摘要：為所有對外提供服務的資源實現了一套通用的符合要求的操作接口，每個服務接口負責處理一類資源對象。該接口最終返回了的和清除操作資源的接口。

Kubernetes v1.5.0

k8s的各個組件與apiServer交互操作各種資源對象，最終都會落入到etcd中。
k8s為所有對外提供服務的Restful資源實現了一套通用的符合Restful要求的etcd操作接口，每個服務接口負責處理一類(Kind)資源對象。
這些資源對象包括pods、bindings、podTemplates、RC、Services等。

要了解etcd操作接口的實現，我們先需要了解下Master.GenericAPIServer.storage結構：

storage map[string]rest.Storage

該storage變量是個map，Key是REST API的path，Value是rest.Storage接口，該接口就是一個通用的符合Restful要求的資源存儲接口。
核心組資源列表的創建要查看pkg/registry/core/rest/storage_core.go中的NewLegacyRESTStorage()接口：
接口調用流程： main --> App.Run --> config.Complete().New() --> m.InstallLegacyAPI() --> NewLegacyRESTStorage()

func (c LegacyRESTStorageProvider) NewLegacyRESTStorage(restOptionsGetter genericapiserver.RESTOptionsGetter) (LegacyRESTStorage, genericapiserver.APIGroupInfo, error) {
    ....
    // 創建podStorage
    podStorage := podetcd.NewStorage(
        restOptionsGetter(api.Resource("pods")),
        nodeStorage.KubeletConnectionInfo,
        c.ProxyTransport,
        podDisruptionClient,
    )

    ...
    // 資源列表
    restStorageMap := map[string]rest.Storage{
        "pods":             podStorage.Pod,
        "pods/attach":      podStorage.Attach,
        "pods/status":      podStorage.Status,
        "pods/log":         podStorage.Log,
        "pods/exec":        podStorage.Exec,
        "pods/portforward": podStorage.PortForward,
        "pods/proxy":       podStorage.Proxy,
        "pods/binding":     podStorage.Binding,
        "bindings":         podStorage.Binding,

        "podTemplates": podTemplateStorage,

        "replicationControllers":        controllerStorage.Controller,
        "replicationControllers/status": controllerStorage.Status,

        "services":        serviceRest.Service,
        "services/proxy":  serviceRest.Proxy,
        "services/status": serviceStatusStorage,

        "endpoints": endpointsStorage,

        ...

        "componentStatuses": componentstatus.NewStorage(componentStatusStorage{c.StorageFactory}.serversToValidate),
    }
    if registered.IsEnabledVersion(unversioned.GroupVersion{Group: "autoscaling", Version: "v1"}) {
        restStorageMap["replicationControllers/scale"] = controllerStorage.Scale
    }
    if registered.IsEnabledVersion(unversioned.GroupVersion{Group: "policy", Version: "v1beta1"}) {
        restStorageMap["pods/eviction"] = podStorage.Eviction
    }
    apiGroupInfo.VersionedResourcesStorageMap["v1"] = restStorageMap

    return restStorage, apiGroupInfo, nil
}

該接口在ApiServer源碼分析的第二章介紹資源注冊的時候已經講過，這里我們主要分析后端存儲etcd操作接口的實現。
我們以Pod資源為例,進行介紹：
路徑： pkg/registry/core/pod/etcd/etcd.go

func NewStorage(opts generic.RESTOptions, k client.ConnectionInfoGetter, proxyTransport http.RoundTripper, podDisruptionBudgetClient policyclient.PodDisruptionBudgetsGetter) PodStorage {
    // 完成prefix
    prefix := "/" + opts.ResourcePrefix

    newListFunc := func() runtime.Object { return &api.PodList{} }
    // 調用接口裝飾器，返回該storage的etcd操作接口及資源delete接口
    // 該opts傳參進來的，需要到上一層查看master.go下的restOptionsFactory.NewFor
    storageInterface, dFunc := opts.Decorator(
        opts.StorageConfig,
        // 這一下的參數都是用于開啟cache時的接口使用
        cachesize.GetWatchCacheSizeByResource(cachesize.Pods),
        &api.Pod{},
        prefix,
        pod.Strategy,
        newListFunc,
        pod.NodeNameTriggerFunc,
    )

