I am trying to create a Tornado application with several chats. The chats should be based on HTML5 websocket. The Websockets communicate nicely, but I always run into the problem that each message is posted twice.
The application uses four classes to handle the chat:
Chat
contains all written messages so far and a list with all waiters
which should be notified
ChatPool
serves as a lookup for new Websockets - it creates a new chat when there is no one with the required scratch_id
or returns an existing chat instance.
ScratchHandler
is the entry point for all HTTP requests - it parses the base template and returns all details of client side.
ScratchWebSocket
queries the database for user information, sets up the connection and notifies the chat instance if a new message has to be spread.
How can I prevent that the messages are posted several times? How can I build a multi chat application with tornado?
import uuid
import tornado.websocket
import tornado.web
import tornado.template
from site import models
from site.handler import auth_handler
class ChatPool(object):
# contains all chats
chats = {}
@classmethod
def get_or_create(cls, scratch_id):
if scratch_id in cls.chats:
return cls.chats[scratch_id]
else:
chat = Chat(scratch_id)
cls.chats[scratch_id] = chat
return chat
@classmethod
def remove_chat(cls, chat_id):
if chat_id not in cls.chats: return
del(cls.chats[chat_id])
class Chat(object):
def __init__(self, scratch_id):
self.scratch_id = scratch_id
self.messages = []
self.waiters = []
def add_websocket(self, websocket):
self.waiters.append(websocket)
def send_updates(self, messages, sending_websocket):
print "WAITERS", self.waiters
for waiter in self.waiters:
waiter.write_message(messages)
self.messages.append(messages)
class ScratchHandler(auth_handler.BaseHandler):
@tornado.web.authenticated
def get(self, scratch_id):
chat = ChatPool.get_or_create(scratch_id)
return self.render('scratch.html', messages=chat.messages,
scratch_id=scratch_id)
class ScratchWebSocket(tornado.websocket.WebSocketHandler):
def allow_draft76(self):
# for iOS 5.0 Safari
return True
def open(self, scratch_id):
self.scratch_id = scratch_id
scratch = models.Scratch.objects.get(scratch_id=scratch_id)
if not scratch:
self.set_status(404)
return
self.scratch_id = scratch.scratch_id
self.title = scratch.title
self.description = scratch.description
self.user = scratch.user
self.chat = ChatPool.get_or_create(scratch_id)
self.chat.add_websocket(self)
def on_close(self):
# this is buggy - only remove the websocket from the chat.
ChatPool.remove_chat(self.scratch_id)
def on_message(self, message):
print 'I got a message'
parsed = tornado.escape.json_decode(message)
chat = {
"id": str(uuid.uuid4()),
"body": parsed["body"],
"from": self.user,
}
chat["html"] = tornado.escape.to_basestring(self.render_string("chat-message.html", message=chat))
self.chat.send_updates(chat, self)
NOTE: After the feedback from @A. Jesse I changed the send_updates
method from Chat
. Unfortunately, it still returns double values.
class Chat(object):
def __init__(self, scratch_id):
self.scratch_id = scratch_id
self.messages = []
self.waiters = []
def add_websocket(self, websocket):
self.waiters.append(websocket)
def send_updates(self, messages, sending_websocket):
for waiter in self.waiters:
if waiter == sending_websocket:
continue
waiter.write_message(messages)
self.messages.append(messages)
2.EDIT: I compared my code with the example provided demos. In the websocket example a new message is spread to the waiters through the WebSocketHandler
subclass and a class method. In my code, it is done with a separated object:
From the demos:
class ChatSocketHandler(tornado.websocket.WebSocketHandler):
@classmethod
def send_updates(cls, chat):
logging.info("sending message to %d waiters", len(cls.waiters))
for waiter in cls.waiters:
try:
waiter.write_message(chat)
except:
logging.error("Error sending message", exc_info=True)
My application using an object and no subclass of WebSocketHandler
class Chat(object):
def send_updates(self, messages, sending_websocket):
for waiter in self.waiters:
if waiter == sending_websocket:
continue
waiter.write_message(messages)
self.messages.append(messages)
If you want to create a multi-chat application based on Tornado I recommend you use some kind of message queue to distribute new message. This way you will be able to launch multiple application process behind a load balancer like nginx. Otherwise you will be stuck to one process only and thus be severely limited in scaling.
I updated my old Tornado Chat Example to support multi-room chats as you asked for. Have a look at the repository:
This simple Tornado application uses Redis Pub/Sub feature and websockets to distribute chat messages to clients. It was very easy to extend the multi-room functionality by simply using the chat room ID as the Pub/Sub channel.