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GraphQL API
This document outlines the styleguide 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 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: false
field :title, GraphQL::STRING_TYPE, null: false
# we also have a method here that we've defined, that extends `field`
markdown_field :title_html, null: true
field :description, GraphQL::STRING_TYPE, null: true
markdown_field :description_html, null: true
We give each type a name (in this case Issue
).
The iid
, title
and description
are scalar GraphQL types.
iid
is a GraphQL::ID_TYPE
, a special string type that signifies a unique ID.
title
and description
are regular GraphQL::STRING_TYPE
types.
When exposing a model through the GraphQL API, we do so by creating a
new type in app/graphql/types
. You can also declare custom GraphQL data types
for scalar data types (e.g. TimeType
).
When exposing properties in a type, make sure to keep the logic inside the definition as minimal as possible. Instead, consider moving any logic into a presenter:
class Types::MergeRequestType < BaseObject
present_using MergeRequestPresenter
name 'MergeRequest'
end
An existing presenter could be used, but it is also possible to create a new presenter specifically for GraphQL.
The presenter is initialized using the object resolved by a field, and the context.
Nullable fields
GraphQL allows fields to be 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
- For instance, the
- 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.
Further reading:
Exposing Global IDs
When exposing an ID
field on a type, we will by default try to
expose a global ID by calling to_global_id
on the resource being
rendered.
To override this behaviour, you can implement an id
method on the
type for which you are exposing an ID. Please make sure that when
exposing a GraphQL::ID_TYPE
using a custom method that it is
globally unique.
The records that are exposing a full_path
as an ID_TYPE
are one of
these exceptions. Since the full path is a unique identifier for a
Project
or Namespace
.
Connection Types
GraphQL uses cursor based pagination to expose collections of items. This provides the clients with a lot of flexibility while also allowing the backend to use different pagination models.
To expose a collection of resources we can use a connection type. This wraps the array with default pagination fields. For example a query for project-pipelines could look like this:
query($project_path: ID!) {
project(fullPath: $project_path) {
pipelines(first: 2) {
pageInfo {
hasNextPage
hasPreviousPage
}
edges {
cursor
node {
id
status
}
}
}
}
}
This would return the first 2 pipelines of a project and related pagination info., ordered by descending ID. The returned data would look like this:
{
"data": {
"project": {
"pipelines": {
"pageInfo": {
"hasNextPage": true,
"hasPreviousPage": false
},
"edges": [
{
"cursor": "Nzc=",
"node": {
"id": "gid://gitlab/Pipeline/77",
"status": "FAILED"
}
},
{
"cursor": "Njc=",
"node": {
"id": "gid://gitlab/Pipeline/67",
"status": "FAILED"
}
}
]
}
}
}
}
To get the next page, the cursor of the last known element could be passed:
query($project_path: ID!) {
project(fullPath: $project_path) {
pipelines(first: 2, after: "Njc=") {
pageInfo {
hasNextPage
hasPreviousPage
}
edges {
cursor
node {
id
status
}
}
}
}
}
To ensure that we get consistent ordering, we will append an ordering on the primary
key, in descending order. This is usually id
, so basically we will add order(id: :desc)
to the end of the relation. A primary key must be available on the underlying table.
Exposing permissions for a type
To expose permissions the current user has on a resource, you can call
the expose_permissions
passing in a separate type representing the
permissions for the resource.
For example:
module Types
class MergeRequestType < BaseObject
expose_permissions Types::MergeRequestPermissionsType
end
end
The permission type inherits from BasePermissionType
which includes
some helper methods, that allow exposing permissions as non-nullable
booleans:
class MergeRequestPermissionsType < BasePermissionType
present_using MergeRequestPresenter
graphql_name 'MergeRequestPermissions'
abilities :admin_merge_request, :update_merge_request, :create_note
ability_field :resolve_note,
description: 'Indicates the user can resolve discussions on the merge request'
permission_field :push_to_source_branch, method: :can_push_to_source_branch?
end
permission_field
: Will act the same asgraphql-ruby
'sfield
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 aspermission_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
Features controlled by feature flags often provide GraphQL functionality. When a feature is enabled or disabled by a feature flag, the related GraphQL functionality should also be enabled or disabled.
