diff --git a/storage/kv.go b/storage/kv.go
index 2cb1587cb..c26917073 100644
--- a/storage/kv.go
+++ b/storage/kv.go
@@ -16,9 +16,40 @@ var (
 	keyBucketName = []byte("key")
 )
 
+type KV interface {
+	// Range gets the keys in the range at rangeIndex.
+	// If rangeIndex <=0, range gets the keys at currentIndex.
+	// If `end` is nil, the request returns the key.
+	// If `end` is not nil, it gets the keys in range [key, range_end).
+	// Limit limits the number of keys returned.
+	Range(key, end []byte, limit, rangeIndex int64) (kvs []storagepb.KeyValue, index int64)
+
+	// Put puts the given key,value into the store.
+	// A put also increases the index of the store, and generates one event in the event history.
+	Put(key, value []byte) (index int64)
+
+	// DeleteRange deletes the given range from the store.
+	// A deleteRange increases the index of the store if any key in the range exists.
+	// The number of key deleted will be returned.
+	// It also generates one event for each key delete in the event history.
+	// if the `end` is nil, deleteRange deletes the key.
+	// if the `end` is not nil, deleteRange deletes the keys in range [key, range_end).
+	DeleteRange(key, end []byte) (n, index int64)
+
+	// TnxBegin begins a tnx. Only Tnx prefixed operation can be executed, others will be blocked
+	// until tnx ends. Only one on-going tnx is allowed.
+	TnxBegin()
+	// TnxEnd ends the on-going tnx.
+	TnxEnd()
+	TnxRange(key, end []byte, limit, rangeIndex int64) (kvs []storagepb.KeyValue, index int64)
+	TnxPut(key, value []byte) (index int64)
+	TnxDeleteRange(key, end []byte) (n, index int64)
+}
+
 type store struct {
 	// read operation MUST hold read lock
 	// write opeartion MUST hold write lock
+	// tnx operation MUST hold write lock
 	sync.RWMutex
 
 	b       backend.Backend