Merge pull request #312 from stevvooe/add-layer-info-cache

registry: integrate layer info cache with registry and storage

docs/doc.go

@@ -1,3 +1,3 @@
 // Package registry is a placeholder package for registry interface
-// destinations and utilities.
+// definitions and utilities.
 package registry

docs/handlers/app.go

@@ -1,10 +1,12 @@
 package handlers
 
 import (
+	"expvar"
 	"fmt"
 	"net"
 	"net/http"
 	"os"
+	"time"
 
 	"code.google.com/p/go-uuid/uuid"
 	"github.com/docker/distribution"
@@ -16,9 +18,11 @@ import (
 	registrymiddleware "github.com/docker/distribution/registry/middleware/registry"
 	repositorymiddleware "github.com/docker/distribution/registry/middleware/repository"
 	"github.com/docker/distribution/registry/storage"
+	"github.com/docker/distribution/registry/storage/cache"
 	storagedriver "github.com/docker/distribution/registry/storage/driver"
 	"github.com/docker/distribution/registry/storage/driver/factory"
 	storagemiddleware "github.com/docker/distribution/registry/storage/driver/middleware"
+	"github.com/garyburd/redigo/redis"
 	"github.com/gorilla/mux"
 	"golang.org/x/net/context"
 )
@@ -44,6 +48,8 @@ type App struct {
 		sink   notifications.Sink
 		source notifications.SourceRecord
 	}
+
+	redis *redis.Pool
 }
 
 // Value intercepts calls context.Context.Value, returning the current app id,
@@ -95,8 +101,32 @@ func NewApp(ctx context.Context, configuration configuration.Configuration) *App
 	}
 
 	app.configureEvents(&configuration)
+	app.configureRedis(&configuration)
+
+	// configure storage caches
+	if cc, ok := configuration.Storage["cache"]; ok {
+		switch cc["layerinfo"] {
+		case "redis":
+			if app.redis == nil {
+				panic("redis configuration required to use for layerinfo cache")
+			}
+			app.registry = storage.NewRegistryWithDriver(app.driver, cache.NewRedisLayerInfoCache(app.redis))
+			ctxu.GetLogger(app).Infof("using redis layerinfo cache")
+		case "inmemory":
+			app.registry = storage.NewRegistryWithDriver(app.driver, cache.NewInMemoryLayerInfoCache())
+			ctxu.GetLogger(app).Infof("using inmemory layerinfo cache")
+		default:
+			if cc["layerinfo"] != "" {
+				ctxu.GetLogger(app).Warnf("unkown cache type %q, caching disabled", configuration.Storage["cache"])
+			}
+		}
+	}
+
+	if app.registry == nil {
+		// configure the registry if no cache section is available.
+		app.registry = storage.NewRegistryWithDriver(app.driver, nil)
+	}
 
-	app.registry = storage.NewRegistryWithDriver(app.driver)
 	app.registry, err = applyRegistryMiddleware(app.registry, configuration.Middleware["registry"])
 	if err != nil {
 		panic(err)
@@ -174,6 +204,88 @@ func (app *App) configureEvents(configuration *configuration.Configuration) {
 	}
 }
 
+func (app *App) configureRedis(configuration *configuration.Configuration) {
+	if configuration.Redis.Addr == "" {
+		ctxu.GetLogger(app).Infof("redis not configured")
+		return
+	}
+
+	pool := &redis.Pool{
+		Dial: func() (redis.Conn, error) {
+			// TODO(stevvooe): Yet another use case for contextual timing.
+			ctx := context.WithValue(app, "redis.connect.startedat", time.Now())
+
+			done := func(err error) {
+				logger := ctxu.GetLoggerWithField(ctx, "redis.connect.duration",
+					ctxu.Since(ctx, "redis.connect.startedat"))
+				if err != nil {
+					logger.Errorf("redis: error connecting: %v", err)
+				} else {
+					logger.Infof("redis: connect %v", configuration.Redis.Addr)
+				}
+			}
+
+			conn, err := redis.DialTimeout("tcp",
+				configuration.Redis.Addr,
+				configuration.Redis.DialTimeout,
+				configuration.Redis.ReadTimeout,
+				configuration.Redis.WriteTimeout)
+			if err != nil {
+				ctxu.GetLogger(app).Errorf("error connecting to redis instance %s: %v",
+					configuration.Redis.Addr, err)
+				done(err)
+				return nil, err
+			}
+
+			// authorize the connection
+			if configuration.Redis.Password != "" {
+				if _, err = conn.Do("AUTH", configuration.Redis.Password); err != nil {
+					defer conn.Close()
+					done(err)
+					return nil, err
+				}
+			}
+
+			// select the database to use
+			if configuration.Redis.DB != 0 {
+				if _, err = conn.Do("SELECT", configuration.Redis.DB); err != nil {
+					defer conn.Close()
+					done(err)
+					return nil, err
+				}
+			}
+
+			done(nil)
+			return conn, nil
+		},
+		MaxIdle:     configuration.Redis.Pool.MaxIdle,
+		MaxActive:   configuration.Redis.Pool.MaxActive,
+		IdleTimeout: configuration.Redis.Pool.IdleTimeout,
+		TestOnBorrow: func(c redis.Conn, t time.Time) error {
+			// TODO(stevvooe): We can probably do something more interesting
+			// here with the health package.
+			_, err := c.Do("PING")
+			return err
+		},
+		Wait: false, // if a connection is not avialable, proceed without cache.
+	}
+
+	app.redis = pool
+
+	// setup expvar
+	registry := expvar.Get("registry")
+	if registry == nil {
+		registry = expvar.NewMap("registry")
+	}
+
+	registry.(*expvar.Map).Set("redis", expvar.Func(func() interface{} {
+		return map[string]interface{}{
+			"Config": configuration.Redis,
+			"Active": app.redis.ActiveCount(),
+		}
+	}))
+}
+
 func (app *App) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	defer r.Body.Close() // ensure that request body is always closed.
 

