diff --git a/hbmqtt/broker.py b/hbmqtt/broker.py
index f6e5009..fbda193 100644
--- a/hbmqtt/broker.py
+++ b/hbmqtt/broker.py
@@ -1,11 +1,11 @@
 # Copyright (c) 2015 Nicolas JOUANIN
 #
 # See the file license.txt for copying permission.
+from typing import Optional, Dict, Union
 import logging
 import ssl
 import websockets
 import asyncio
-import sys
 import re
 from asyncio import CancelledError
 from collections import deque
@@ -90,7 +90,7 @@ class Server:
                 % (self.listener_name, self.conn_count)
             )
 
-    def release_connection(self):
+    def release_connection(self) -> None:
         if self.semaphore:
             self.semaphore.release()
         self.conn_count -= 1
@@ -117,7 +117,7 @@ class BrokerContext(BaseContext):
     It act as an adapter to broker services from plugins developed for HBMQTT broker
     """
 
-    def __init__(self, broker):
+    def __init__(self, broker: "Broker") -> None:
         super().__init__()
         self.config = None
         self._broker_instance = broker
@@ -205,7 +205,7 @@ class Broker:
         except KeyError as ke:
             raise BrokerException("Listener config not found invalid: %s" % ke)
 
-    def _init_states(self):
+    def _init_states(self) -> None:
         self.transitions = Machine(states=Broker.states, initial="new")
         self.transitions.add_transition(trigger="start", source="new", dest="starting")
         self.transitions.add_transition(
@@ -741,7 +741,13 @@ class Broker:
         # If all plugins returned True, authentication is success
         return topic_result
 
-    def retain_message(self, source_session, topic_name, data, qos=None):
+    def retain_message(
+        self,
+        source_session: Session,
+        topic_name: str,
+        data: bytearray,
+        qos: Optional[int] = None,
+    ) -> None:
         if data is not None and data != b"":
             # If retained flag set, store the message for further subscriptions
             self.logger.debug("Retaining message on topic %s" % topic_name)
@@ -794,7 +800,7 @@ class Broker:
         except KeyError:
             return 0x80
 
-    def _del_subscription(self, a_filter, session):
+    def _del_subscription(self, a_filter: str, session: Session) -> int:
         """
         Delete a session subscription on a given topic
         :param a_filter:
@@ -819,7 +825,7 @@ class Broker:
         finally:
             return deleted
 
-    def _del_all_subscriptions(self, session):
+    def _del_all_subscriptions(self, session: Session) -> None:
         """
         Delete all topic subscriptions for a given session
         :param session:
@@ -980,7 +986,7 @@ class Broker:
             % (subscription[0], format_client_message(session=session))
         )
 
-    def delete_session(self, client_id):
+    def delete_session(self, client_id: str) -> None:
         """
         Delete an existing session data, for example due to clean session set in CONNECT
         :param client_id: