GraphQL pagination
Types of pagination
GitLab uses two primary types of pagination: offset and keyset (sometimes called cursor-based) pagination. The GraphQL API mainly uses keyset pagination, falling back to offset pagination when needed.
Performance considerations
See the general pagination guidelines section for more information.
Offset pagination
This is the traditional, page-by-page pagination, that is most common, and used across much of GitLab. You can recognize it by a list of page numbers near the bottom of a page, which, when selected, take you to that page of results.
For example, when you select Page 100, we send 100
to the
backend. For example, if each page has say 20 items, the
backend calculates 20 * 100 = 2000
,
and it queries the database by offsetting (skipping) the first 2000
records and pulls the next 20.
page number * page size = where to find my records
There are a couple of problems with this:
-
Performance. When we query for page 100 (which gives an offset of 2000), then the database has to scan through the table to that specific offset, and then pick up the next 20 records. As the offset increases, the performance degrades quickly. Read more in The SQL I Love <3. Efficient pagination of a table with 100M records.
-
Data stability. When you get the 20 items for page 100 (at offset 2000), GitLab shows those 20 items. If someone then deletes or adds records in page 99 or before, the items at offset 2000 become a different set of items. You can even get into a situation where, when paginating, you could skip over items, because the list keeps changing. Read more in Pagination: You're (Probably) Doing It Wrong.
Keyset pagination
Given any specific record, if you know how to calculate what comes after it, you can query the database for those specific records.
For example, suppose you have a list of issues sorted by creation date. If you know the first item on a page has a specific date (say Jan 1), you can ask for all records that were created after that date and take the first 20. It no longer matters if many are deleted or added, as you always ask for the ones after that date, and so get the correct items.
Unfortunately, there is no easy way to know if the issue created on Jan 1 is on page 20 or page 100.
Some of the benefits and tradeoffs of keyset pagination are
-
Performance is much better.
-
More data stability for end-users since records are not missing from lists due to deletions or insertions.
-
It's the best way to do infinite scrolling.
-
It's more difficult to program and maintain. Easy for
updated_at
andsort_order
, complicated (or impossible) for complex sorting scenarios.
Implementation
When pagination is supported for a query, GitLab defaults to using
keyset pagination. You can see where this is configured in
pagination/connections.rb
.
If a query returns ActiveRecord::Relation
, keyset pagination is automatically used.
This was a conscious decision to support performance and data stability.
However, there are some cases where we have to use the offset
pagination connection, OffsetActiveRecordRelationConnection
, such as when
sorting by label priority in issues, due to the complexity of the sort.
If you return a relation from a resolver that is not suitable for keyset
pagination (due to the sort order for example), then you can use the
BaseResolver#offset_pagination
method to wrap the relation in the correct
connection type. For example:
def resolve(**args)
result = Finder.new(object, current_user, args).execute
result = offset_pagination(result) if needs_offset?(args[:sort])
result
end
Keyset pagination
The keyset pagination implementation is a subclass of GraphQL::Pagination::ActiveRecordRelationConnection
,
which is a part of the graphql
gem. This is installed as the default for all ActiveRecord::Relation
.
However, instead of using a cursor based on an offset (which is the default), GitLab uses a more specialized cursor.
The cursor is created by encoding a JSON object which contains the relevant ordering fields. For example:
ordering = {"id"=>"72410125", "created_at"=>"2020-10-08 18:05:21.953398000 UTC"}
json = ordering.to_json
cursor = Base64.urlsafe_encode64(json, padding: false)
"eyJpZCI6IjcyNDEwMTI1IiwiY3JlYXRlZF9hdCI6IjIwMjAtMTAtMDggMTg6MDU6MjEuOTUzMzk4MDAwIFVUQyJ9"
json = Base64.urlsafe_decode64(cursor)
Gitlab::Json.parse(json)
{"id"=>"72410125", "created_at"=>"2020-10-08 18:05:21.953398000 UTC"}
The benefits of storing the order attribute values in the cursor:
- If only the ID of the object were stored, the object and its attributes could be queried. That would require an additional query, and if the object is no longer there, then the needed attributes are not available.
- If an attribute is
NULL
, then one SQL query can be used. If it's notNULL
, then a different SQL query can be used.
Based on whether the main attribute field being sorted on is NULL
in the cursor, the proper query
condition is built. The last ordering field is considered to be unique (a primary key), meaning the
column never contains NULL
values.
Limitations of query complexity
We only support two ordering fields, and one of those fields needs to be the primary key.
Here are two examples of pseudocode for the query:
-
Two-condition query.