docs/handlers/app_test.go

@@ -13,6 +13,7 @@ import (
 	"github.com/docker/distribution/registry/auth"
 	_ "github.com/docker/distribution/registry/auth/silly"
 	"github.com/docker/distribution/registry/storage"
+	"github.com/docker/distribution/registry/storage/cache"
 	"github.com/docker/distribution/registry/storage/driver/inmemory"
 	"golang.org/x/net/context"
 )
@@ -28,7 +29,7 @@ func TestAppDispatcher(t *testing.T) {
 		Context:  context.Background(),
 		router:   v2.Router(),
 		driver:   driver,
-		registry: storage.NewRegistryWithDriver(driver),
+		registry: storage.NewRegistryWithDriver(driver, cache.NewInMemoryLayerInfoCache()),
 	}
 	server := httptest.NewServer(app)
 	router := v2.Router()

docs/storage/blobstore.go

@@ -18,8 +18,9 @@ import (
 // abstraction, providing utility methods that support creating and traversing
 // backend links.
 type blobStore struct {
-	*registry
-	ctx context.Context
+	driver storagedriver.StorageDriver
+	pm     *pathMapper
+	ctx    context.Context
 }
 
 // exists reports whether or not the path exists. If the driver returns error

docs/storage/cache/cache.go

@@ -0,0 +1,98 @@
+// Package cache provides facilities to speed up access to the storage
+// backend. Typically cache implementations deal with internal implementation
+// details at the backend level, rather than generalized caches for
+// distribution related interfaces. In other words, unless the cache is
+// specific to the storage package, it belongs in another package.
+package cache
+
+import (
+	"fmt"
+
+	"github.com/docker/distribution/digest"
+	"golang.org/x/net/context"
+)
+
+// ErrNotFound is returned when a meta item is not found.
+var ErrNotFound = fmt.Errorf("not found")
+
+// LayerMeta describes the backend location and length of layer data.
+type LayerMeta struct {
+	Path   string
+	Length int64
+}
+
+// LayerInfoCache is a driver-aware cache of layer metadata. Basically, it
+// provides a fast cache for checks against repository metadata, avoiding
+// round trips to backend storage. Note that this is different from a pure
+// layer cache, which would also provide access to backing data, as well. Such
+// a cache should be implemented as a middleware, rather than integrated with
+// the storage backend.
+//
+// Note that most implementations rely on the caller to do strict checks on on
+// repo and dgst arguments, since these are mostly used behind existing
+// implementations.
+type LayerInfoCache interface {
+	// Contains returns true if the repository with name contains the layer.
+	Contains(ctx context.Context, repo string, dgst digest.Digest) (bool, error)
+
+	// Add includes the layer in the given repository cache.
+	Add(ctx context.Context, repo string, dgst digest.Digest) error
+
+	// Meta provides the location of the layer on the backend and its size. Membership of a
+	// repository should be tested before using the result, if required.
+	Meta(ctx context.Context, dgst digest.Digest) (LayerMeta, error)
+
+	// SetMeta sets the meta data for the given layer.
+	SetMeta(ctx context.Context, dgst digest.Digest, meta LayerMeta) error
+}
+
+// base implements common checks between cache implementations. Note that
+// these are not full checks of input, since that should be done by the
+// caller.
+type base struct {
+	LayerInfoCache
+}
+
+func (b *base) Contains(ctx context.Context, repo string, dgst digest.Digest) (bool, error) {
+	if repo == "" {
+		return false, fmt.Errorf("cache: cannot check for empty repository name")
+	}
+
+	if dgst == "" {
+		return false, fmt.Errorf("cache: cannot check for empty digests")
+	}
+
+	return b.LayerInfoCache.Contains(ctx, repo, dgst)
+}
+
+func (b *base) Add(ctx context.Context, repo string, dgst digest.Digest) error {
+	if repo == "" {
+		return fmt.Errorf("cache: cannot add empty repository name")
+	}
+
+	if dgst == "" {
+		return fmt.Errorf("cache: cannot add empty digest")
+	}
+
+	return b.LayerInfoCache.Add(ctx, repo, dgst)
+}
+
+func (b *base) Meta(ctx context.Context, dgst digest.Digest) (LayerMeta, error) {
+	if dgst == "" {
+		return LayerMeta{}, fmt.Errorf("cache: cannot get meta for empty digest")
+	}
+
+	return b.LayerInfoCache.Meta(ctx, dgst)
+}
+
+func (b *base) SetMeta(ctx context.Context, dgst digest.Digest, meta LayerMeta) error {
+	if dgst == "" {
+		return fmt.Errorf("cache: cannot set meta for empty digest")
+	}
+
+	if meta.Path == "" {
+		return fmt.Errorf("cache: cannot set empty path for meta")
+	}
+
+	return b.LayerInfoCache.SetMeta(ctx, dgst, meta)
+}

