diff --git a/gnes/client/base.py b/gnes/client/base.py
index ce4bbd66..aaaa6862 100644
--- a/gnes/client/base.py
+++ b/gnes/client/base.py
@@ -63,9 +63,11 @@ def get_default_fn(r_type):
                 fn = self.routes.get(resp_type)
             else:
                 fn = get_default_fn(type(resp))
-
-        self.logger.info('handling response with %s' % fn.__name__)
-        return fn(self._context, resp)
+            self.logger.info('handling response with %s' % fn.__name__)
+            return fn(self._context, resp)
+        else:
+            self.logger.warning('the received message is not response')
+            return None
 
 
 class ZmqClient:
diff --git a/gnes/client/stream.py b/gnes/client/stream.py
index 7adde956..6df6051e 100644
--- a/gnes/client/stream.py
+++ b/gnes/client/stream.py
@@ -44,7 +44,7 @@ class StreamingClient(GrpcClient):
     def __init__(self, args):
         super().__init__(args)
 
-        self._request_queue = queue.Queue(maxsize=1000)
+        self._request_queue = queue.Queue(maxsize=10)
         self._is_streaming = threading.Event()
 
         self._dispatch_thread = threading.Thread(target=self._start)
@@ -63,16 +63,16 @@ def _start(self):
         self._is_streaming.clear()
 
     def _request_generator(self):
-        while self._is_streaming.is_set():
+        while True:
             try:
                 request = self._request_queue.get(block=True, timeout=5.0)
                 if request is None:
                     break
                 yield request
             except queue.Empty:
-                continue
+                break
             except Exception as e:
-                print('exception: %s' % str(e))
+                self.logger.error('exception: %s' % str(e))
                 break
 
     @handler.register(NotImplementedError)
diff --git a/gnes/preprocessor/video/frame_select.py b/gnes/preprocessor/video/frame_select.py
index d55aacbb..1e3fe6b6 100644
--- a/gnes/preprocessor/video/frame_select.py
+++ b/gnes/preprocessor/video/frame_select.py
@@ -46,6 +46,7 @@ def apply(self, doc: 'gnes_pb2.Document') -> None:
                     else:
                         idx = np.sort(np.random.choice(len(images), self.sframes, replace=False))
                         chunk.blob.CopyFrom(array2blob(images[idx]))
+                del images
         else:
             self.logger.error(
                 'bad document: "doc.chunks" is empty!')
diff --git a/gnes/service/frontend.py b/gnes/service/frontend.py
index b74c2727..6d62bae0 100644
--- a/gnes/service/frontend.py
+++ b/gnes/service/frontend.py
@@ -125,6 +125,9 @@ def get_response(num_recv, blocked=False):
             with self.zmq_context as zmq_client:
 
                 for request in request_iterator:
+                    num_recv = max(self.pending_request - self.args.max_pending_request, 0)
+                    yield from get_response(num_recv, num_recv > 0)
+
                     zmq_client.send_message(self.add_envelope(request, zmq_client), **self.send_recv_kwargs)
                     self.pending_request += 1