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========
LongPoll
========

.. contents::

Overview
========

Here is an overview of all the components involved in long polling::


                              +------------+  LP.cache.longpoll.{uri, key}
                              |     LP     |<-----------(1)-----------------+
                              +------------+                                |
                                    ^                                       v
 +-----------+                      |                           +--------------------------+
 |           |<---LongPollEvent-(3)-+                           |        Browser           |
 | RabbitMQ  |                                                  |--------------------------|
 |  Server   |                                                  |                          |
 |           |<---RabbitEvent---(4)-+                           | longpolljs  (JSEvent (6))|
 +-----------+                      |                           +--------------------------+                   
                                    v                                       ^
                              +------------+  /{uri}?uuid={key}&sequence=0  |
                              | txlongpoll |<------------(2)----------------+
                              +------------+           --(5)->
                                                       <-(7)--

The workflow for a typical usage of the LongPoll system of LP will go like this:

1) The LP web server serves a web page with LP.cache.longpoll.uri and LP.cache.longpoll.key populated. (If these are not populated, the LongPoll machinery will stop here)
2) The javascript longpoll library then uses these to initiate a long lasting XHR request to the twisted based LongPoll server.
3) (Some time later) The LP app server uses LongPollEvent.emit(payload) to send a message to the RabbitMQ server (if the server is available)
4) The event will then be published by RabbitMQ to the txlongpoll server.
5) The long lasting XHR request to the txlongpoll server returns (with the JSONified payload from the LongPollEvent).
6) This is handled by the javascript longpoll library which triggers a javascript event with the payload.
7) The javascript longpoll library now reconnects to be able to handle other events.

RabbitMQ
========

RabbitMQ_ is the messaging system used to connect events between discrete components.  In this case it connects long running jobs, that emit an event when finishing, to the "longpoll" server which informs the browser.

.. _RabbitMQ: RabbitMQ

LongPoll server
===============

See `lp:txlongpoll`_ for a project that describes a so-called long-polling server. It is Twisted-based and maps HTTP requests to consumption of messages on Rabbit queues.  It is a completely separate service to Launchpad.

.. _lp:txlongpoll: https://launchpad.net/txlongpoll

This means we can have Javascript in the browser doing XHR to the LongPoll server and block on long-running jobs until the job emits a message saying it's done. There is currently work in progress to make merge proposal diffs appear as soon as the job completes so that the end user doesn't have to refresh the MP page.

LongPoll request format
=======================

The LongPoll server awaits connections of the form '/uuid={queue_name}&sequence={sequence_id}'. The uuid parameter will be used to connect to a specific queue. The sequence parameter is a sequential integer that RabbitMQ requires to fetch successive events from the same queue.

  **How is this sequence driven, exactly? Why is it required?** -- *Julian*

  **It is required by ``txlongpoll`` and I think serves one main purpose: cache busting.** -- *Gavin*

----

Writing code on the server side
===============================

LongPollEvent class
-------------------

`LongPollEvent`_ is the abstract adapter base class that should be used to create custom event adapters.
   **why?** -- *Julian*

.. _LongPollEvent: http://bazaar.launchpad.net/~launchpad-pqm/launchpad/devel/view/head:/lib/lp/services/longpoll/adapters/event.py

Sub-classes need to define the `event_key` property and declare something along the lines of::

 class LongPollAwesomeThingEvent(LongPollEvent):
     adapts(IAwesomeThing)
     implements(ILongPollEvent)

**what is event_key for?** -- *Julian*

Alternatively, use the `long_poll_event` class decorator::

 @long_poll_event(IAwesomeThing)
 class LongPollAwesomeThingEvent(LongPollEvent):
     ...

In both cases the adapter should be registered in a `configure.zcml` somewhere sensible::

 <adapter factory=".adapters.LongPollAwesomeThingEvent" />

**EXAMPLE NEEDED HERE** -- *Julian*

zope.event bridge
-----------------

As a convenience, adapters for the storm events ObjectCreatedEvent, ObjectDeletedEvent and ObjectModifiedEvent already exist. They generate events named "longpoll.event.{table_name}.{object_primary_key}" with the type of event ("created", "deleted" or "modified") in the event payload.

