diff --git a/solutions/object_oriented_design/online_chat/online_chat.ipynb b/solutions/object_oriented_design/online_chat/online_chat.ipynb
index b9f84ef4..b3a243a6 100644
--- a/solutions/object_oriented_design/online_chat/online_chat.ipynb
+++ b/solutions/object_oriented_design/online_chat/online_chat.ipynb
@@ -4,7 +4,9 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "This notebook was prepared by [Donne Martin](https://github.com/donnemartin). Source and license info is on [GitHub](https://github.com/donnemartin/system-design-primer)."
+    "This notebook was prepared by [Donne Martin](https://github.com/donnemartin). Source and license info is on [GitHub](https://github.com/donnemartin/system-design-primer).\n",
+    "\n",
+    "This notebook was enhanced by [Parvez M Robin](https://github.com/parvezmrobin)."
    ]
   },
   {
@@ -36,7 +38,7 @@
     "        * Post a message to a group chat\n",
     "    * Private 1-1 chat\n",
     "        * Invite a friend to a private chat\n",
-    "        * Post a meesage to a private chat\n",
+    "        * Post a message to a private chat\n",
     "* No need to worry about scaling initially"
    ]
   },
@@ -65,6 +67,7 @@
    "source": [
     "%%writefile online_chat.py\n",
     "from abc import ABCMeta\n",
+    "from enum import Enum\n",
     "\n",
     "\n",
     "class UserService(object):\n",
@@ -72,11 +75,20 @@
     "    def __init__(self):\n",
     "        self.users_by_id = {}  # key: user id, value: User\n",
     "\n",
-    "    def add_user(self, user_id, name, pass_hash):  # ...\n",
-    "    def remove_user(self, user_id):  # ...\n",
-    "    def add_friend_request(self, from_user_id, to_user_id):  # ...\n",
-    "    def approve_friend_request(self, from_user_id, to_user_id):  # ...\n",
-    "    def reject_friend_request(self, from_user_id, to_user_id):  # ...\n",
+    "    def add_user(self, user_id, name, pass_hash):\n",
+    "        pass\n",
+    "\n",
+    "    def remove_user(self, user_id):\n",
+    "        pass\n",
+    "\n",
+    "    def add_friend_request(self, from_user_id, to_user_id):\n",
+    "        pass\n",
+    "\n",
+    "    def approve_friend_request(self, from_user_id, to_user_id):\n",
+    "        pass\n",
+    "\n",
+    "    def reject_friend_request(self, from_user_id, to_user_id):\n",
+    "        pass\n",
     "\n",
     "\n",
     "class User(object):\n",
@@ -91,34 +103,62 @@
     "        self.received_friend_requests_by_friend_id = {}  # key: friend id, value: AddRequest\n",
     "        self.sent_friend_requests_by_friend_id = {}  # key: friend id, value: AddRequest\n",
     "\n",
-    "    def message_user(self, friend_id, message):  # ...\n",
-    "    def message_group(self, group_id, message):  # ...\n",
-    "    def send_friend_request(self, friend_id):  # ...\n",
-    "    def receive_friend_request(self, friend_id):  # ...\n",
-    "    def approve_friend_request(self, friend_id):  # ...\n",
-    "    def reject_friend_request(self, friend_id):  # ...\n",
+    "    def message_user(self, friend_id, message):\n",
+    "        pass\n",
+    "\n",
+    "    def message_group(self, group_id, message):\n",
+    "        pass\n",
+    "\n",
+    "    def send_friend_request(self, friend_id):\n",
+    "        pass\n",
+    "\n",
+    "    def check_pending_friend_requests(self):\n",
+    "        pass\n",
+    "\n",
+    "    def approve_friend_request(self, friend_id):\n",
+    "        pass\n",
+    "\n",
+    "    def reject_friend_request(self, friend_id):\n",
+    "        pass\n",
+    "\n",
+    "    def remove_friend(self, friend_id):\n",
+    "        pass\n",
     "\n",
     "\n",
     "class Chat(metaclass=ABCMeta):\n",
     "\n",
-    "    def __init__(self, chat_id):\n",
+    "    def __init__(self, chat_id, max_member):\n",
     "        self.chat_id = chat_id\n",
     "        self.users = []\n",
     "        self.messages = []\n",
+    "        self.max_member = max_member\n",
+    "\n",
+    "    def add_user(self, user):\n",
+    "        if len(self.users) == self.max_member:\n",
+    "            raise OverflowError('Chat {} already has maximum number of users'.format(self.chat_id))\n",
+    "        self.users.append(user)\n",
+    "\n",
+    "    def remove_user(self, user):\n",
+    "        self.users.remove(user)\n",
     "\n",
     "\n",
     "class PrivateChat(Chat):\n",
     "\n",
-    "    def __init__(self, first_user, second_user):\n",
-    "        super(PrivateChat, self).__init__()\n",
-    "        self.users.append(first_user)\n",
-    "        self.users.append(second_user)\n",
+    "    def __init__(self, chat_id, first_user, second_user):\n",
+    "        super(PrivateChat, self).__init__(chat_id, max_member=2)\n",
+    "        self.add_user(first_user)\n",
+    "        self.add_user(second_user)\n",
+    "\n",
+    "    def add_user(self, user):\n",
+    "        if len(self.users) == self.max_member:\n",
+    "            raise OverflowError(\n",
+    "                'PrivateChat {} cannot have more than {} members'.format(self.chat_id, self.max_member)\n",
+    "            )\n",
+    "        super(PrivateChat, self).add_user(user)\n",
     "\n",
     "\n",
     "class GroupChat(Chat):\n",
-    "\n",
-    "    def add_user(self, user):  # ...\n",
-    "    def remove_user(self, user):  # ... \n",
+    "    pass\n",
     "\n",
     "\n",
     "class Message(object):\n",
@@ -139,7 +179,6 @@
     "\n",
     "\n",
     "class RequestStatus(Enum):\n",
-    "\n",
     "    UNREAD = 0\n",
     "    READ = 1\n",
     "    ACCEPTED = 2\n",
diff --git a/solutions/object_oriented_design/online_chat/online_chat.py b/solutions/object_oriented_design/online_chat/online_chat.py
index 7063ca04..49dc288b 100644
--- a/solutions/object_oriented_design/online_chat/online_chat.py
+++ b/solutions/object_oriented_design/online_chat/online_chat.py
@@ -44,7 +44,7 @@ class User(object):
     def send_friend_request(self, friend_id):
         pass
 