docs/storage/cache/cache_test.go

@@ -0,0 +1,86 @@
+package cache
+
+import (
+	"testing"
+
+	"golang.org/x/net/context"
+)
+
+// checkLayerInfoCache takes a cache implementation through a common set of
+// operations. If adding new tests, please add them here so new
+// implementations get the benefit.
+func checkLayerInfoCache(t *testing.T, lic LayerInfoCache) {
+	ctx := context.Background()
+
+	exists, err := lic.Contains(ctx, "", "fake:abc")
+	if err == nil {
+		t.Fatalf("expected error checking for cache item with empty repo")
+	}
+
+	exists, err = lic.Contains(ctx, "foo/bar", "")
+	if err == nil {
+		t.Fatalf("expected error checking for cache item with empty digest")
+	}
+
+	exists, err = lic.Contains(ctx, "foo/bar", "fake:abc")
+	if err != nil {
+		t.Fatalf("unexpected error checking for cache item: %v", err)
+	}
+
+	if exists {
+		t.Fatalf("item should not exist")
+	}
+
+	if err := lic.Add(ctx, "", "fake:abc"); err == nil {
+		t.Fatalf("expected error adding cache item with empty name")
+	}
+
+	if err := lic.Add(ctx, "foo/bar", ""); err == nil {
+		t.Fatalf("expected error adding cache item with empty digest")
+	}
+
+	if err := lic.Add(ctx, "foo/bar", "fake:abc"); err != nil {
+		t.Fatalf("unexpected error adding item: %v", err)
+	}
+
+	exists, err = lic.Contains(ctx, "foo/bar", "fake:abc")
+	if err != nil {
+		t.Fatalf("unexpected error checking for cache item: %v", err)
+	}
+
+	if !exists {
+		t.Fatalf("item should exist")
+	}
+
+	_, err = lic.Meta(ctx, "")
+	if err == nil || err == ErrNotFound {
+		t.Fatalf("expected error getting meta for cache item with empty digest")
+	}
+
+	_, err = lic.Meta(ctx, "fake:abc")
+	if err != ErrNotFound {
+		t.Fatalf("expected unknown layer error getting meta for cache item with empty digest")
+	}
+
+	if err = lic.SetMeta(ctx, "", LayerMeta{}); err == nil {
+		t.Fatalf("expected error setting meta for cache item with empty digest")
+	}
+
+	if err = lic.SetMeta(ctx, "foo/bar", LayerMeta{}); err == nil {
+		t.Fatalf("expected error setting meta for cache item with empty meta")
+	}
+
+	expected := LayerMeta{Path: "/foo/bar", Length: 20}
+	if err := lic.SetMeta(ctx, "foo/bar", expected); err != nil {
+		t.Fatalf("unexpected error setting meta: %v", err)
+	}
+
+	meta, err := lic.Meta(ctx, "foo/bar")
+	if err != nil {
+		t.Fatalf("unexpected error getting meta: %v", err)
+	}
+
+	if meta != expected {
+		t.Fatalf("retrieved meta data did not match: %v", err)
+	}
+}