    // 創建Store對象
    store := ®istry.Store{
        NewFunc:     func() runtime.Object { return &api.Pod{} },
        NewListFunc: newListFunc,
        KeyRootFunc: func(ctx api.Context) string {
            return registry.NamespaceKeyRootFunc(ctx, prefix)
        },
        KeyFunc: func(ctx api.Context, name string) (string, error) {
            return registry.NamespaceKeyFunc(ctx, prefix, name)
        },
        ObjectNameFunc: func(obj runtime.Object) (string, error) {
            return obj.(*api.Pod).Name, nil
        },
        PredicateFunc:           pod.MatchPod,
        QualifiedResource:       api.Resource("pods"),
        EnableGarbageCollection: opts.EnableGarbageCollection,
        DeleteCollectionWorkers: opts.DeleteCollectionWorkers,

        CreateStrategy:      pod.Strategy,
        UpdateStrategy:      pod.Strategy,
        DeleteStrategy:      pod.Strategy,
        ReturnDeletedObject: true,

        Storage:     storageInterface,
        DestroyFunc: dFunc,
    }

    statusStore := *store
    statusStore.UpdateStrategy = pod.StatusStrategy

    return PodStorage{
        Pod:         &REST{store, proxyTransport},
        Binding:     &BindingREST{store: store},
        Eviction:    newEvictionStorage(store, podDisruptionBudgetClient),
        Status:      &StatusREST{store: &statusStore},
        Log:         &podrest.LogREST{Store: store, KubeletConn: k},
        Proxy:       &podrest.ProxyREST{Store: store, ProxyTransport: proxyTransport},
        Exec:        &podrest.ExecREST{Store: store, KubeletConn: k},
        Attach:      &podrest.AttachREST{Store: store, KubeletConn: k},
        PortForward: &podrest.PortForwardREST{Store: store, KubeletConn: k},
    }
}

該接口中調用了opts.Decorator()接口返回了關鍵的storage interface及清除操作資源的接口。
要看該接口的實現，我們得先從opts的創建開始。
restOptionsGetter(api.Resource("pods"))該步完成了opts的創建，api.Resource("pods")其實就是拼接了一個GroupResource的結構，我們需要從頭開始介紹restOptionsGetter接口的由來。
路徑：pkg/master/master.go

func (c completedConfig) New() (*Master, error) {
...
    restOptionsFactory := restOptionsFactory{
        deleteCollectionWorkers: c.DeleteCollectionWorkers,
        enableGarbageCollection: c.GenericConfig.EnableGarbageCollection,
        storageFactory:          c.StorageFactory,
    }

    // 判斷是否使能了用于Watch的Cache
    // 有無cache賦值的是不同的接口實現
    // restOptionsFactory.storageDecorator：是一個各個資源的REST interface(CRUD)裝飾者
    // 后面調用NewStorage()時會用到該接口，并輸出對應的CRUD接口及銷毀接口。
    // 可以參考pkg/registry/core/pod/etcd/etcd.go中的NewStorage()
    // 其實這里有無cache的接口差異就在于：有cache的話，就提供操作cache的接口；無cache的話，就提供直接操作etcd的接口
    if c.EnableWatchCache {
        restOptionsFactory.storageDecorator = registry.StorageWithCacher
    } else {
        restOptionsFactory.storageDecorator = generic.UndecoratedStorage
    }
    // install legacy rest storage
    if c.GenericConfig.APIResourceConfigSource.AnyResourcesForVersionEnabled(apiv1.SchemeGroupVersion) {
        legacyRESTStorageProvider := corerest.LegacyRESTStorageProvider{
            StorageFactory:       c.StorageFactory,
            ProxyTransport:       c.ProxyTransport,
            KubeletClientConfig:  c.KubeletClientConfig,
            EventTTL:             c.EventTTL,
            ServiceIPRange:       c.ServiceIPRange,
            ServiceNodePortRange: c.ServiceNodePortRange,
            LoopbackClientConfig: c.GenericConfig.LoopbackClientConfig,
        }
        m.InstallLegacyAPI(c.Config, restOptionsFactory.NewFor, legacyRESTStorageProvider)
    }
...
}