For instance, if storm_object is an object from a table named FakeTable with a primary id of 1234, calling::

 notify(ObjectDeletedEvent(storm_object))

... will result in an event being fired with the following data::

 {"event_key": "longpoll.event.faketable.1234",
  "what": "deleted"})

Testing
-------

To test that your code triggers the appropriate long poll events, use the class `LongPollEventRecord`_ and the context manager `capture_longpoll_emissions`_::

 from lp.services.longpoll.testing import (
     capture_longpoll_emissions,
     LongPollEventRecord,
     )

 with capture_longpoll_emissions() as log:
     pass
     # Here something should trigger a long poll event.

 expected = LongPollEventRecord(
     "my-event-key", {
         "event_key": "my-event-key",
         "something": "somedata"})

 self.assertEqual(expected, log)

.. _`LongPollEventRecord`: http://bazaar.launchpad.net/~launchpad-pqm/launchpad/devel/view/head:/lib/lp/services/longpoll/testing.py

.. _`capture_longpoll_emissions`: http://bazaar.launchpad.net/~launchpad-pqm/launchpad/devel/view/head:/lib/lp/services/longpoll/testing.py

----

LongPoll subscription
=====================

As the time of this writing only a request can be used as a subscriber using LongPollApplicationRequestSubscriber_

    **I don't understand what a "request" and a "subscriber" is in this context** -- *Julian*

.. _LongPollApplicationRequestSubscriber: http://bazaar.launchpad.net/~launchpad-pqm/launchpad/devel/view/head:/lib/lp/services/longpoll/adapters/subscriber.py

Once a LongPollApplicationRequestSubscriber object is created, individual events can be subscribed to it::

 subscriber = LongPollApplicationRequestSubscriber(request)
 subscriber.subscribe(FakeEvent())

After the first event has been subscribed, LP.cache.longpoll.key will be populated with a key (identifying the RabbitMQ queue) specific to this request.

    **I don't understand how this stuff fits in and where it is used, can you explain with an example?** -- *Julian*

----

LongPoll Javascript library
===========================

The `long poll Javascript library`_ is responsible for connecting to the LongPoll server and firing Javascript events to reflect the state of this connection and make RabbitMQ events available to the Javascript layer.

.. _`long poll Javascript library`: http://bazaar.launchpad.net/~launchpad-pqm/launchpad/devel/view/head:/lib/lp/app/longpoll/javascript/longpoll.js

Connection
----------

The long poll library will issue a connection to the long poll server iff LP.cache.longpoll.uri and LP.cache.longpoll.key are populated in the JSON cache.
After each successful or failed connection, the long poll library reconnects to the long poll server to be able to handle other events.

Events
------

Two kind of events are fired by the long poll Javascript library. Events deriving from LongPollEvents and "system" events indicating the state of the long polling connection. Here is a list of all the possible events fired by the Javascript LongPoll library:

*event_key*
  fired each time RabbitMQ passes an event created using the LongPollEvent adapter (event_key being the name of the RabbitMQ event).

  For instance, this in LP python code::

   # subscription code omitted.
   class FakeEvent(LongPollEvent):

       implements(ILongPollEvent)

       @property
       def event_key(self):
          return "event-key-test"

   event = FakeEvent("source")
   event_data = {"hello": 1234}
   event.emit(**event_data)

  ... will result in an event being fired on the Javascript side::

   Y.fire("event-key-test", {"hello": 1234});

*'lp.app.longpoll.failure'*
  fired each time a long poll transaction fails. This might be because of connection problems
  or because the library failed to parse the event payload.

  To avoid hammering the server in case of an outage, the Long poll Javascript library waits
  for 1 second before reconnecting.  Also, after 5 failed connections, the library will wait
  3 minutes before trying to reconnect. The following events can be used to make the end user
  aware of the connection status.


*'lp.app.longpoll.longdelay'*
  fired each time the library switches in 'longdelay' mode (i.e. after 5 failed connections).


*'lp.app.longpoll.shortdelay'*
  fired each time the library switches back in 'shortdelay' mode (i.e. after at least 5
  failed connections and then after a successful connection).

Testing
-------

Since all the long poll Javascript library will be doing is firing Javascript events, testing a Javascript application that interacts with this library is pretty straightforward. Simply fire the events and make sure the application reacts appropriately::

 // Fire "event-key-test".
 Y.fire("event-key-test", {"hello": 1234});

 // Simulate a failure.
 Y.fire("lp.app.longpoll.failure");

 // Simulate the library going into 'longdelay' mode.
 Y.fire("lp.app.longpoll.longdelay");