docs/storage/cache/memory.go

@@ -0,0 +1,63 @@
+package cache
+
+import (
+	"github.com/docker/distribution/digest"
+	"golang.org/x/net/context"
+)
+
+// inmemoryLayerInfoCache is a map-based implementation of LayerInfoCache.
+type inmemoryLayerInfoCache struct {
+	membership map[string]map[digest.Digest]struct{}
+	meta       map[digest.Digest]LayerMeta
+}
+
+// NewInMemoryLayerInfoCache provides an implementation of LayerInfoCache that
+// stores results in memory.
+func NewInMemoryLayerInfoCache() LayerInfoCache {
+	return &base{&inmemoryLayerInfoCache{
+		membership: make(map[string]map[digest.Digest]struct{}),
+		meta:       make(map[digest.Digest]LayerMeta),
+	}}
+}
+
+func (ilic *inmemoryLayerInfoCache) Contains(ctx context.Context, repo string, dgst digest.Digest) (bool, error) {
+	members, ok := ilic.membership[repo]
+	if !ok {
+		return false, nil
+	}
+
+	_, ok = members[dgst]
+	return ok, nil
+}
+
+// Add adds the layer to the redis repository blob set.
+func (ilic *inmemoryLayerInfoCache) Add(ctx context.Context, repo string, dgst digest.Digest) error {
+	members, ok := ilic.membership[repo]
+	if !ok {
+		members = make(map[digest.Digest]struct{})
+		ilic.membership[repo] = members
+	}
+
+	members[dgst] = struct{}{}
+
+	return nil
+}
+
+// Meta retrieves the layer meta data from the redis hash, returning
+// ErrUnknownLayer if not found.
+func (ilic *inmemoryLayerInfoCache) Meta(ctx context.Context, dgst digest.Digest) (LayerMeta, error) {
+	meta, ok := ilic.meta[dgst]
+	if !ok {
+		return LayerMeta{}, ErrNotFound
+	}
+
+	return meta, nil
+}
+
+// SetMeta sets the meta data for the given digest using a redis hash. A hash
+// is used here since we may store unrelated fields about a layer in the
+// future.
+func (ilic *inmemoryLayerInfoCache) SetMeta(ctx context.Context, dgst digest.Digest, meta LayerMeta) error {
+	ilic.meta[dgst] = meta
+	return nil
+}

docs/storage/cache/memory_test.go

@@ -0,0 +1,9 @@
+package cache
+
+import "testing"
+
+// TestInMemoryLayerInfoCache checks the in memory implementation is working
+// correctly.
+func TestInMemoryLayerInfoCache(t *testing.T) {
+	checkLayerInfoCache(t, NewInMemoryLayerInfoCache())
+}

