Add single flight for cache not just origin

store/caching.go

@@ -24,7 +24,7 @@ func NewCachingStore(component string, origin, cache BlobStore) *CachingStore {
 	return &CachingStore{
 		component: component,
 		origin:    WithSingleFlight(component, origin),
-		cache:     cache,
+		cache:     WithSingleFlight(component, cache),
 	}
 }
 

store/singleflight.go

@@ -80,6 +80,43 @@ func (s *singleflightStore) getter(hash string) func() (interface{}, error) {
 	}
 }
 
+// Put ensures that only one request per hash is sent to the origin at a time,
+// thereby protecting against https://en.wikipedia.org/wiki/Thundering_herd_problem
+func (s *singleflightStore) Put(hash string, blob stream.Blob) error {
+	metrics.CacheWaitingRequestsCount.With(metrics.CacheLabels(s.Name(), s.component)).Inc()
+	defer metrics.CacheWaitingRequestsCount.With(metrics.CacheLabels(s.Name(), s.component)).Dec()
+
+	_, err, _ := s.sf.Do(hash, s.putter(hash, blob))
+	if err != nil {
+		return err
+	}
+	return nil
+}
+
+// putter returns a function that puts a blob from the origin
+// only one putter per hash will be executing at a time
+func (s *singleflightStore) putter(hash string, blob stream.Blob) func() (interface{}, error) {
+	return func() (interface{}, error) {
+		metrics.CacheOriginRequestsCount.With(metrics.CacheLabels(s.Name(), s.component)).Inc()
+		defer metrics.CacheOriginRequestsCount.With(metrics.CacheLabels(s.Name(), s.component)).Dec()
+
+		start := time.Now()
+		err := s.BlobStore.Put(hash, blob)
+		if err != nil {
+			return nil, err
+		}
+
+		rate := float64(len(blob)) / 1024 / 1024 / time.Since(start).Seconds()
+		metrics.CacheRetrievalSpeed.With(map[string]string{
+			metrics.LabelCacheType: s.Name(),
+			metrics.LabelComponent: s.component,
+			metrics.LabelSource:    "origin",
+		}).Set(rate)
+
+		return nil, nil
+	}
+}
+
 // Shutdown shuts down the store gracefully
 func (s *singleflightStore) Shutdown() {
 	s.BlobStore.Shutdown()