diff --git a/docs/conf.py b/docs/conf.py
index d4c48f5a..bfb9fdaa 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -13,8 +13,9 @@
 # documentation root, use os.path.abspath to make it absolute, like shown here.
 #
 import os
-from os import path
 import sys
+from os import path
+
 sys.path.insert(0, os.path.abspath('..'))
 
 # -- Project information -----------------------------------------------------
@@ -51,6 +52,7 @@
 # ones.
 extensions = [
     'sphinx.ext.autodoc',
+    'sphinx_autodoc_typehints',
     'sphinx.ext.viewcode',
     'sphinxcontrib.apidoc',
     'sphinxarg.ext',
diff --git a/docs/requirements.txt b/docs/requirements.txt
index 1f36fde0..8e5e8e72 100644
--- a/docs/requirements.txt
+++ b/docs/requirements.txt
@@ -1,2 +1,3 @@
 sphinx-argparse
-sphinxcontrib-apidoc
\ No newline at end of file
+sphinxcontrib-apidoc
+sphinx-autodoc-typehints
\ No newline at end of file
diff --git a/gnes/base/__init__.py b/gnes/base/__init__.py
index 617a7d7e..bc92c313 100644
--- a/gnes/base/__init__.py
+++ b/gnes/base/__init__.py
@@ -217,6 +217,7 @@ def dump_full_path(self):
     def yaml_full_path(self):
         """
         Get the file path of the yaml config
+
         :return:
         """
         return os.path.join(self.work_dir, '%s.yml' % self.name)
@@ -247,6 +248,7 @@ def train(self, *args, **kwargs):
     def dump(self, filename: str = None) -> None:
         """
         Serialize the object to a binary file
+
         :param filename: file path of the serialized file, if not given then :py:attr:`dump_full_path` is used
         """
         f = filename or self.dump_full_path
@@ -260,6 +262,7 @@ def dump(self, filename: str = None) -> None:
     def dump_yaml(self, filename: str = None) -> None:
         """
         Serialize the object to a yaml file
+
         :param filename: file path of the yaml file, if not given then :py:attr:`dump_yaml_path` is used
         """
         f = filename or self.yaml_full_path
diff --git a/gnes/client/cli.py b/gnes/client/cli.py
index 53236554..999ed215 100644
--- a/gnes/client/cli.py
+++ b/gnes/client/cli.py
@@ -17,7 +17,7 @@
 import sys
 import time
 import zipfile
-from typing import Generator
+from typing import Iterator
 
 from termcolor import colored
 
@@ -78,6 +78,7 @@ def query_callback(self, req: 'gnes_pb2.Request', resp: 'gnes_pb2.Response'):
         """
         callback after get the query result
         override this method to customize query behavior
+
         :param resp: response
         :param req: query
         :return:
@@ -86,14 +87,14 @@ def query_callback(self, req: 'gnes_pb2.Request', resp: 'gnes_pb2.Response'):
         print(resp)
 
     @property
-    def bytes_generator(self) -> Generator[bytes, None, None]:
+    def bytes_generator(self) -> Iterator[bytes]:
         if self._bytes_generator:
             return self._bytes_generator
         else:
             raise ValueError('bytes_generator is empty or not set')
 
     @bytes_generator.setter
-    def bytes_generator(self, bytes_gen: Generator[bytes, None, None]):
+    def bytes_generator(self, bytes_gen: Iterator[bytes]):
         if self._bytes_generator:
             self.logger.warning('bytes_generator is not empty, overrided')
         self._bytes_generator = bytes_gen
diff --git a/gnes/flow/__init__.py b/gnes/flow/__init__.py
index 057484d1..504a29d9 100644
--- a/gnes/flow/__init__.py
+++ b/gnes/flow/__init__.py
@@ -3,7 +3,7 @@
 from collections import OrderedDict, defaultdict
 from contextlib import ExitStack
 from functools import wraps
-from typing import Union, Tuple, List, Optional, Generator
+from typing import Union, Tuple, List, Optional, Iterator
 
 from ..cli.parser import set_router_parser, set_indexer_parser, \
     set_frontend_parser, set_preprocessor_parser, \
@@ -78,9 +78,10 @@ class Flow:
             flow.index()
             ...
 
-    You can also use the shortcuts, e.g. :py:meth:add_encoder , :py:meth:add_preprocessor
+    You can also use the shortcuts, e.g. :py:meth:`add_encoder`, :py:meth:`add_preprocessor`.
 
