Add limited count to VSA

This exposes the count of issues or MRs per value stream stage. To
avoid counting large amount of DB rows, we use a limit count.

ee/app/controllers/groups/analytics/cycle_analytics/stages_controller.rb

@@ -9,7 +9,8 @@ module Groups
 
         before_action :load_group
         before_action :load_value_stream
-        before_action :validate_params, only: %i[median average records duration_chart average_duration_chart]
+        before_action :validate_params, only: %i[median average records duration_chart average_duration_chart count]
+        before_action :authorize_read_group_stage, only: %i[median average records duration_chart average_duration_chart count]
 
         def index
           return render_403 unless can?(current_user, :read_group_cycle_analytics, @group)
@@ -42,20 +43,14 @@ module Groups
         end
 
         def median
-          return render_403 unless can?(current_user, :read_group_stage, @group)
-
           render json: { value: data_collector.median.seconds }
         end
 
         def average
-          return render_403 unless can?(current_user, :read_group_stage, @group)
-
           render json: { value: data_collector.average.seconds }
         end
 
         def records
-          return render_403 unless can?(current_user, :read_group_stage, @group)
-
           serialized_records = data_collector.serialized_records do |relation|
             add_pagination_headers(relation)
           end
@@ -64,17 +59,17 @@ module Groups
         end
 
         def duration_chart
-          return render_403 unless can?(current_user, :read_group_stage, @group)
-
           render json: ::Analytics::CycleAnalytics::DurationChartItemEntity.represent(data_collector.duration_chart_data)
         end
 
         def average_duration_chart
-          return render_403 unless can?(current_user, :read_group_stage, @group)
-
           render json: ::Analytics::CycleAnalytics::DurationChartAverageItemEntity.represent(data_collector.duration_chart_average_data)
         end
 
+        def count
+          render json: { count: data_collector.count }
+        end
+
         private
 
         def data_collector
@@ -156,6 +151,10 @@ module Groups
             params: permitted_cycle_analytics_params
           ).execute(exclude_total_headers: true, data_without_counts: true)
         end
+
+        def authorize_read_group_stage
+          return render_403 unless can?(current_user, :delete_group_stage, @group)
+        end
       end
     end
   end

ee/config/routes/group.rb

@@ -32,6 +32,7 @@ constraints(::Constraints::GroupUrlConstrainer.new) do
             get :median
             get :average
             get :records
+            get :count
           end
         end
         resources :value_streams, only: [:index, :create, :update, :destroy] do
@@ -42,6 +43,7 @@ constraints(::Constraints::GroupUrlConstrainer.new) do
               get :median
               get :average
               get :records
+              get :count
             end
           end
         end

ee/spec/lib/gitlab/analytics/cycle_analytics/data_collector_spec.rb

@@ -110,6 +110,12 @@ RSpec.describe Gitlab::Analytics::CycleAnalytics::DataCollector do
         })
       end
     end
+
+    describe '#count' do
+      subject(:count) { data_collector.count }
+
+      it { is_expected.to eq(3) }
+    end
   end
 
   shared_examples 'test various start and end event combinations' do
@@ -692,4 +698,52 @@ RSpec.describe Gitlab::Analytics::CycleAnalytics::DataCollector do
       end
     end
   end
+
+  describe 'limit count' do
+    let_it_be(:user) { create(:user) }
+    let_it_be(:group) { create(:group) }
+    let_it_be(:project) { create(:project, :repository, group: group) }
+
+    let(:merge_request) { merge_requests.first }
+
+    let(:stage) do
+      Analytics::CycleAnalytics::GroupStage.new(
+        name: 'My Stage',
+        group: group,
+        start_event_identifier: :merge_request_created,
+        end_event_identifier: :merge_request_merged
+      )
+    end
+
+    before do
+      merge_requests = create_list(:merge_request, 3, :unique_branches, target_project: project, source_project: project)
+      merge_requests.each { |mr| mr.metrics.update!(merged_at: 10.days.from_now) }
+
+      project.add_user(user, Gitlab::Access::DEVELOPER)
+    end
+
+    subject(:count) do
+      described_class.new(stage: stage, params: {
+        from: 5.months.ago,
+        to: 5.months.from_now,
+        current_user: user
+      }).count
+    end
+
+    context 'when limit is reached' do
+      before do
+        stub_const('Gitlab::Analytics::CycleAnalytics::DataCollector::MAX_COUNT', 2)
+      end
+
+      it 'shows the MAX COUNT' do
+        is_expected.to eq(2)
+      end
+    end
+
+    context 'when limit is not reached' do
+      it 'shows the actual count' do
+        is_expected.to eq(3)
+      end
+    end
+  end
 end

ee/spec/support/shared_examples/controllers/analytics/cycle_analytics/shared_stage_shared_examples.rb

@@ -280,6 +280,20 @@ RSpec.shared_examples 'Value Stream Analytics Stages controller' do
       include_examples 'Value Stream Analytics data endpoint examples'
       include_examples 'group permission check on the controller level'
     end
+
+    describe 'GET #count' do
+      subject { get :count, params: params }
+
+      it 'matches the response schema' do
+        subject
+
+        expect(response).to be_successful
+        expect(json_response['count']).to eq(0)
+      end
+
+      include_examples 'Value Stream Analytics data endpoint examples'
+      include_examples 'group permission check on the controller level'
+    end
   end
 end
 

lib/gitlab/analytics/cycle_analytics/data_collector.rb

@@ -12,6 +12,8 @@ module Gitlab
       class DataCollector
         include Gitlab::Utils::StrongMemoize
 
+        MAX_COUNT = 1001
+
         delegate :serialized_records, to: :records_fetcher
 
         def initialize(stage:, params: {})
@@ -37,6 +39,12 @@ module Gitlab
           end
         end
 
+        def count
+          strong_memoize(:count) do
+            limit_count
+          end
+        end
+
         private
 
         attr_reader :stage, :params
@@ -44,6 +52,13 @@ module Gitlab
         def query
           BaseQueryBuilder.new(stage: stage, params: params).build
         end
+
+        # Limiting the maximum number of records so the COUNT(*) query stays efficient for large groups.
+        # COUNT = 1001, show 1000+ on the UI
+        # COUNT < 1001, show the actual number on the UI
+        def limit_count
+          query.limit(MAX_COUNT).count
+        end
       end
     end
   end