diff --git a/server/apps/exam/views.py b/server/apps/exam/views.py
index d3efa0b..5a530c4 100644
--- a/server/apps/exam/views.py
+++ b/server/apps/exam/views.py
@@ -56,7 +56,7 @@ def dectry(p):
 
 def acquire_lock(lock_name, timeout=60):
     identifier = str(uuid.uuid4())
-    lock = cache.set(lock_name, identifier, timeout=timeout)
+    lock = cache.add(lock_name, identifier, timeout=timeout)
     print(lock, identifier)
     return identifier if lock else None