// Config is configuration for creating a storage backend.
type Config struct {
// Type defines the type of storage backend. Default ("") is "etcd3".
Type string
// Prefix is the prefix to all keys passed to storage.Interface methods.
Prefix string
// 存放连接etcd的证书，etcd集群地址等信息
// Transport holds all connection related info, i.e. equal TransportConfig means equal servers we talk to.
Transport TransportConfig
// Paging indicates whether the server implementation should allow paging (if it is
// supported). This is generally configured by feature gating, or by a specific
// resource type not wishing to allow paging, and is not intended for end users to
// set.
Paging bool
Codec runtime.Codec
// EncodeVersioner is the same groupVersioner used to build the
// storage encoder. Given a list of kinds the input object might belong
// to, the EncodeVersioner outputs the gvk the object will be
// converted to before persisted in etcd.
EncodeVersioner runtime.GroupVersioner
// Transformer allows the value to be transformed prior to persisting into etcd.
Transformer value.Transformer
// CompactionInterval is an interval of requesting compaction from apiserver.
// If the value is 0, no compaction will be issued.
CompactionInterval time.Duration
// CountMetricPollPeriod specifies how often should count metric be updated
CountMetricPollPeriod time.Duration
// DBMetricPollInterval specifies how often should storage backend metric be updated.
DBMetricPollInterval time.Duration
// HealthcheckTimeout specifies the timeout used when checking health
HealthcheckTimeout time.Duration
LeaseManagerConfig etcd3.LeaseManagerConfig
}
CompactionInterval
Since etcd keeps an exact history of its keyspace, this history should be periodically compacted to avoid performance degradation and eventual storage space exhaustion. Compacting the keyspace history drops all information about keys superseded prior to a given keyspace revision. The space used by these keys then becomes available for additional writes to the keyspace.
etcd需要周期性进行compact操作用于压缩空间，因为etcd会存储对象的每个版本。这会导致空间会一直增长，当超过限制之后etcd会触发集群内警报，警报触发后，etcd集群将只接收读请求和删除请求。
那compact操作，就是传入一个对象版本，所有这个版本之前的数据全部都删除。
# compact up to revision 3
$ etcdctl compact 3
compact之后，所有版本3之前的版本再访问就会报错了。
$ etcdctl get --rev=2 somekey
Error: rpc error: code = 11 desc = etcdserver: mvcc: required revision has been compacted
CompactionInterval就是周期性往etcd发送compact请求，防止因为etcd存储空间耗尽apiserver不能进行写操作。
Transformer
这个对象主要用于在对象持久化到etcd之前，进行格式转换。比如加密 etc.
// Transformer allows a value to be transformed before being read from or written to the underlying store. The methods
// must be able to undo the transformation caused by the other.
type Transformer interface {
// TransformFromStorage may transform the provided data from its underlying storage representation or return an error.
// Stale is true if the object on disk is stale and a write to etcd should be issued, even if the contents of the object
// have not changed.
TransformFromStorage(data []byte, context Context) (out []byte, stale bool, err error)
// TransformToStorage may transform the provided data into the appropriate form in storage or return an error.
TransformToStorage(data []byte, context Context) (out []byte, err error)
}