X
represents the values from the cursor.C
represents the columns in the database, sorted in ascending order, using an:after
cursor, and withNULL
values sorted last.X1 IS NOT NULL AND (C1 > X1) OR (C1 IS NULL) OR (C1 = X1 AND C2 > X2) X1 IS NULL AND (C1 IS NULL AND C2 > X2)
Below is an example based on the relation
Issue.order(relative_position: :asc).order(id: :asc)
with an after cursor ofrelative_position: 1500, id: 500
:when cursor[relative_position] is not NULL ("issues"."relative_position" > 1500) OR ( "issues"."relative_position" = 1500 AND "issues"."id" > 500 ) OR ("issues"."relative_position" IS NULL) when cursor[relative_position] is NULL "issues"."relative_position" IS NULL AND "issues"."id" > 500
-
Three-condition query. The example below is not complete, but shows the complexity of adding one more condition.
X
represents the values from the cursor.C
represents the columns in the database, sorted in ascending order, using an:after
cursor, and withNULL
values sorted last.X1 IS NOT NULL AND (C1 > X1) OR (C1 IS NULL) OR (C1 = X1 AND C2 > X2) OR (C1 = X1 AND X2 IS NOT NULL AND ((C2 > X2) OR (C2 IS NULL) OR (C2 = X2 AND C3 > X3) OR X2 IS NULL.....
By using
Gitlab::Graphql::Pagination::Keyset::QueryBuilder
,
we're able to build the necessary SQL conditions and apply them to the Active Record relation.
Complex queries can be difficult or impossible to use. For example,
in issuable.rb
,
the order_due_date_and_labels_priority
method creates a very complex query.
These types of queries are not supported. In these instances, you can use offset pagination.
Gotchas
Do not define a collection's order using the string syntax:
# Bad
items.order('created_at DESC')
Instead, use the hash syntax:
# Good
items.order(created_at: :desc)
The first example won't correctly embed the sort information (created_at
, in
the example above) into the pagination cursors, which will result in an
incorrect sort order.
Offset pagination
There are times when the complexity of sorting is more than our keyset pagination can handle.
For example, in ProjectIssuesResolver
,
when sorting by priority_asc
, we can't use keyset pagination as the ordering is much
too complex. For more information, read issuable.rb
.
In cases like this, we can fall back to regular offset pagination by returning a
Gitlab::Graphql::Pagination::OffsetActiveRecordRelationConnection
instead of an ActiveRecord::Relation
:
def resolve(parent, finder, **args)
issues = apply_lookahead(Gitlab::Graphql::Loaders::IssuableLoader.new(parent, finder).batching_find_all)
if non_stable_cursor_sort?(args[:sort])
# Certain complex sorts are not supported by the stable cursor pagination yet.
# In these cases, we use offset pagination, so we return the correct connection.
offset_pagination(issues)
else
issues
end
end
External pagination
There may be times where you need to return data through the GitLab API that is stored in another system. In these cases you may have to paginate a third-party's API.
An example of this is with our Error Tracking implementation, where we proxy Sentry errors through the GitLab API. We do this by calling the Sentry API which enforces its own pagination rules. This means we cannot access the collection within GitLab to perform our own custom pagination.
For consistency, we manually set the pagination cursors based on values returned by the external API, using Gitlab::Graphql::ExternallyPaginatedArray.new(previous_cursor, next_cursor, *items)
.
You can see an example implementation in the following files:
-
types/error__tracking/sentry_error_collection_type.rb
which adds an extension tofield :errors
. -
resolvers/error_tracking/sentry_errors_resolver.rb
which returns the data from the resolver.
Testing
Any GraphQL field that supports pagination and sorting should be tested
using the sorted paginated query shared example found in
graphql/sorted_paginated_query_shared_examples.rb
.
It helps verify that your sort keys are compatible and that cursors
work properly.
This is particularly important when using keyset pagination, as some sort keys might not be supported.
Add a section to your request specs like this:
describe 'sorting and pagination' do
...
end
You can then use
issues_spec.rb
as an example to construct your tests.
graphql/sorted_paginated_query_shared_examples.rb
also contains some documentation on how to use the shared examples.
The shared example requires certain let
variables and methods to be set up:
describe 'sorting and pagination' do
let_it_be(:sort_project) { create(:project, :public) }
let(:data_path) { [:project, :issues] }
def pagination_query(params)
graphql_query_for( :project, { full_path: sort_project.full_path },
query_nodes(:issues, :id, include_pagination_info: true, args: params))
)
end
def pagination_results_data(nodes)
nodes.map { |issue| issue['iid'].to_i }
end
context 'when sorting by weight' do
let_it_be(:issues) { make_some_issues_with_weights }
context 'when ascending' do
let(:ordered_issues) { issues.sort_by(&:weight) }
it_behaves_like 'sorted paginated query' do
let(:sort_param) { :WEIGHT_ASC }
let(:first_param) { 2 }
let(:all_records) { ordered_issues.map(&:iid) }
end
end
end