該接口初始化了一個restOptionsFactory變量，里面指定了最大的刪除回收資源的協程數，是否使能GC和storageFactory,還根據是否使能了WatchCache來完成NewStorage()接口中調用的裝飾器接口的賦值。
restOptionsFactory.NewForj接口一直被往下傳，直到NewLegacyRESTStorage()接口中被調用然后創建了opts，我們看下該接口實現：
路徑： pkg/master/master.go

type restOptionsFactory struct {
    deleteCollectionWorkers int
    enableGarbageCollection bool
    storageFactory          genericapiserver.StorageFactory
    storageDecorator        generic.StorageDecorator
}

func (f restOptionsFactory) NewFor(resource unversioned.GroupResource) generic.RESTOptions {
    // 創建該資源的Storage Config
    storageConfig, err := f.storageFactory.NewConfig(resource)
    if err != nil {
        glog.Fatalf("Unable to find storage destination for %v, due to %v", resource, err.Error())
    }
    // 最終返回的就是RESTOptions, 就是前面的opts的類型
    // 需要關注f.storageDecorator的由來
    return generic.RESTOptions{
        // 用于生成Storage的config
        StorageConfig:           storageConfig,
        Decorator:               f.storageDecorator,
        DeleteCollectionWorkers: f.deleteCollectionWorkers,
        EnableGarbageCollection: f.enableGarbageCollection,
        ResourcePrefix:          f.storageFactory.ResourcePrefix(resource),
    }
}

該接口比較簡單，初始化了一個generic.RESTOptions變量，即opts。我們需要找出opts.Decorator的由來,就只需要看下上一個接口判斷EnableWatchCache時就明白了。
opts.Decorator該接口最終返回了storage的interface和清除操作資源的接口。可以想一下帶緩沖和不帶緩沖的接口實現肯定不一致，所以這里需要進行區分:

registry.StorageWithCacher：該接口是返回了操作cache的接口，和清除cache的操作接口

generic.UndecoratedStorage: 該接口會根據你配置的后端類型(etcd2/etcd3等)，來返回不同的etcd操作接口，其實是為所有的資源對象創建了etcd的鏈接，然后通過該鏈接發送不同的命令，最后還返回了斷開該鏈接的接口。

所以實現完全不一樣，一個操作cache，一個操作實際的etcd。

先看registry.StorageWithCacher()接口實現:
路徑： pkg/registry/generic/registry/storage_factory.go

func StorageWithCacher(
    storageConfig *storagebackend.Config,
    capacity int,
    objectType runtime.Object,
    resourcePrefix string,
    scopeStrategy rest.NamespaceScopedStrategy,
    newListFunc func() runtime.Object,
    triggerFunc storage.TriggerPublisherFunc) (storage.Interface, factory.DestroyFunc) {
    
    // storageConfig是后端存儲的config，定義了存儲類型，存儲服務器List，TLS證書信息，Cache大小等。
    // 該接口就是generic.UndecoratedStorage()接口的實現，StorageWithCacher()接口就是多了下面的cacher操作
    s, d := generic.NewRawStorage(storageConfig)
    // TODO: we would change this later to make storage always have cacher and hide low level KV layer inside.
    // Currently it has two layers of same storage interface -- cacher and low level kv.
    cacherConfig := storage.CacherConfig{
        CacheCapacity:        capacity,
        Storage:              s,
        Versioner:            etcdstorage.APIObjectVersioner{},
        Type:                 objectType,
        ResourcePrefix:       resourcePrefix,
        NewListFunc:          newListFunc,
        TriggerPublisherFunc: triggerFunc,
        Codec:                storageConfig.Codec,
    }
    // 根據是否有namespace來進行區分賦值
    // KeyFunc函數用于獲取該object的Key: 
    // 有namespace的話，key的格式：prefix + "/" + Namespace + "/" + name
    // 無namespace的話，key的格式：prefix + "/" + name
    if scopeStrategy.NamespaceScoped() {
        cacherConfig.KeyFunc = func(obj runtime.Object) (string, error) {
            return storage.NamespaceKeyFunc(resourcePrefix, obj)
        }
    } else {
        cacherConfig.KeyFunc = func(obj runtime.Object) (string, error) {
            return storage.NoNamespaceKeyFunc(resourcePrefix, obj)
        }
    }
    // 根據之前初始化的Cacher的config，進行cacher創建
    // 比較關鍵，后面進行介紹
    cacher := storage.NewCacherFromConfig(cacherConfig)
    destroyFunc := func() {
        cacher.Stop()
        d()
    }

