========
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*

    All that's needed is an implementation of ``ILongPollEvent``;
    subclassing ``LongPollEvent`` is not *required*. However, it has a
    sensible implementation of ``emit`` that is either usable as-is,
    or worth reusing via calling-up from subclasses. -- *Gavin*

.. _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*

    See `ILongPollEvent`_ ;) It is the key with which events will be
    emitted. It should be predictable and stable. This is the key to
    which queues can be subscribed. An example could be
    ``longpoll.event.branchmergeproposal.1234``. -- *Gavin*

.. _ILongPollEvent: http://bazaar.launchpad.net/~launchpad-pqm/launchpad/devel/view/head:/lib/lp/services/longpoll/interfaces.py

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*

    See `LongPollStormEvent`_ -- *Gavin*

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

zope.event bridge for Storm life-cycle events
---------------------------------------------

As a convenience, subscription handlers for `life-cycle events`_
(IObjectCreatedEvent, IObjectDeletedEvent, IObjectModifiedEvent) of
Storm model objects already exist. They generate long-poll events
named "longpoll.event.{table_name}.{object_primary_key}" with the type
of event ("created", "deleted" or "modified") in the event payload.

.. _life-cycle events: http://bazaar.launchpad.net/~lazr-developers/lazr.lifecycle/trunk/view/head:/src/lazr/lifecycle/interfaces.py

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
=====================

``ILongPollSubscriber`` describes a subscriber to long-poll events. It
defines a ``subscribe_key`` property:

  "The key which the subscriber must know in order to be able to
  long-poll for subscribed events. Should be infeasible to guess, a
  UUID for example."

It also defines a ``subscribe`` method that can be passed an object
that provides ``ILongPollEvent``. In the back-end this ends up
creating a queue named ``subscribe_key`` which is bound to event's
``event_key`` (not exclusively; it can be bound to multiple events).

As the time of this writing only a request (as in
``IApplicationRequest``) can be used as a subscriber using
LongPollApplicationRequestSubscriber_

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

    I think I've clarified that above. -- *Gavin*

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

This adapter ensures that the request's JSON cache is stuffed with the
necessary information for the JavaScript component of LongPoll can
operate.

A request can be subscribed to a events quite simply::

 ILongPollSubscriber(request).subscribe(FakeEvent())

However, there's an even easier way, using
``lp.app.longpoll.subscribe``:

  subscribe(FakeEvent())

It discovers the current request automagically so that's all there is
to it. Obviously something in the page needs to know what to do with
the events that will now arrive, but the back-end plumbing is done
now.

After the first event in a request has been subscribed, the JSON cache
will contain a ``longpoll.key`` value (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");