Cache the number of open issues and merge requests

Every project page displays a navigation menu that in turn displays the
number of open issues and merge requests. This means that for every
project page we run two COUNT(*) queries, each taking up roughly 30
milliseconds on GitLab.com. By caching these numbers and refreshing them
whenever necessary we can reduce loading times of all these pages by up
to roughly 60 milliseconds.

The number of open issues does not include confidential issues. This is
a trade-off to keep the code simple and to ensure refreshing the data
only needs 2 COUNT(*) queries instead of 3. A downside is that if a
project only has 5 confidential issues the counter will be set to 0.

Because we now have 3 similar counting service classes the code
previously used in Projects::ForksCountService has mostly been moved to
Projects::CountService, which in turn is reused by the various service
classes.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/36622

app/models/issue.rb

@@ -53,7 +53,10 @@ class Issue < ActiveRecord::Base
   scope :preload_associations, -> { preload(:labels, project: :namespace) }
+  scope :public_only, -> { where(confidential: false) }
+
   after_save :expire_etag_cache
+  after_commit :update_project_counter_caches, on: :destroy
   attr_spammable :title, spam_title: true
   attr_spammable :description, spam_description: true
@@ -269,6 +272,10 @@ class Issue < ActiveRecord::Base
     end
   end
+  def update_project_counter_caches
+    Projects::OpenIssuesCountService.new(project).refresh_cache
+  end
+
   private
   # Returns `true` if the given User can read the current Issue.

app/models/merge_request.rb

@@ -32,6 +32,7 @@ class MergeRequest < ActiveRecord::Base
   after_create :ensure_merge_request_diff, unless: :importing?
   after_update :reload_diff_if_branch_changed
+  after_commit :update_project_counter_caches, on: :destroy
   # When this attribute is true some MR validation is ignored
   # It allows us to close or modify broken merge requests
@@ -937,6 +938,10 @@ class MergeRequest < ActiveRecord::Base
     true
   end
+  def update_project_counter_caches
+    Projects::OpenMergeRequestsCountService.new(target_project).refresh_cache
+  end
+
   private
   def write_ref

app/models/project.rb

@@ -1158,7 +1158,11 @@ class Project < ActiveRecord::Base
   end
   def open_issues_count
-    issues.opened.count
+    Projects::OpenIssuesCountService.new(self).count
+  end
+
+  def open_merge_requests_count
+    Projects::OpenMergeRequestsCountService.new(self).count
   end
   def visibility_level_allowed_as_fork?(level = self.visibility_level)

app/services/issuable_base_service.rb

@@ -192,6 +192,8 @@ class IssuableBaseService < BaseService
   def after_create(issuable)
     # To be overridden by subclasses
+
+    issuable.update_project_counter_caches
   end
   def before_update(issuable)
@@ -200,6 +202,8 @@ class IssuableBaseService < BaseService
   def after_update(issuable)
     # To be overridden by subclasses
+
+    issuable.update_project_counter_caches
   end
   def update(issuable)

app/services/issues/create_service.rb

@@ -27,6 +27,8 @@ module Issues
       todo_service.new_issue(issuable, current_user)
       user_agent_detail_service.create
       resolve_discussions_with_issue(issuable)
+
+      super
     end
     def resolve_discussions_with_issue(issue)

app/services/merge_requests/create_service.rb

@@ -28,6 +28,8 @@ module MergeRequests
       todo_service.new_merge_request(issuable, current_user)
       issuable.cache_merge_request_closes_issues!(current_user)
       update_merge_requests_head_pipeline(issuable)
+
+      super
     end
     private

app/services/projects/count_service.rb

+module Projects
+  # Base class for the various service classes that count project data (e.g.
+  # issues or forks).
+  class CountService
+    def initialize(project)
+      @project = project
+    end
+
+    def relation_for_count
+      raise(
+        NotImplementedError,
+        '"relation_for_count" must be implemented and return an ActiveRecord::Relation'
+      )
+    end
+
+    def count
+      Rails.cache.fetch(cache_key) { uncached_count }
+    end
+
+    def refresh_cache
+      Rails.cache.write(cache_key, uncached_count)
+    end
+
+    def uncached_count
+      relation_for_count.count
+    end
+
+    def delete_cache
+      Rails.cache.delete(cache_key)
+    end
+
+    def cache_key_name
+      raise(
+        NotImplementedError,
+        '"cache_key_name" must be implemented and return a String'
+      )
+    end
+
+    def cache_key
+      ['projects', @project.id, cache_key_name]
+    end
+  end
+end