docs/storage/cache/redis.go

@@ -0,0 +1,98 @@
+package cache
+
+import (
+	ctxu "github.com/docker/distribution/context"
+	"github.com/docker/distribution/digest"
+	"github.com/garyburd/redigo/redis"
+	"golang.org/x/net/context"
+)
+
+// redisLayerInfoCache provides an implementation of storage.LayerInfoCache
+// based on redis. Layer info is stored in two parts. The first provide fast
+// access to repository membership through a redis set for each repo. The
+// second is a redis hash keyed by the digest of the layer, providing path and
+// length information. Note that there is no implied relationship between
+// these two caches. The layer may exist in one, both or none and the code
+// must be written this way.
+type redisLayerInfoCache struct {
+	pool *redis.Pool
+
+	// TODO(stevvooe): We use a pool because we don't have great control over
+	// the cache lifecycle to manage connections. A new connection if fetched
+	// for each operation. Once we have better lifecycle management of the
+	// request objects, we can change this to a connection.
+}
+
+// NewRedisLayerInfoCache returns a new redis-based LayerInfoCache using the
+// provided redis connection pool.
+func NewRedisLayerInfoCache(pool *redis.Pool) LayerInfoCache {
+	return &base{&redisLayerInfoCache{
+		pool: pool,
+	}}
+}
+
+// Contains does a membership check on the repository blob set in redis. This
+// is used as an access check before looking up global path information. If
+// false is returned, the caller should still check the backend to if it
+// exists elsewhere.
+func (rlic *redisLayerInfoCache) Contains(ctx context.Context, repo string, dgst digest.Digest) (bool, error) {
+	conn := rlic.pool.Get()
+	defer conn.Close()
+
+	ctxu.GetLogger(ctx).Debugf("(*redisLayerInfoCache).Contains(%q, %q)", repo, dgst)
+	return redis.Bool(conn.Do("SISMEMBER", rlic.repositoryBlobSetKey(repo), dgst))
+}
+
+// Add adds the layer to the redis repository blob set.
+func (rlic *redisLayerInfoCache) Add(ctx context.Context, repo string, dgst digest.Digest) error {
+	conn := rlic.pool.Get()
+	defer conn.Close()
+
+	ctxu.GetLogger(ctx).Debugf("(*redisLayerInfoCache).Add(%q, %q)", repo, dgst)
+	_, err := conn.Do("SADD", rlic.repositoryBlobSetKey(repo), dgst)
+	return err
+}
+
+// Meta retrieves the layer meta data from the redis hash, returning
+// ErrUnknownLayer if not found.
+func (rlic *redisLayerInfoCache) Meta(ctx context.Context, dgst digest.Digest) (LayerMeta, error) {
+	conn := rlic.pool.Get()
+	defer conn.Close()
+
+	reply, err := redis.Values(conn.Do("HMGET", rlic.blobMetaHashKey(dgst), "path", "length"))
+	if err != nil {
+		return LayerMeta{}, err
+	}
+
+	if len(reply) < 2 || reply[0] == nil || reply[1] == nil {
+		return LayerMeta{}, ErrNotFound
+	}
+
+	var meta LayerMeta
+	if _, err := redis.Scan(reply, &meta.Path, &meta.Length); err != nil {
+		return LayerMeta{}, err
+	}
+
+	return meta, nil
+}
+
+// SetMeta sets the meta data for the given digest using a redis hash. A hash
+// is used here since we may store unrelated fields about a layer in the
+// future.
+func (rlic *redisLayerInfoCache) SetMeta(ctx context.Context, dgst digest.Digest, meta LayerMeta) error {
+	conn := rlic.pool.Get()
+	defer conn.Close()
+
+	_, err := conn.Do("HMSET", rlic.blobMetaHashKey(dgst), "path", meta.Path, "length", meta.Length)
+	return err
+}
+
+// repositoryBlobSetKey returns the key for the blob set in the cache.
+func (rlic *redisLayerInfoCache) repositoryBlobSetKey(repo string) string {
+	return "repository::" + repo + "::blobs"
+}
+
+// blobMetaHashKey returns the cache key for immutable blob meta data.
+func (rlic *redisLayerInfoCache) blobMetaHashKey(dgst digest.Digest) string {
+	return "blobs::" + dgst.String()
+}

docs/storage/cache/redis_test.go

@@ -0,0 +1,50 @@
+package cache
+
+import (
+	"flag"
+	"os"
+	"testing"
+	"time"
+
+	"github.com/garyburd/redigo/redis"
+)
+
+var redisAddr string
+
+func init() {
+	flag.StringVar(&redisAddr, "test.registry.storage.cache.redis.addr", "", "configure the address of a test instance of redis")
+}
+
+// TestRedisLayerInfoCache exercises a live redis instance using the cache
+// implementation.
+func TestRedisLayerInfoCache(t *testing.T) {
+	if redisAddr == "" {
+		// fallback to an environement variable
+		redisAddr = os.Getenv("TEST_REGISTRY_STORAGE_CACHE_REDIS_ADDR")
+	}
+
+	if redisAddr == "" {
+		// skip if still not set
+		t.Skip("please set -registry.storage.cache.redis to test layer info cache against redis")
+	}
+
+	pool := &redis.Pool{
+		Dial: func() (redis.Conn, error) {
+			return redis.Dial("tcp", redisAddr)
+		},
+		MaxIdle:   1,
+		MaxActive: 2,
+		TestOnBorrow: func(c redis.Conn, t time.Time) error {
+			_, err := c.Do("PING")
+			return err
+		},
+		Wait: false, // if a connection is not avialable, proceed without cache.
+	}
+
+	// Clear the database
+	if _, err := pool.Get().Do("FLUSHDB"); err != nil {
+		t.Fatalf("unexpected error flushing redis db: %v", err)
+	}
+
+	checkLayerInfoCache(t, NewRedisLayerInfoCache(pool))
+}

docs/storage/filereader.go

@@ -27,8 +27,8 @@ type fileReader struct {
 
 	// identifying fields
 	path    string
-	size    int64 // size is the total layer size, must be set.
-	modtime time.Time
+	size    int64     // size is the total size, must be set.
+	modtime time.Time // TODO(stevvooe): This is not needed anymore.
 
