distribution/docs/storage/layercache.go

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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() {
expvar.Publish("layerinfocache", 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
}))
}