app/services/projects/forks_count_service.rb

 module Projects
   # Service class for getting and caching the number of forks of a project.
-  class ForksCountService
-    def initialize(project)
-      @project = project
+  class ForksCountService < CountService
+    def relation_for_count
+      @project.forks
     end
-    def count
-      Rails.cache.fetch(cache_key) { uncached_count }
-    end
-
-    def refresh_cache
-      Rails.cache.write(cache_key, uncached_count)
-    end
-
-    def delete_cache
-      Rails.cache.delete(cache_key)
-    end
-
-    private
-
-    def uncached_count
-      @project.forks.count
-    end
-
-    def cache_key
-      ['projects', @project.id, 'forks_count']
+    def cache_key_name
+      'forks_count'
     end
   end
 end

app/services/projects/open_issues_count_service.rb

+module Projects
+  # Service class for counting and caching the number of open issues of a
+  # project.
+  class OpenIssuesCountService < CountService
+    def relation_for_count
+      # We don't include confidential issues in this number since this would
+      # expose the number of confidential issues to non project members.
+      @project.issues.opened.public_only
+    end
+
+    def cache_key_name
+      'open_issues_count'
+    end
+  end
+end

app/services/projects/open_merge_requests_count_service.rb

+module Projects
+  # Service class for counting and caching the number of open merge requests of
+  # a project.
+  class OpenMergeRequestsCountService < CountService
+    def relation_for_count
+      @project.merge_requests.opened
+    end
+
+    def cache_key_name
+      'open_merge_requests_count'
+    end
+  end
+end

app/views/layouts/nav/_new_project_sidebar.html.haml

@@ -86,7 +86,8 @@
             %span.nav-item-name
               Issues
             - if @project.issues_enabled?
-              %span.badge.count.issue_counter= number_with_delimiter(IssuesFinder.new(current_user, project_id: @project.id).execute.opened.count)
+              %span.badge.count.issue_counter
+                = number_with_delimiter(@project.open_issues_count)
           %ul.sidebar-sub-level-items
             = nav_link(controller: :issues) do
@@ -116,7 +117,8 @@
               = custom_icon('mr_bold')
             %span.nav-item-name
               Merge Requests
-            %span.badge.count.merge_counter.js-merge-counter= number_with_delimiter(MergeRequestsFinder.new(current_user, project_id: @project.id).execute.opened.count)
+            %span.badge.count.merge_counter.js-merge-counter
+              = number_with_delimiter(@project.open_merge_requests_count)
       - if project_nav_tab? :pipelines
         = nav_link(controller: [:pipelines, :builds, :jobs, :pipeline_schedules, :environments, :artifacts]) do

app/views/layouts/nav/_project.html.haml

@@ -28,7 +28,8 @@
           %span
             Issues
             - if @project.issues_enabled?
-              %span.badge.count.issue_counter= number_with_delimiter(issuables_count_for_state(:issues, :opened, finder: IssuesFinder.new(current_user, project_id: @project.id)))
+              %span.badge.count.issue_counter
+                = number_with_delimiter(@project.open_issues_count)
     - if project_nav_tab? :merge_requests
       - controllers = [:merge_requests, 'projects/merge_requests/conflicts']
@@ -37,7 +38,8 @@
         = link_to project_merge_requests_path(@project), title: 'Merge Requests', class: 'shortcuts-merge_requests' do
           %span
             Merge Requests
-            %span.badge.count.merge_counter.js-merge-counter= number_with_delimiter(issuables_count_for_state(:merge_requests, :opened, finder: MergeRequestsFinder.new(current_user, project_id: @project.id)))
+            %span.badge.count.merge_counter.js-merge-counter
+              = number_with_delimiter(@project.open_merge_requests_count)
     - if project_nav_tab? :pipelines
       = nav_link(controller: [:pipelines, :builds, :environments, :artifacts]) do

changelogs/unreleased/cache-issue-and-mr-counts.yml

+---
+title: Cache the number of open issues and merge requests
+merge_request:
+author:
+type: other

spec/models/issue_spec.rb

@@ -751,4 +751,22 @@ describe Issue do
       end
     end
   end
+
+  describe 'removing an issue' do
+    it 'refreshes the number of open issues of the project' do
+      project = subject.project
+
+      expect { subject.destroy }
+        .to change { project.open_issues_count }.from(1).to(0)
+    end
+  end
+
+  describe '.public_only' do
+    it 'only returns public issues' do
+      public_issue = create(:issue)
+      create(:issue, confidential: true)
+
+      expect(described_class.public_only).to eq([public_issue])
+    end
+  end
 end