-    def receive_friend_request(self, friend_id):
+    def check_pending_friend_requests(self):
         pass
 
     def approve_friend_request(self, friend_id):
@@ -53,30 +53,44 @@ class User(object):
     def reject_friend_request(self, friend_id):
         pass
 
+    def remove_friend(self, friend_id):
+        pass
+
 
 class Chat(metaclass=ABCMeta):
 
-    def __init__(self, chat_id):
+    def __init__(self, chat_id, max_member):
         self.chat_id = chat_id
         self.users = []
         self.messages = []
+        self.max_member = max_member
+
+    def add_user(self, user):
+        if len(self.users) == self.max_member:
+            raise OverflowError('Chat {} already has maximum number of users'.format(self.chat_id))
+        self.users.append(user)
+
+    def remove_user(self, user):
+        self.users.remove(user)
 
 
 class PrivateChat(Chat):
 
-    def __init__(self, first_user, second_user):
-        super(PrivateChat, self).__init__()
-        self.users.append(first_user)
-        self.users.append(second_user)
+    def __init__(self, chat_id, first_user, second_user):
+        super(PrivateChat, self).__init__(chat_id, max_member=2)
+        self.add_user(first_user)
+        self.add_user(second_user)
+
+    def add_user(self, user):
+        if len(self.users) == self.max_member:
+            raise OverflowError(
+                'PrivateChat {} cannot have more than {} members'.format(self.chat_id, self.max_member)
+            )
+        super(PrivateChat, self).add_user(user)
 
 
 class GroupChat(Chat):
-
-    def add_user(self, user):
-        pass
-
-    def remove_user(self, user):
-        pass
+    pass
 
 
 class Message(object):
@@ -97,7 +111,6 @@ class AddRequest(object):
 
 
 class RequestStatus(Enum):
-
     UNREAD = 0
     READ = 1
     ACCEPTED = 2