 	// mutable fields
 	rc     io.ReadCloser // remote read closer

docs/storage/layer_test.go

@@ -11,6 +11,7 @@ import (
 
 	"github.com/docker/distribution"
 	"github.com/docker/distribution/digest"
+	"github.com/docker/distribution/registry/storage/cache"
 	storagedriver "github.com/docker/distribution/registry/storage/driver"
 	"github.com/docker/distribution/registry/storage/driver/inmemory"
 	"github.com/docker/distribution/testutil"
@@ -35,7 +36,7 @@ func TestSimpleLayerUpload(t *testing.T) {
 	ctx := context.Background()
 	imageName := "foo/bar"
 	driver := inmemory.New()
-	registry := NewRegistryWithDriver(driver)
+	registry := NewRegistryWithDriver(driver, cache.NewInMemoryLayerInfoCache())
 	repository, err := registry.Repository(ctx, imageName)
 	if err != nil {
 		t.Fatalf("unexpected error getting repo: %v", err)
@@ -143,7 +144,7 @@ func TestSimpleLayerRead(t *testing.T) {
 	ctx := context.Background()
 	imageName := "foo/bar"
 	driver := inmemory.New()
-	registry := NewRegistryWithDriver(driver)
+	registry := NewRegistryWithDriver(driver, cache.NewInMemoryLayerInfoCache())
 	repository, err := registry.Repository(ctx, imageName)
 	if err != nil {
 		t.Fatalf("unexpected error getting repo: %v", err)
@@ -180,7 +181,7 @@ func TestSimpleLayerRead(t *testing.T) {
 		t.Fatalf("unexpected error fetching non-existent layer: %v", err)
 	}
 
-	randomLayerDigest, err := writeTestLayer(driver, ls.(*layerStore).repository.pm, imageName, dgst, randomLayerReader)
+	randomLayerDigest, err := writeTestLayer(driver, defaultPathMapper, imageName, dgst, randomLayerReader)
 	if err != nil {
 		t.Fatalf("unexpected error writing test layer: %v", err)
 	}
@@ -252,7 +253,7 @@ func TestLayerUploadZeroLength(t *testing.T) {
 	ctx := context.Background()
 	imageName := "foo/bar"
 	driver := inmemory.New()
-	registry := NewRegistryWithDriver(driver)
+	registry := NewRegistryWithDriver(driver, cache.NewInMemoryLayerInfoCache())
 	repository, err := registry.Repository(ctx, imageName)
 	if err != nil {
 		t.Fatalf("unexpected error getting repo: %v", err)