     It is recommend to use flow in the context manner as showed above.
+
     Note the different default copy behaviors in :py:meth:`.add()` and :py:meth:`.build()`:
     :py:meth:`.add()` always copy the flow by default, whereas :py:meth:`.build()` modify the flow in place.
     You can change this behavior by giving an argument `copy_flow=False`.
@@ -169,18 +170,38 @@ def to_mermaid(self, left_right: bool = True):
             'copy-paste the output and visualize it with: https://mermaidjs.github.io/mermaid-live-editor/')
         return mermaid_str
 
-    def train(self, bytes_gen: Generator[bytes, None, None] = None, **kwargs):
+    def train(self, bytes_gen: Iterator[bytes] = None, **kwargs):
+        """Do training on the current flow
+
+        It will start a :py:class:`CLIClient` and call :py:func:`train`.
+
+        :param bytes_gen: An iterator of bytes. If not given, then you have to specify it in `kwargs`.
+        :param kwargs: accepts all keyword arguments of `gnes client` CLI
+        """
         self._call_client(bytes_gen, mode='train', **kwargs)
 
-    def index(self, bytes_gen: Generator[bytes, None, None] = None, **kwargs):
+    def index(self, bytes_gen: Iterator[bytes] = None, **kwargs):
+        """Do indexing on the current flow
+
+        It will start a :py:class:`CLIClient` and call :py:func:`index`.
+
+        :param bytes_gen: An iterator of bytes. If not given, then you have to specify it in `kwargs`.
+        :param kwargs: accepts all keyword arguments of `gnes client` CLI
+        """
         self._call_client(bytes_gen, mode='index', **kwargs)
 
-    def query(self, bytes_gen: Generator[bytes, None, None] = None, **kwargs):
+    def query(self, bytes_gen: Iterator[bytes] = None, **kwargs):
+        """Do indexing on the current flow
+
+        It will start a :py:class:`CLIClient` and call :py:func:`query`.
+
+        :param bytes_gen: An iterator of bytes. If not given, then you have to specify it in `kwargs`.
+        :param kwargs: accepts all keyword arguments of `gnes client` CLI
+        """
         self._call_client(bytes_gen, mode='query', **kwargs)
 
     @_build_level(BuildLevel.RUNTIME)
-    def _call_client(self, bytes_gen: Generator[bytes, None, None] = None, **kwargs):
-
+    def _call_client(self, bytes_gen: Iterator[bytes] = None, **kwargs):
         os.unsetenv('http_proxy')
         os.unsetenv('https_proxy')
         args, p_args = self._get_parsed_args(self, set_client_cli_parser, kwargs)
@@ -192,26 +213,26 @@ def _call_client(self, bytes_gen: Generator[bytes, None, None] = None, **kwargs)
         c.start()
 
     def add_frontend(self, *args, **kwargs) -> 'Flow':
-        """Add a frontend to the current flow, a shortcut of add(Service.Frontend)
+        """Add a frontend to the current flow, a shortcut of :py:meth:`add(Service.Frontend)`.
         Usually you dont need to call this function explicitly, a flow object contains a frontend service by default.
         This function is useful when you build a flow without the frontend and want to customize the frontend later.
         """
         return self.add(Service.Frontend, *args, **kwargs)
 
     def add_encoder(self, *args, **kwargs) -> 'Flow':
-        """Add an encoder to the current flow, a shortcut of add(Service.Encoder)"""
+        """Add an encoder to the current flow, a shortcut of :py:meth:`add(Service.Encoder)`"""
         return self.add(Service.Encoder, *args, **kwargs)
 
     def add_indexer(self, *args, **kwargs) -> 'Flow':
-        """Add an indexer to the current flow, a shortcut of add(Service.Indexer)"""
+        """Add an indexer to the current flow, a shortcut of :py:meth:`add(Service.Indexer)`"""
         return self.add(Service.Indexer, *args, **kwargs)
 
     def add_preprocessor(self, *args, **kwargs) -> 'Flow':
-        """Add a router to the current flow, a shortcut of add(Service.Preprocessor)"""
+        """Add a preprocessor to the current flow, a shortcut of :py:meth:`add(Service.Preprocessor)`"""
         return self.add(Service.Preprocessor, *args, **kwargs)
 
     def add_router(self, *args, **kwargs) -> 'Flow':
-        """Add a preprocessor to the current flow, a shortcut of add(Service.Router)"""
+        """Add a router to the current flow, a shortcut of :py:meth:`add(Service.Router)`"""
         return self.add(Service.Router, *args, **kwargs)
 
