Open cache interface

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)

registry/storage/blobcachemetrics.go

@@ -0,0 +1,60 @@
+package storage
+
+import (
+	"expvar"
+	"sync/atomic"
+
+	"github.com/docker/distribution/registry/storage/cache"
+)
+
+type blobStatCollector struct {
+	metrics cache.Metrics
+}
+
+func (bsc *blobStatCollector) Hit() {
+	atomic.AddUint64(&bsc.metrics.Requests, 1)
+	atomic.AddUint64(&bsc.metrics.Hits, 1)
+}
+
+func (bsc *blobStatCollector) Miss() {
+	atomic.AddUint64(&bsc.metrics.Requests, 1)
+	atomic.AddUint64(&bsc.metrics.Misses, 1)
+}
+
+func (bsc *blobStatCollector) Metrics() cache.Metrics {
+	return bsc.metrics
+}
+
+// blobStatterCacheMetrics keeps track of cache metrics for blob descriptor
+// cache requests. Note this is kept globally and made available via expvar.
+// For more detailed metrics, its recommend to instrument a particular cache
+// implementation.
+var blobStatterCacheMetrics cache.MetricsTracker = &blobStatCollector{}
+
+func init() {
+	registry := expvar.Get("registry")
+	if registry == nil {
+		registry = expvar.NewMap("registry")
+	}
+
+	cache := registry.(*expvar.Map).Get("cache")
+	if cache == nil {
+		cache = &expvar.Map{}
+		cache.(*expvar.Map).Init()
+		registry.(*expvar.Map).Set("cache", cache)
+	}
+
+	storage := cache.(*expvar.Map).Get("storage")
+	if storage == nil {
+		storage = &expvar.Map{}
+		storage.(*expvar.Map).Init()
+		cache.(*expvar.Map).Set("storage", storage)
+	}
+
+	storage.(*expvar.Map).Set("blobdescriptor", expvar.Func(func() interface{} {
+		// no need for synchronous access: the increments are atomic and
+		// during reading, we don't care if the data is up to date. The
+		// numbers will always *eventually* be reported correctly.
+		return blobStatterCacheMetrics
+	}))
+}