docs/storage/layercache.go

@@ -0,0 +1,202 @@
+package storage
+
+import (
+	"expvar"
+	"sync/atomic"
+	"time"
+
+	"github.com/docker/distribution"
+	ctxu "github.com/docker/distribution/context"
+	"github.com/docker/distribution/digest"
+	"github.com/docker/distribution/registry/storage/cache"
+	"github.com/docker/distribution/registry/storage/driver"
+	"golang.org/x/net/context"
+)
+
+// cachedLayerService implements the layer service with path-aware caching,
+// using a LayerInfoCache interface.
+type cachedLayerService struct {
+	distribution.LayerService // upstream layer service
+	repository                distribution.Repository
+	ctx                       context.Context
+	driver                    driver.StorageDriver
+	*blobStore                // global blob store
+	cache                     cache.LayerInfoCache
+}
+
+// Exists checks for existence of the digest in the cache, immediately
+// returning if it exists for the repository. If not, the upstream is checked.
+// When a positive result is found, it is written into the cache.
+func (lc *cachedLayerService) Exists(dgst digest.Digest) (bool, error) {
+	ctxu.GetLogger(lc.ctx).Debugf("(*cachedLayerService).Exists(%q)", dgst)
+	now := time.Now()
+	defer func() {
+		// TODO(stevvooe): Replace this with a decent context-based metrics solution
+		ctxu.GetLoggerWithField(lc.ctx, "blob.exists.duration", time.Since(now)).
+			Infof("(*cachedLayerService).Exists(%q)", dgst)
+	}()
+
+	atomic.AddUint64(&layerInfoCacheMetrics.Exists.Requests, 1)
+	available, err := lc.cache.Contains(lc.ctx, lc.repository.Name(), dgst)
+	if err != nil {
+		ctxu.GetLogger(lc.ctx).Errorf("error checking availability of %v@%v: %v", lc.repository.Name(), dgst, err)
+		goto fallback
+	}
+
+	if available {
+		atomic.AddUint64(&layerInfoCacheMetrics.Exists.Hits, 1)
+		return true, nil
+	}
+
+fallback:
+	atomic.AddUint64(&layerInfoCacheMetrics.Exists.Misses, 1)
+	exists, err := lc.LayerService.Exists(dgst)
+	if err != nil {
+		return exists, err
+	}
+
+	if exists {
+		// we can only cache this if the existence is positive.
+		if err := lc.cache.Add(lc.ctx, lc.repository.Name(), dgst); err != nil {
+			ctxu.GetLogger(lc.ctx).Errorf("error adding %v@%v to cache: %v", lc.repository.Name(), dgst, err)
+		}
+	}
+
+	return exists, err
+}
+
+// Fetch checks for the availability of the layer in the repository via the
+// cache. If present, the metadata is resolved and the layer is returned. If
+// any operation fails, the layer is read directly from the upstream. The
+// results are cached, if possible.
+func (lc *cachedLayerService) Fetch(dgst digest.Digest) (distribution.Layer, error) {
+	ctxu.GetLogger(lc.ctx).Debugf("(*layerInfoCache).Fetch(%q)", dgst)
+	now := time.Now()
+	defer func() {
+		ctxu.GetLoggerWithField(lc.ctx, "blob.fetch.duration", time.Since(now)).
+			Infof("(*layerInfoCache).Fetch(%q)", dgst)
+	}()
+
+	atomic.AddUint64(&layerInfoCacheMetrics.Fetch.Requests, 1)
+	available, err := lc.cache.Contains(lc.ctx, lc.repository.Name(), dgst)
+	if err != nil {
+		ctxu.GetLogger(lc.ctx).Errorf("error checking availability of %v@%v: %v", lc.repository.Name(), dgst, err)
+		goto fallback
+	}
+
+	if available {
+		// fast path: get the layer info and return
+		meta, err := lc.cache.Meta(lc.ctx, dgst)
+		if err != nil {
+			ctxu.GetLogger(lc.ctx).Errorf("error fetching %v@%v from cache: %v", lc.repository.Name(), dgst, err)
+			goto fallback
+		}
+
+		atomic.AddUint64(&layerInfoCacheMetrics.Fetch.Hits, 1)
+		return newLayerReader(lc.driver, dgst, meta.Path, meta.Length)
+	}
+
+	// NOTE(stevvooe): Unfortunately, the cache here only makes checks for
+	// existing layers faster. We'd have to provide more careful
+	// synchronization with the backend to make the missing case as fast.
+
+fallback:
+	atomic.AddUint64(&layerInfoCacheMetrics.Fetch.Misses, 1)
+	layer, err := lc.LayerService.Fetch(dgst)
+	if err != nil {
+		return nil, err
+	}
+
+	// add the layer to the repository
+	if err := lc.cache.Add(lc.ctx, lc.repository.Name(), dgst); err != nil {
+		ctxu.GetLogger(lc.ctx).
+			Errorf("error caching repository relationship for %v@%v: %v", lc.repository.Name(), dgst, err)
+	}
+
+	// lookup layer path and add it to the cache, if it succeds. Note that we
+	// still return the layer even if we have trouble caching it.
+	if path, err := lc.resolveLayerPath(layer); err != nil {
+		ctxu.GetLogger(lc.ctx).
+			Errorf("error resolving path while caching %v@%v: %v", lc.repository.Name(), dgst, err)
+	} else {
+		// add the layer to the cache once we've resolved the path.
+		if err := lc.cache.SetMeta(lc.ctx, dgst, cache.LayerMeta{Path: path, Length: layer.Length()}); err != nil {
+			ctxu.GetLogger(lc.ctx).Errorf("error adding meta for %v@%v to cache: %v", lc.repository.Name(), dgst, err)
+		}
+	}
+
+	return layer, err
+}
+
+// extractLayerInfo pulls the layerInfo from the layer, attempting to get the
+// path information from either the concrete object or by resolving the
+// primary blob store path.
+func (lc *cachedLayerService) resolveLayerPath(layer distribution.Layer) (path string, err error) {
+	// try and resolve the type and driver, so we don't have to traverse links
+	switch v := layer.(type) {
+	case *layerReader:
+		// only set path if we have same driver instance.
+		if v.driver == lc.driver {
+			return v.path, nil
+		}
+	}
+
+	ctxu.GetLogger(lc.ctx).Warnf("resolving layer path during cache lookup (%v@%v)", lc.repository.Name(), layer.Digest())
+	// we have to do an expensive stat to resolve the layer location but no
+	// need to check the link, since we already have layer instance for this
+	// repository.
+	bp, err := lc.blobStore.path(layer.Digest())
+	if err != nil {
+		return "", err
+	}
+
+	return bp, nil
+}
+
+// layerInfoCacheMetrics keeps track of cache metrics for layer info 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 layerInfoCacheMetrics struct {
+	// Exists tracks calls to the Exists caches.
+	Exists struct {
+		Requests uint64
+		Hits     uint64
+		Misses   uint64
+	}
+
+	// Fetch tracks calls to the fetch caches.
+	Fetch struct {
+		Requests uint64
+		Hits     uint64
+		Misses   uint64
+	}
+}
+
+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("layerinfo", 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 layerInfoCacheMetrics
+	}))
+}

