diff --git a/quarkchain/cluster/shard_state.py b/quarkchain/cluster/shard_state.py
index e6ef41d8b..24b15c2db 100644
--- a/quarkchain/cluster/shard_state.py
+++ b/quarkchain/cluster/shard_state.py
@@ -1,6 +1,7 @@
 import asyncio
 import json
 import time
+from cachetools import LRUCache
 from collections import Counter, deque, defaultdict
 from fractions import Fraction
 from typing import Dict, List, Optional, Tuple, Union
diff --git a/requirements.txt b/requirements.txt
index 2343b0249..71ade7850 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,6 +1,7 @@
 aiohttp==3.1.3
 aioprocessing==1.0.1
 async_armor==0.0.2
+cachetools==3.1.1
 coincurve==7.1.0
 decorator==4.2.1
 ecdsa==0.13