Cache the number of forks of a project

The number of forks of a project doesn't change very frequently and
running a COUNT(*) every time this information is requested can be quite
expensive. We also end up running such a COUNT(*) query at least twice
on the homepage of a project.

By caching this data and refreshing it when necessary we can reduce
project homepage loading times by around 60 milliseconds (based on the
timings of https://gitlab.com/gitlab-org/gitlab-ce).

4 files changed, 44 insertions, 0 deletions

diff --git a/app/services/projects/destroy_service.rb b/app/services/projects/destroy_service.rb
index 11ad4838471..54eb75ab9bf 100644
--- a/app/services/projects/destroy_service.rb
+++ b/app/services/projects/destroy_service.rb
@@ -128,6 +128,8 @@ module Projects
       project.repository.before_delete
 
       Repository.new(wiki_path, project, disk_path: repo_path).before_delete
+
+      Projects::ForksCountService.new(project).delete_cache
     end
   end
 end
diff --git a/app/services/projects/fork_service.rb b/app/services/projects/fork_service.rb
index a2b23ea6171..ad67e68a86a 100644
--- a/app/services/projects/fork_service.rb
+++ b/app/services/projects/fork_service.rb
@@ -21,11 +21,17 @@ module Projects
       builds_access_level = @project.project_feature.builds_access_level
       new_project.project_feature.update_attributes(builds_access_level: builds_access_level)
 
+      refresh_forks_count
+
       new_project
     end
 
     private
 
+    def refresh_forks_count
+      Projects::ForksCountService.new(@project).refresh_cache
+    end
+
     def allowed_visibility_level
       project_level = @project.visibility_level
 
diff --git a/app/services/projects/forks_count_service.rb b/app/services/projects/forks_count_service.rb
new file mode 100644
index 00000000000..e2e2b1da91d
--- /dev/null
+++ b/app/services/projects/forks_count_service.rb
@@ -0,0 +1,30 @@
+module Projects
+  # Service class for getting and caching the number of forks of a project.
+  class ForksCountService
+    def initialize(project)
+      @project = project
+    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']
+    end
+  end
+end
diff --git a/app/services/projects/unlink_fork_service.rb b/app/services/projects/unlink_fork_service.rb
index f385e426827..f30b40423c8 100644
--- a/app/services/projects/unlink_fork_service.rb
+++ b/app/services/projects/unlink_fork_service.rb
@@ -13,7 +13,13 @@ module Projects
         ::MergeRequests::CloseService.new(@project, @current_user).execute(mr)
       end
 
+      refresh_forks_count(@project.forked_from_project)
+
       @project.forked_project_link.destroy
     end
+
+    def refresh_forks_count(project)
+      Projects::ForksCountService.new(project).refresh_cache
+    end
   end
 end