:schema) { GraphQL::Schema.define(query: query_type, mutation: nil)}

  • Use GraphqlHelpers#query_double(schema: nil) of double('query', schema: nil). For example:

    # good
let(:query) { query_double(schema: GitlabSchema) }

# bad
let(:query) { double('Query', schema: GitlabSchema) }

  • Avoid false positives:

    Authenticating a user with the current_user: argument for post_graphql generates more queries on the first request than on subsequent requests on that same user. If you are testing for N+1 queries using QueryRecorder, use a different user for each request.

    The below example shows how a test for avoiding N+1 queries should look:

    RSpec.describe 'Query.project(fullPath).pipelines' do
  include GraphqlHelpers

  let(:project) { create(:project) }

  let(:query) do
    %(
      {
        project(fullPath: "#{project.full_path}") {
          pipelines {
            nodes {
              id
            }
          }
        }
      }
    )
  end

  it 'avoids N+1 queries' do
    first_user = create(:user)
    second_user = create(:user)
    create(:ci_pipeline, project: project)

    control_count = ActiveRecord::QueryRecorder.new do
      post_graphql(query, current_user: first_user)
    end

    create(:ci_pipeline, project: project)

    expect do
      post_graphql(query, current_user: second_user)  # use a different user to avoid a false positive from authentication queries
    end.not_to exceed_query_limit(control_count)
  end
end

  • Mimic the folder structure of app/graphql/types:

    For example, tests for fields on Types::Ci::PipelineType in app/graphql/types/ci/pipeline_type.rb should be stored in spec/requests/api/graphql/ci/pipeline_spec.rb regardless of the query being used to fetch the pipeline data.

  • When testing resolvers using GraphqlHelpers#resolve, arguments for the resolver can be handled two ways.

    1. 95% of the resolver specs use arguments that are Ruby objects, as opposed to when using the GraphQL API only strings and integers are used. This works fine in most cases.

    2. If your resolver takes arguments that use a prepare proc, such as a resolver that accepts time frame arguments (TimeFrameArguments), you must pass the arg_style: :internal_prepared parameter into the resolve method. This tells the code to convert the arguments into strings and integers and pass them through regular argument handling, ensuring that the prepare proc is called correctly. For example in iterations_resolver_spec.rb:

      def resolve_group_iterations(args = {}, obj = group, context = { current_user: current_user })
  resolve(described_class, obj: obj, args: args, ctx: context, arg_style: :internal_prepared)
end

      One additional caveat is that if you are passing enums as a resolver argument, you must use the external representation of the enum, rather than the internal. For example:

      # good
resolve_group_iterations({ search: search, in: ['CADENCE_TITLE'] })

# bad
resolve_group_iterations({ search: search, in: [:cadence_title] })

    The use of :internal_prepared was added as a bridge for the GraphQL gem upgrade. Testing resolvers directly will eventually be removed, and writing unit tests for resolvers/mutations is already deprecated

Notes about Query flow and GraphQL infrastructure

The GitLab 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 can be used for objects that are expensive (such as 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.

To help our readers, you should also add a new page to our GraphQL API documentation. For guidance, see the GraphQL API page.

Include a changelog entry

All client-facing changes must include a changelog entry.

Laziness

One important technique unique to GraphQL for managing performance is using lazy values. Lazy values represent the promise of a result, allowing their action to be run later, which enables batching of queries in different parts of the query tree. The main example of lazy values in our code is the GraphQL BatchLoader.

To manage lazy values directly, read Gitlab::Graphql::Lazy, and in particular Gitlab::Graphql::Laziness. This contains #force and #delay, which help implement the basic operations of creation and elimination of laziness, where needed.

For dealing with lazy values without forcing them, use Gitlab::Graphql::Lazy.with_value.