Open cache interface

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

docs/storage/cache/cachedblobdescriptorstore.go

@@ -0,0 +1,80 @@
+package cache
+
+import (
+	"github.com/docker/distribution/context"
+	"github.com/docker/distribution/digest"
+
+	"github.com/docker/distribution"
+)
+
+// Metrics is used to hold metric counters
+// related to the number of times a cache was
+// hit or missed.
+type Metrics struct {
+	Requests uint64
+	Hits     uint64
+	Misses   uint64
+}
+
+// MetricsTracker represents a metric tracker
+// which simply counts the number of hits and misses.
+type MetricsTracker interface {
+	Hit()
+	Miss()
+	Metrics() Metrics
+}
+
+type cachedBlobStatter struct {
+	cache   distribution.BlobDescriptorService
+	backend distribution.BlobStatter
+	tracker MetricsTracker
+}
+
+// NewCachedBlobStatter creates a new statter which prefers a cache and
+// falls back to a backend.
+func NewCachedBlobStatter(cache distribution.BlobDescriptorService, backend distribution.BlobStatter) distribution.BlobStatter {
+	return &cachedBlobStatter{
+		cache:   cache,
+		backend: backend,
+	}
+}
+
+// NewCachedBlobStatterWithMetrics creates a new statter which prefers a cache and
+// falls back to a backend. Hits and misses will send to the tracker.
+func NewCachedBlobStatterWithMetrics(cache distribution.BlobDescriptorService, backend distribution.BlobStatter, tracker MetricsTracker) distribution.BlobStatter {
+	return &cachedBlobStatter{
+		cache:   cache,
+		backend: backend,
+		tracker: tracker,
+	}
+}
+
+func (cbds *cachedBlobStatter) Stat(ctx context.Context, dgst digest.Digest) (distribution.Descriptor, error) {
+	desc, err := cbds.cache.Stat(ctx, dgst)
+	if err != nil {
+		if err != distribution.ErrBlobUnknown {
+			context.GetLogger(ctx).Errorf("error retrieving descriptor from cache: %v", err)
+		}
+
+		goto fallback
+	}
+
+	if cbds.tracker != nil {
+		cbds.tracker.Hit()
+	}
+	return desc, nil
+fallback:
+	if cbds.tracker != nil {
+		cbds.tracker.Miss()
+	}
+	desc, err = cbds.backend.Stat(ctx, dgst)
+	if err != nil {
+		return desc, err
+	}
+
+	if err := cbds.cache.SetDescriptor(ctx, dgst, desc); err != nil {
+		context.GetLogger(ctx).Errorf("error adding descriptor %v to cache: %v", desc.Digest, err)
+	}
+
+	return desc, err
+}