Cache the number of forks of a projectforks-count-cache

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).

1 files changed, 30 insertions, 0 deletions

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