15 KiB
GraphQL API
Deep Dive
In March 2019, Nick Thomas hosted a Deep Dive on GitLab's [GraphQL API] to share his domain specific knowledge with anyone who may work in this part of the code base in the future. You can find the recording on YouTube, and the slides on Google Slides and in PDF. Everything covered in this deep dive was accurate as of GitLab 11.9, and while specific details may have changed since then, it should still serve as a good introduction.
Authentication
Authentication happens through the GraphqlController
, right now this
uses the same authentication as the Rails application. So the session
can be shared.
It is also possible to add a private_token
to the querystring, or
add a HTTP_PRIVATE_TOKEN
header.
Types
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.
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
}
}
}
}
}
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: 'Whether or not the user can resolve disussions 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 takes 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.
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.
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
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.
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.
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 can include the
Gitlab::Graphql::Authorize::AuthorizeResource
concern in the
mutation.
This allows us to 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.
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-ce',
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