    return cacher, destroyFunc
}

先調用NewRawStorage()接口創建了一個存儲后端，我們先看下這個接口實現：
路徑： pkg/registry/generic/storage_decorator.go

func NewRawStorage(config *storagebackend.Config) (storage.Interface, factory.DestroyFunc) {
    s, d, err := factory.Create(*config)
    if err != nil {
        glog.Fatalf("Unable to create storage backend: config (%v), err (%v)", config, err)
    }
    return s, d
}

沒啥好說的，繼續看Create()：
路徑： pkg/storage/storagebackend/factory/factory.go

func Create(c storagebackend.Config) (storage.Interface, DestroyFunc, error) {
    // 判斷下存儲類型：etcd2 、etcd3
    switch c.Type {
    case storagebackend.StorageTypeUnset, storagebackend.StorageTypeETCD2:
        return newETCD2Storage(c)
    case storagebackend.StorageTypeETCD3:
        // TODO: We have the following features to implement:
        // - Support secure connection by using key, cert, and CA files.
        // - Honor "https" scheme to support secure connection in gRPC.
        // - Support non-quorum read.
        return newETCD3Storage(c)
    default:
        return nil, nil, fmt.Errorf("unknown storage type: %s", c.Type)
    }
}

挑個etcd2看下實現：
路徑： pkg/storage/storagebackend/factory/etcd2.go

func newETCD2Storage(c storagebackend.Config) (storage.Interface, DestroyFunc, error) {
    // 根據配置的TLS證書信息創建http.Transport
    tr, err := newTransportForETCD2(c.CertFile, c.KeyFile, c.CAFile)
    if err != nil {
        return nil, nil, err
    }
    // 創建etcd2 client，返回的是httpClusterClient結構
    client, err := newETCD2Client(tr, c.ServerList)
    if err != nil {
        return nil, nil, err
    }
    // 根據入參初始化一個實現了storage.Interface接口的etcdHelper變量
    s := etcd.NewEtcdStorage(client, c.Codec, c.Prefix, c.Quorum, c.DeserializationCacheSize)
    // 返回etcdHelper變量，及關閉鏈接的函數
    return s, tr.CloseIdleConnections, nil
}

前兩步都是為了創建與etcd連接的client，后一步比較關鍵：
路徑： pkg/storage/etcd/etcd_helper.go

func NewEtcdStorage(client etcd.Client, codec runtime.Codec, prefix string, quorum bool, cacheSize int) storage.Interface {
    return &etcdHelper{
        // 創建一個httpMembersAPI變量，附帶很多方法
        etcdMembersAPI: etcd.NewMembersAPI(client),
        // 創建一個httpKeysAPI變量，同樣附帶各類方法
        etcdKeysAPI:    etcd.NewKeysAPI(client),
        // 編解碼使用
        codec:          codec,
        versioner:      APIObjectVersioner{},
        // 用于序列化反序列化，版本間轉換，兼容等
        copier:         api.Scheme,
        pathPrefix:     path.Join("/", prefix),
        quorum:         quorum,
        // 創建cache結構
        cache:          utilcache.NewCache(cacheSize),
    }
}

