diff --git a/core/txpool/interface.go b/core/txpool/interface.go
index 7a3c807..df82a3f 100644
--- a/core/txpool/interface.go
+++ b/core/txpool/interface.go
@@ -26,17 +26,16 @@ type ItxPool interface {
 	CheckTxn(stxn *SignedTxn) error
 
 	Insert(txn *SignedTxn) error
+	InsertWithTopic(topic string, stxn *SignedTxn) error
+
 	SetOrder(order map[int]Hash)
 	SortTxns(fn func(txns []*SignedTxn) []*SignedTxn)
 
-	InsertWithTopic(topic string, stxn *SignedTxn) error
-	GetWithTopic(topic string, numLimit uint64) []*SignedTxn
-	GetWithTopicFor(topic string, numLimit uint64, filter func(txn *SignedTxn) bool) []*SignedTxn
-
 	// Pack packs some txns to send to tripods
 	Pack(numLimit uint64) ([]*SignedTxn, error)
-
 	PackFor(numLimit uint64, filter func(txn *SignedTxn) bool) ([]*SignedTxn, error)
+	PackWithTopic(topic string, numLimit uint64) ([]*SignedTxn, error)
+	PackWithTopicFor(topic string, numLimit uint64, filter func(txn *SignedTxn) bool) ([]*SignedTxn, error)
 
 	// GetTxn returns unpacked txn
 	GetTxn(hash Hash) (*SignedTxn, error)
diff --git a/core/txpool/txpool.go b/core/txpool/txpool.go
index 6aaef23..4b133db 100644
--- a/core/txpool/txpool.go
+++ b/core/txpool/txpool.go
@@ -110,20 +110,20 @@ func (tp *TxPool) InsertWithTopic(topic string, stxn *SignedTxn) error {
 	return tp.topicUnpackedTxns[topic].Insert(stxn)
 }
 
-func (tp *TxPool) GetWithTopic(topic string, numLimit uint64) []*SignedTxn {
+func (tp *TxPool) PackWithTopic(topic string, numLimit uint64) ([]*SignedTxn, error) {
 	if unpack, ok := tp.topicUnpackedTxns[topic]; ok {
-		return unpack.Gets(numLimit, tp.filter)
+		return unpack.Gets(numLimit, tp.filter), nil
 	}
-	return nil
+	return nil, nil
 }
 
-func (tp *TxPool) GetWithTopicFor(topic string, numLimit uint64, filter func(txn *SignedTxn) bool) []*SignedTxn {
+func (tp *TxPool) PackWithTopicFor(topic string, numLimit uint64, filter func(txn *SignedTxn) bool) ([]*SignedTxn, error) {
 	tp.topicLock.RLock()
 	defer tp.topicLock.RUnlock()
 	if unpack, ok := tp.topicUnpackedTxns[topic]; ok {
-		return unpack.Gets(numLimit, filter)
+		return unpack.Gets(numLimit, filter), nil
 	}
-	return nil
+	return nil, nil
 }
 
 func (tp *TxPool) SetOrder(order map[int]Hash) {