diff --git a/zerver/lib/socket.py b/zerver/lib/socket.py
index 7edf8f9b41..c9fe049619 100644
--- a/zerver/lib/socket.py
+++ b/zerver/lib/socket.py
@@ -1,5 +1,5 @@
 from __future__ import absolute_import
-from typing import Any
+from typing import Any, Union, Optional
 
 from django.conf import settings
 from django.utils.importlib import import_module
@@ -7,6 +7,7 @@ from django.utils import timezone
 from django.contrib.sessions.models import Session as djSession
 
 import sockjs.tornado
+from sockjs.tornado.session import ConnectionInfo
 import tornado.ioloop
 import ujson
 import logging
@@ -27,6 +28,7 @@ logger = logging.getLogger('zulip.socket')
 
 djsession_engine = import_module(settings.SESSION_ENGINE)
 def get_user_profile(session_id):
+    # type: (str) -> Optional[UserProfile]
     if session_id is None:
         return None
 
@@ -41,12 +43,14 @@ def get_user_profile(session_id):
     except (UserProfile.DoesNotExist, KeyError):
         return None
 
-connections = dict() # type: Dict[int, SocketConnection]
+connections = dict() # type: Dict[Union[int, str], SocketConnection]
 
 def get_connection(id):
+    # type: (Union[int, str]) -> SocketConnection
     return connections.get(id)
 
 def register_connection(id, conn):
+    # type: (Union[int, str], SocketConnection) -> None
     # Kill any old connections if they exist
     if id in connections:
         connections[id].close()
@@ -55,26 +59,31 @@ def register_connection(id, conn):
     connections[conn.client_id] = conn
 
 def deregister_connection(conn):
+    # type: (SocketConnection) -> None
     del connections[conn.client_id]
 
 redis_client = get_redis_client()
 
 def req_redis_key(req_id):
+    # type: (str) -> str
     return 'socket_req_status:%s' % (req_id,)
 
 class SocketAuthError(Exception):
     def __init__(self, msg):
+        # type: (str) -> None
         self.msg = msg
 
 class CloseErrorInfo(object):
     def __init__(self, status_code, err_msg):
+        # type: (int, str) -> None
         self.status_code = status_code
         self.err_msg = err_msg
 
 class SocketConnection(sockjs.tornado.SockJSConnection):
-    client_id = None # type: str
+    client_id = None # type: Union[int, str]
 
     def on_open(self, info):
+        # type: (ConnectionInfo) -> None
         log_data = dict(extra='[transport=%s]' % (self.session.transport_name,))
         record_request_start_data(log_data)
 
@@ -97,6 +106,7 @@ class SocketConnection(sockjs.tornado.SockJSConnection):
             return
 
         def auth_timeout():
+            # type: () -> None
             self.close_info = CloseErrorInfo(408, "Timeout while waiting for authentication")
             self.close()
 
@@ -105,6 +115,7 @@ class SocketConnection(sockjs.tornado.SockJSConnection):
                        remote_ip=info.ip, email='unknown', client_name='?')
 
     def authenticate_client(self, msg):
+        # type: (Dict[str, Any]) -> None
         if self.authenticated:
             self.session.send_message({'req_id': msg['req_id'], 'type': 'response',
                                        'response': {'result': 'error', 'msg': 'Already authenticated'}})
@@ -151,10 +162,11 @@ class SocketConnection(sockjs.tornado.SockJSConnection):
         ioloop = tornado.ioloop.IOLoop.instance()
         ioloop.remove_timeout(self.timeout_handle)
 
-    def on_message(self, msg):
+    def on_message(self, msg_raw):
+        # type: (str) -> None
         log_data = dict(extra='[transport=%s' % (self.session.transport_name,))
         record_request_start_data(log_data)
-        msg = ujson.loads(msg)
+        msg = ujson.loads(msg_raw)
 
         if self.did_close:
             logger.info("Received message on already closed socket! transport=%s user=%s client_id=%s"
@@ -211,6 +223,7 @@ class SocketConnection(sockjs.tornado.SockJSConnection):
                            fake_message_sender)
 
     def on_close(self):
+        # type: () -> None
         log_data = dict(extra='[transport=%s]' % (self.session.transport_name,))
         record_request_start_data(log_data)
         if self.close_info is not None:
@@ -229,6 +242,7 @@ class SocketConnection(sockjs.tornado.SockJSConnection):
         self.did_close = True
 
 def fake_message_sender(event):
+    # type: (Dict[str, Any]) -> None
     log_data = dict() # type: Dict[str, Any]
     record_request_start_data(log_data)
 
@@ -254,6 +268,7 @@ def fake_message_sender(event):
     respond_send_message(result)
 
 def respond_send_message(data):
+    # type: (Dict[str, Any]) -> None
     log_data = data['server_meta']['log_data']
     record_request_restart_data(log_data)
 
@@ -286,4 +301,5 @@ sockjs_router = sockjs.tornado.SockJSRouter(SocketConnection, "/sockjs",
                                             {'sockjs_url': 'https://%s/static/third/sockjs/sockjs-0.3.4.js' % (settings.EXTERNAL_HOST,),
                                              'disabled_transports': ['eventsource', 'htmlfile']})
 def get_sockjs_router():
+    # type: () -> sockjs.tornado.SockJSRouter
     return sockjs_router