Fields can be put behind a feature flag so they can conditionally return the value for the field depending on if the feature has been enabled or not.
GraphQL feature flags use the common
GitLab feature flag system, and can be added to a
field using the feature_flag
property.
For example:
field :test_field, type: GraphQL::STRING_TYPE,
null: false,
description: 'Some test field',
feature_flag: :some_feature_flag
In the above example, the test_field
field will only be returned if
the some_feature_flag
feature flag is enabled.
If the feature flag is not enabled, an error will be returned saying the field does not exist.
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 styleguide
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 stringEnum
.
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:
Description styleguide
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 withThe
. - 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 typeTypes::TimeType
. This lets the reader know that the format of the property will beTime
, rather than justDate
. - 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. Othewise use a globally unique ID.
We already have a FullPathLoader
that can be included in other
resolvers to quickly find Projects and Namespaces which will have a
lot of dependant objects.
To limit the amount of queries performed, we can use BatchLoader
.
Mutations
Mutations are used to change any stored values, or to trigger actions. In the same way a GET-request should not modify data, we cannot modify data in a regular GraphQL-query. We can however in a mutation.
To find objects for a mutation, arguments need to be specified. As with resolvers, prefer using internal ID or, if needed, a global ID rather than the database ID.
Fields
In the most common situations, a mutation would return 2 fields:
- 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
andDateTime
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: false, description: 'Timestamp of when the issue was created'
Testing
full stack tests for a graphql query or mutation live in
spec/requests/api/graphql
.
When adding a query, the a working graphql query
shared example can
be used to test if the query renders valid results.
Using the GraphqlHelpers#all_graphql_fields_for
-helper, a query
including all available fields can be constructed. This makes it easy
to add a test rendering all possible fields for a query.
To test GraphQL mutation requests, GraphqlHelpers
provides 2
helpers: graphql_mutation
which takes the name of the mutation, and
a hash with the input for the mutation. This will return a struct with
a mutation query, and prepared variables.
This struct can then be passed to the post_graphql_mutation
helper,
that will post the request with the correct params, like a GraphQL
client would do.
To access the response of a mutation, the graphql_mutation_response
helper is available.
Using these helpers, we can build specs like this:
let(:mutation) do
graphql_mutation(
:merge_request_set_wip,
project_path: 'gitlab-org/gitlab-foss',
iid: '1',
wip: true
)
end
it 'returns a successful response' do
post_graphql_mutation(mutation, current_user: user)
expect(response).to have_gitlab_http_status(:success)
expect(graphql_mutation_response(:merge_request_set_wip)['errors']).to be_empty
end
Notes about Query flow and GraphQL infrastructure
GitLab's GraphQL infrastructure can be found in lib/gitlab/graphql
.
Instrumentation is functionality
that wraps around a query being executed. It is implemented as a module that uses the Instrumentation
class.
Example: Present
module Present
#... some code above...
def self.use(schema_definition)
schema_definition.instrument(:field, Instrumentation.new)
end
end
A Query Analyzer contains a series of callbacks to validate queries before they are executed. Each field can pass through the analyzer, and the final value is also available to you.
Multiplex queries enable multiple queries to be sent in a single request. This reduces the number of requests sent to the server. (there are custom Multiplex Query Analyzers and Multiplex Instrumentation provided by graphql-ruby).
Query limits
Queries and mutations are limited by depth, complexity, and recursion to protect server resources from overly ambitious or malicious queries. These values can be set as defaults and overridden in specific queries as needed. The complexity values can be set per object as well, and the final query complexity is evaluated based on how many objects are being returned. This is useful for objects that are expensive (e.g. requiring Gitaly calls).
For example, a conditional complexity method in a resolver:
def self.resolver_complexity(args, child_complexity:)
complexity = super
complexity += 2 if args[:labelName]
complexity
end
More about complexity: graphql-ruby docs
Documentation and Schema
Our schema is located at app/graphql/gitlab_schema.rb
.
See the schema reference for details.
This generated GraphQL documentation needs to be updated when the schema changes. For information on generating GraphQL documentation and schema files, see updating the schema documentation.