spec/models/merge_request_spec.rb

@@ -1692,4 +1692,13 @@ describe MergeRequest do
       expect(subject.ref_fetched?).to be_falsey
     end
   end
+
+  describe 'removing a merge request' do
+    it 'refreshes the number of open merge requests of the target project' do
+      project = subject.target_project
+
+      expect { subject.destroy }
+        .to change { project.open_merge_requests_count }.from(1).to(0)
+    end
+  end
 end

spec/services/issues/close_service_spec.rb

@@ -42,6 +42,11 @@ describe Issues::CloseService do
       service.execute(issue)
     end
+    it 'refreshes the number of open issues' do
+      expect { service.execute(issue) }
+        .to change { project.open_issues_count }.from(1).to(0)
+    end
+
     it 'invalidates counter cache for assignees' do
       expect_any_instance_of(User).to receive(:invalidate_issue_cache_counts)

spec/services/issues/create_service_spec.rb

@@ -35,6 +35,10 @@ describe Issues::CreateService do
         expect(issue.due_date).to eq Date.tomorrow
       end
+      it 'refreshes the number of open issues' do
+        expect { issue }.to change { project.open_issues_count }.from(0).to(1)
+      end
+
       context 'when current user cannot admin issues in the project' do
         let(:guest) { create(:user) }

spec/services/issues/reopen_service_spec.rb

@@ -34,6 +34,13 @@ describe Issues::ReopenService do
         described_class.new(project, user).execute(issue)
       end
+      it 'refreshes the number of opened issues' do
+        service = described_class.new(project, user)
+
+        expect { service.execute(issue) }
+          .to change { project.open_issues_count }.from(0).to(1)
+      end
+
       context 'when issue is not confidential' do
         it 'executes issue hooks' do
           expect(project).to receive(:execute_hooks).with(an_instance_of(Hash), :issue_hooks)

spec/services/merge_requests/close_service_spec.rb

@@ -52,6 +52,13 @@ describe MergeRequests::CloseService do
       end
     end
+    it 'refreshes the number of open merge requests for a valid MR' do
+      service = described_class.new(project, user, {})
+
+      expect { service.execute(merge_request) }
+        .to change { project.open_merge_requests_count }.from(1).to(0)
+    end
+
     context 'current user is not authorized to close merge request' do
       before do
         perform_enqueued_jobs do

spec/services/merge_requests/create_service_spec.rb

@@ -18,31 +18,35 @@ describe MergeRequests::CreateService do
       end
       let(:service) { described_class.new(project, user, opts) }
+      let(:merge_request) { service.execute }
       before do
         project.team << [user, :master]
         project.team << [assignee, :developer]
         allow(service).to receive(:execute_hooks)
-
-        @merge_request = service.execute
       end
       it 'creates an MR' do
-        expect(@merge_request).to be_valid
-        expect(@merge_request.title).to eq('Awesome merge_request')
-        expect(@merge_request.assignee).to be_nil
-        expect(@merge_request.merge_params['force_remove_source_branch']).to eq('1')
+        expect(merge_request).to be_valid
+        expect(merge_request.title).to eq('Awesome merge_request')
+        expect(merge_request.assignee).to be_nil
+        expect(merge_request.merge_params['force_remove_source_branch']).to eq('1')
       end
       it 'executes hooks with default action' do
-        expect(service).to have_received(:execute_hooks).with(@merge_request)
+        expect(service).to have_received(:execute_hooks).with(merge_request)
+      end
+
+      it 'refreshes the number of open merge requests' do
+        expect { service.execute }
+          .to change { project.open_merge_requests_count }.from(0).to(1)
       end
       it 'does not creates todos' do
         attributes = {
           project: project,
-          target_id: @merge_request.id,
-          target_type: @merge_request.class.name
+          target_id: merge_request.id,
+          target_type: merge_request.class.name
         }
         expect(Todo.where(attributes).count).to be_zero
@@ -51,8 +55,8 @@ describe MergeRequests::CreateService do
       it 'creates exactly 1 create MR event' do
         attributes = {
           action: Event::CREATED,
-          target_id: @merge_request.id,
-          target_type: @merge_request.class.name
+          target_id: merge_request.id,
+          target_type: merge_request.class.name
         }
         expect(Event.where(attributes).count).to eq(1)
@@ -69,15 +73,15 @@ describe MergeRequests::CreateService do
           }
         end
-        it { expect(@merge_request.assignee).to eq assignee }
+        it { expect(merge_request.assignee).to eq assignee }
         it 'creates a todo for new assignee' do
           attributes = {
             project: project,
             author: user,
             user: assignee,
-            target_id: @merge_request.id,
-            target_type: @merge_request.class.name,
+            target_id: merge_request.id,
+            target_type: merge_request.class.name,
             action: Todo::ASSIGNED,
             state: :pending
           }