     def add(self, service: 'Service',
@@ -416,7 +437,8 @@ def _build_graph(self, copy_flow: bool) -> 'Flow':
     def build(self, backend: Optional[str] = 'thread', copy_flow: bool = False, *args, **kwargs) -> 'Flow':
         """
         Build the current flow and make it ready to use
-        :param backend: supported 'thread', 'process', 'swarm', 'k8s', 'shell'
+
+        :param backend: supported 'thread', 'process', 'swarm', 'k8s', 'shell', if None then only build graph only
         :param copy_flow: return the copy of the current flow
         :return: the current flow (by default)
         """
diff --git a/gnes/indexer/base.py b/gnes/indexer/base.py
index 9748680a..a275bab0 100644
--- a/gnes/indexer/base.py
+++ b/gnes/indexer/base.py
@@ -75,6 +75,7 @@ def __init__(self, helper_indexer: 'BaseChunkIndexerHelper' = None, *args, **kwa
     def add(self, keys: List[Tuple[int, int]], vectors: np.ndarray, weights: List[float], *args, **kwargs):
         """
         adding new chunks and their vector representations
+
         :param keys: list of (doc_id, offset) tuple
         :param vectors: vector representations
         :param weights: weight of the chunks
@@ -160,6 +161,7 @@ class BaseDocIndexer(BaseIndexer):
     def add(self, keys: List[int], docs: List['gnes_pb2.Document'], *args, **kwargs):
         """
         adding new docs and their protobuf representation
+
         :param keys: list of doc_id
         :param docs: list of protobuf Document objects
         """
diff --git a/gnes/indexer/chunk/annoy.py b/gnes/indexer/chunk/annoy.py
index 360dbe1e..92e96cca 100644
--- a/gnes/indexer/chunk/annoy.py
+++ b/gnes/indexer/chunk/annoy.py
@@ -27,6 +27,7 @@ class AnnoyIndexer(BCI):
     def __init__(self, num_dim: int, data_path: str, metric: str = 'angular', n_trees: int = 10, *args, **kwargs):
         """
         Initialize an AnnoyIndexer
+
         :param num_dim: when set to -1, then num_dim is auto decided on first .add()
         :param data_path: index data file managed by the annoy indexer
         :param metric:
diff --git a/gnes/indexer/chunk/faiss.py b/gnes/indexer/chunk/faiss.py
index ac694292..23a2c47d 100644
--- a/gnes/indexer/chunk/faiss.py
+++ b/gnes/indexer/chunk/faiss.py
@@ -28,6 +28,7 @@ class FaissIndexer(BCI):
     def __init__(self, num_dim: int, index_key: str, data_path: str, *args, **kwargs):
         """
         Initialize an FaissIndexer
+
         :param num_dim: when set to -1, then num_dim is auto decided on first .add()
         :param data_path: index data file managed by the faiss indexer
         """
diff --git a/gnes/indexer/doc/filesys.py b/gnes/indexer/doc/filesys.py
index 349de83e..d77a565b 100644
--- a/gnes/indexer/doc/filesys.py
+++ b/gnes/indexer/doc/filesys.py
@@ -38,6 +38,7 @@ def add(self, keys: List[int], docs: List['gnes_pb2.Document'], *args, **kwargs)
         """
         write GIFs of each document into disk
         folder structure: /data_path/doc_id/0.gif, 1.gif...
+
         :param keys: list of doc id
         :param docs: list of docs
         """
@@ -55,6 +56,8 @@ def add(self, keys: List[int], docs: List['gnes_pb2.Document'], *args, **kwargs)
 
     def query(self, keys: List[int], *args, **kwargs) -> List['gnes_pb2.Document']:
         """
+        Find the doc according to the keys
+
         :param keys: list of doc id
         :return: list of documents whose chunks field contain all the GIFs of this doc(one GIF per chunk)
         """
diff --git a/gnes/router/base.py b/gnes/router/base.py
index 0a1ca20f..dccea73e 100644
--- a/gnes/router/base.py
+++ b/gnes/router/base.py
@@ -101,6 +101,7 @@ def reduce_embedding(self, accum_msgs: List['gnes_pb2.Message'], msg_type: str,
     def apply(self, msg: 'gnes_pb2.Message', accum_msgs: List['gnes_pb2.Message'], *args, **kwargs) -> None:
         """
         reduce embeddings from encoders (means, concat ....)
+
         :param msg: the current message
         :param accum_msgs: accumulated messages
         """
diff --git a/gnes/service/frontend.py b/gnes/service/frontend.py
index a60b497b..051f5350 100644
--- a/gnes/service/frontend.py
+++ b/gnes/service/frontend.py
@@ -125,8 +125,8 @@ 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)
+                    num_recv = max(self.pending_request - self.args.max_pending_request, 1)
+                    yield from get_response(num_recv, num_recv > 1)
 
                     zmq_client.send_message(self.add_envelope(request, zmq_client), **self.send_recv_kwargs)
                     self.pending_request += 1