docs/storage/layerreader.go

@@ -17,6 +17,21 @@ type layerReader struct {
 	digest digest.Digest
 }
 
+// newLayerReader returns a new layerReader with the digest, path and length,
+// eliding round trips to the storage backend.
+func newLayerReader(driver driver.StorageDriver, dgst digest.Digest, path string, length int64) (*layerReader, error) {
+	fr := &fileReader{
+		driver: driver,
+		path:   path,
+		size:   length,
+	}
+
+	return &layerReader{
+		fileReader: *fr,
+		digest:     dgst,
+	}, nil
+}
+
 var _ distribution.Layer = &layerReader{}
 
 func (lr *layerReader) Digest() digest.Digest {

docs/storage/manifeststore_test.go

@@ -6,6 +6,8 @@ import (
 	"reflect"
 	"testing"
 
+	"github.com/docker/distribution/registry/storage/cache"
+
 	"github.com/docker/distribution"
 	"github.com/docker/distribution/digest"
 	"github.com/docker/distribution/manifest"
@@ -28,7 +30,7 @@ type manifestStoreTestEnv struct {
 func newManifestStoreTestEnv(t *testing.T, name, tag string) *manifestStoreTestEnv {
 	ctx := context.Background()
 	driver := inmemory.New()
-	registry := NewRegistryWithDriver(driver)
+	registry := NewRegistryWithDriver(driver, cache.NewInMemoryLayerInfoCache())
 
 	repo, err := registry.Repository(ctx, name)
 	if err != nil {

docs/storage/registry.go

@@ -3,6 +3,7 @@ package storage
 import (
 	"github.com/docker/distribution"
 	"github.com/docker/distribution/registry/api/v2"
+	"github.com/docker/distribution/registry/storage/cache"
 	storagedriver "github.com/docker/distribution/registry/storage/driver"
 	"golang.org/x/net/context"
 )
@@ -10,28 +11,29 @@ import (
 // registry is the top-level implementation of Registry for use in the storage
 // package. All instances should descend from this object.
 type registry struct {
-	driver    storagedriver.StorageDriver
-	pm        *pathMapper
-	blobStore *blobStore
+	driver         storagedriver.StorageDriver
+	pm             *pathMapper
+	blobStore      *blobStore
+	layerInfoCache cache.LayerInfoCache
 }
 
 // NewRegistryWithDriver creates a new registry instance from the provided
 // driver. The resulting registry may be shared by multiple goroutines but is
 // cheap to allocate.
-func NewRegistryWithDriver(driver storagedriver.StorageDriver) distribution.Registry {
-	bs := &blobStore{}
+func NewRegistryWithDriver(driver storagedriver.StorageDriver, layerInfoCache cache.LayerInfoCache) distribution.Registry {
+	bs := &blobStore{
+		driver: driver,
+		pm:     defaultPathMapper,
+	}
 
-	reg := &registry{
+	return &registry{
 		driver:    driver,
 		blobStore: bs,
 
 		// TODO(sday): This should be configurable.
-		pm: defaultPathMapper,
+		pm:             defaultPathMapper,
+		layerInfoCache: layerInfoCache,
 	}
-
-	reg.blobStore.registry = reg
-
-	return reg
 }
 
 // Repository returns an instance of the repository tied to the registry.
@@ -83,9 +85,29 @@ func (repo *repository) Manifests() distribution.ManifestService {
 // may be context sensitive in the future. The instance should be used similar
 // to a request local.
 func (repo *repository) Layers() distribution.LayerService {
-	return &layerStore{
+	ls := &layerStore{
 		repository: repo,
 	}
+
+	if repo.registry.layerInfoCache != nil {
+		// TODO(stevvooe): This is not the best place to setup a cache. We would
+		// really like to decouple the cache from the backend but also have the
+		// manifeset service use the layer service cache. For now, we can simply
+		// integrate the cache directly. The main issue is that we have layer
+		// access and layer data coupled in a single object. Work is already under
+		// way to decouple this.
+
+		return &cachedLayerService{
+			LayerService: ls,
+			repository:   repo,
+			ctx:          repo.ctx,
+			driver:       repo.driver,
+			blobStore:    repo.blobStore,
+			cache:        repo.registry.layerInfoCache,
+		}
+	}
+
+	return ls
 }
 
 func (repo *repository) Signatures() distribution.SignatureService {