該接口很簡單的初始化，需要關注的是etcdHelper附帶的通用的RESTFul 方法：

可以看到storage.Interface接口所需要的方法都實現了。

繼續回到StorageWithCacher()接口，在往下走就是CacherConfig的初始化，就不介紹了,直接進入cacher的創建接口：
路徑： pkg/storage/cacher.go

func NewCacherFromConfig(config CacherConfig) *Cacher {
    watchCache := newWatchCache(config.CacheCapacity, config.KeyFunc)
    listerWatcher := newCacherListerWatcher(config.Storage, config.ResourcePrefix, config.NewListFunc)

    // Give this error when it is constructed rather than when you get the
    // first watch item, because it"s much easier to track down that way.
    if obj, ok := config.Type.(runtime.Object); ok {
        if err := runtime.CheckCodec(config.Codec, obj); err != nil {
            panic("storage codec doesn"t seem to match given type: " + err.Error())
        }
    }

    cacher := &Cacher{
        ready:       newReady(),
        storage:     config.Storage,
        objectType:  reflect.TypeOf(config.Type),
        watchCache:  watchCache,
        reflector:   cache.NewReflector(listerWatcher, config.Type, watchCache, 0),
        versioner:   config.Versioner,
        triggerFunc: config.TriggerPublisherFunc,
        watcherIdx:  0,
        watchers: indexedWatchers{
            allWatchers:   make(map[int]*cacheWatcher),
            valueWatchers: make(map[string]watchersMap),
        },
        // TODO: Figure out the correct value for the buffer size.
        incoming: make(chan watchCacheEvent, 100),
        // We need to (potentially) stop both:
        // - wait.Until go-routine
        // - reflector.ListAndWatch
        // and there are no guarantees on the order that they will stop.
        // So we will be simply closing the channel, and synchronizing on the WaitGroup.
        stopCh: make(chan struct{}),
    }
    watchCache.SetOnEvent(cacher.processEvent)
    go cacher.dispatchEvents()

    stopCh := cacher.stopCh
    cacher.stopWg.Add(1)
    go func() {
        defer cacher.stopWg.Done()
        wait.Until(
            func() {
                if !cacher.isStopped() {
                    cacher.startCaching(stopCh)
                }
            }, time.Second, stopCh,
        )
    }()
    return cacher
}

該接口主要用于開啟cacher,而該cache只用于WATCH和LIST的request。
我們在看下Cacher結構體：

該接口必然也實現了storage.Interface接口所需要的方法。
因為該Cacher只用于WATCH和LIST的request，所以你可以看下cacher提供的API,除了WATCH和LIST相關的之外的接口都是調用了之前創建的storage的API。
查看下cacher.Create和Delete:

func (c *Cacher) Create(ctx context.Context, key string, obj, out runtime.Object, ttl uint64) error {
    return c.storage.Create(ctx, key, obj, out, ttl)
}

func (c *Cacher) Delete(ctx context.Context, key string, out runtime.Object, preconditions *Preconditions) error {
    return c.storage.Delete(ctx, key, out, preconditions)
}

到這里registry.StorageWithCacher()接口就結束了，我們繼續回到前面講的另外一個接口generic.UndecoratedStorage()：
路徑：pkg/registry/generic/storage_decorator.go

func UndecoratedStorage(
    config *storagebackend.Config,
    capacity int,
    objectType runtime.Object,
    resourcePrefix string,
    scopeStrategy rest.NamespaceScopedStrategy,
    newListFunc func() runtime.Object,
    trigger storage.TriggerPublisherFunc) (storage.Interface, factory.DestroyFunc) {
    return NewRawStorage(config)
}

發現registry.StorageWithCacher()接口也是調用了NewRawStorage()接口，其實現就少了cache。

這里接觸到了cache，下節會專門介紹該cache實現。

--watch-cache： 該apiServer的參數默認就是true的，用于打開watch cache

--watch-cache-sizes: 既然有enable cache，那就少不了cache sizes，而且該size可以指定各類資源所使用的cache size。格式： resource#size

--storage-backend: 后端持久化存儲類型，可選項為etcd2(默認)、etcd3