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mutation_cache.go
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mutation_cache.go
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package controllers
import (
lru "github.com/hashicorp/golang-lru"
"k8s.io/apimachinery/pkg/runtime"
utilruntime "k8s.io/apimachinery/pkg/util/runtime"
"k8s.io/apiserver/pkg/storage/etcd"
"k8s.io/client-go/tools/cache"
"k8s.io/kubernetes/pkg/controller"
)
// MutationCache is able to take the result of update operations and stores them in an LRU
// that can be used to provide a more current view of a requested object. It requires interpretting
// resourceVersions for comparisons.
// Implementations must be thread-safe.
type MutationCache interface {
GetByKey(key string) (interface{}, bool, error)
Mutation(interface{})
}
type ResourceVersionComparator interface {
CompareResourceVersion(lhs, rhs runtime.Object) int
}
// NewEtcdMutationCache gives back a MutationCache that understands how to deal with etcd backed objects
func NewEtcdMutationCache(backingCache cache.Store) MutationCache {
lru, err := lru.New(100)
if err != nil {
// errors only happen on invalid sizes, this would be programmer error
panic(err)
}
return &mutationCache{
backingCache: backingCache,
mutationCache: lru,
comparator: etcd.APIObjectVersioner{},
}
}
// mutationCache doesn't guarantee that it returns values added via Mutation since they can page out and
// since you can't distinguish between, "didn't observe create" and "was deleted after create",
// if the key is missing from the backing cache, we always return it as missing
type mutationCache struct {
backingCache cache.Store
mutationCache *lru.Cache
comparator ResourceVersionComparator
}
// GetByKey is never guaranteed to return back the value set in Mutation. It could be paged out, it could
// be older than another copy, the backingCache may be more recent or, you might have written twice into the same key.
// You get a value that was valid at some snapshot of time and will always return the newer of backingCache and mutationCache.
func (c *mutationCache) GetByKey(key string) (interface{}, bool, error) {
obj, exists, err := c.backingCache.GetByKey(key)
if err != nil {
return nil, false, err
}
if !exists {
// we can't distinguish between, "didn't observe create" and "was deleted after create", so
// if the key is missing, we always return it as missing
return nil, false, nil
}
objRuntime, ok := obj.(runtime.Object)
if !ok {
return obj, true, nil
}
mutatedObj, exists := c.mutationCache.Get(key)
if !exists {
return obj, true, nil
}
mutatedObjRuntime, ok := mutatedObj.(runtime.Object)
if !ok {
return obj, true, nil
}
if c.comparator.CompareResourceVersion(objRuntime, mutatedObjRuntime) >= 0 {
c.mutationCache.Remove(key)
return obj, true, nil
}
return mutatedObj, true, nil
}
// Mutation adds a change to the cache that can be returned in GetByKey if it is newer than the backingCache
// copy. If you call Mutation twice with the same object on different threads, one will win, but its not defined
// which one. This doesn't affect correctness, since the GetByKey guaranteed of "later of these two caches" is
// preserved, but you may not get the version of the object you want. The object you get is only guaranteed to
// "one that was valid at some point in time", not "the one that I want".
func (c *mutationCache) Mutation(obj interface{}) {
key, err := controller.KeyFunc(obj)
if err != nil {
// this is a "nice to have", so failures shouldn't do anything weird
utilruntime.HandleError(err)
return
}
c.mutationCache.Add(key, obj)
}