tornado_chat_example_diagram

                    +-----------+                           +-----------+
                    |           |                           |           |
                    |  client   |                           |  server   |
                    +-----------+                           +-----------+                         +-------------------------+
                         +                                        +                               |   global_message_buffer |
                         |                                        |+----------------------------->|                         |
                         | POST button is clicked                 |                               +-----------+-------------+
                         |  uri: a/message/new                    |                                           +
                         |  POST, param={'body': 'text444'}       |                                           |
                       +--+                                     +--+ MessageNewHandler                        |
                       |  |+----------------------------------->|  | create message with id / body            |
                       |  |                                     |  |                                          |
                       |  |                                     |  |                                          |
                       |  |                                     |  |                                          |
                       |  |  Message id / body / html-form-body |  |                                          |
                       |  |<------------------------------------|  |                                          |
                       |  |                                     |  | new_message([message])                 +--+
                       |  |                                     |  |--------------------------------------->|  |
                       |  |                                     |  |                                        |  |
                       |  |                                     +-++                                        |  |+---------------------------------+
   updater.newMessage  |  |                                       |                                         |  ||   [loop: self.waiters]          |
    - show all message |  |                                       |                                         |  ||   future.set_result(message )   |
                       |  |                                       |                                         |  ||                                 |
                       |  |                                       |                                         |  ||                                 |
                       |  |    in chat.js/updater.poll            |                                         |  |+---------------------------------+
                       |  |    uri: a/message/updates             |                                         |  |
       updater.poll    |  |    POST,param={cursor:last_msg_id}/   |                                         |  |init-self.waiters()
                       |  |    keep pending                       |                                         |  |message --> cache
                       |  |-------------------------------------> |                                         |  |trim when the cache is over capacity
                       |  |                                       |                                         +--+
                       |  |                                       |                                           |
                       |  |                                       | wait_for_messages(cursor=cursor)        +--+
                       |  |                                       |+--------------------------------------->|  |
                       |  |                                       |                                         |  | result_future = Future()
                       |  |                                       |                                         |  | if message id exist
                       |  |                                       |                                         |  |  not-delivered-messages <--- cache
                       |  |                                       |                                         |  |  result_future.set_result(not-delivered-messages)
                       |  |                                       |                                         |  |
                       |  |                                       |       result_future                     |  | self.waiters.add(result_future)
                       |  |    messages =[{'body':'dddd',         |<----------------------------------------|  |
                       |  |                'id': '342ds',...},...]|                                         +--+
                       |  |<-------------------------------------+|                                           |
                       |  |                                       |  yield future                             |
                       +--+                                       |                                           |
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