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Revision 1 as of 2011-09-26 09:09:15
Size: 8991
Editor: rvb
Comment: Refactor LongPoll documentation on another page.
Revision 21 as of 2011-10-04 15:23:05
Size: 9906
Editor: allenap
Comment: Move the JavaScript stuff off to its own page.
Deletions are marked like this. Additions are marked like this.
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.. contents::
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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)
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 will now return (will the JSONified payload from the LongPollEvent).
6) This will be handled by the javascript longpoll library which will trigger a javascript event with the payload.
7) The javascript longpoll library will now reconnect to be able to handle other events.
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.

LongPoll's usage
================

- `Merge proposal's diff update`_: merge proposal's diffs are updated via XHR as soon as new version is available (no more page reloads!)

.. _`Merge proposal's diff update`: /MPDiffUpdate


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
Line 40: Line 69:
================

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.
===============

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.
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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. 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.
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========================

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.

LongPoll events
===============

LongPollEvent
-------------

`LongPollEvent`_ is the abstract adapter base class that should be used to create a custom event adapters.
=======================

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 ``txlongpoll`` requires and is used to better support load-balancing the async frontend.

----

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

  **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
Line 75: Line 136:
In both cases the adapter should be registered in a `configure.zcml` somewhere sensible:: In both cases the adapter should be registered in a `configure.zcml`
somewhere sensible::
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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 payload.

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

  **EXAMPLE NEEDED HERE** -- *Julian*

    See `LongPollStormEvent`_ -- *Gavin*

.. _LongPollStormEvent: http://ba
zaar.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`_
(I
ObjectCreatedEvent, 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::
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Will result in this event being fired:: ... will result in an event being fired with the following data::
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To test that your code triggers the appropriate long poll events, use the class `LongPollEventRecord`_ and the context manager `capture_longpoll_emissions`_:: To test that your code triggers the appropriate long poll events, use
the class `LongPollEventRecord`_ and the context manager
`capture_longpoll_emissions`_::
Line 112: Line 191:
 self.assertEqual(expected, log)  self.assertEqual([expected], log)
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----
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======================

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

``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."

An example could be
``longpoll.subscribe.a966e051-8817-4897-840f-9de18404c8f0``.

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*
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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.

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

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

- event_key: fired each time RabbitMQ passed an event created using the LongPollEvent adapter.

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. These events can be used to make the end user aware of the connection problem.

- '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");
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())

Or, when ``some_object`` can be adapted to ``ILongPollEvent``::

  subscribe(some_object)

For example, if ``some_object`` is a Storm model object (i.e. inherits
from Storm, or indirectly via SQLBase) then this single call arranges
the following:

1. Declares a new subscribe queue with an unguessable name (see
   ``subscribe_key``).

2. Binds the subscribe queue to the ``event_key`` defined by the
   ``ILongPollEvent`` adapter of ``some_object``.

3. Puts the ``subscribe_key`` in the request's JSON cache as
   ``longpoll.key``, sets ``longpoll.uri`` so that the in-page
   JavaScript knows where to connect, and adds the ``subscribe_key``
   to the ``longpoll.subscriptions`` list.

That's the back-end plumbing, now it's up to the in-page JavaScript to
know what to do with the events that it'll be able to long-poll for...

Next: `/JavaScript`_
Rendering of reStructured text is not possible, please install Docutils.

========
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.

LongPoll's usage
================

- `Merge proposal's diff update`_: merge proposal's diffs are updated via XHR as soon as new version is available (no more page reloads!)

.. _`Merge proposal's diff update`: /MPDiffUpdate 


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 ``txlongpoll`` requires and is used to better support load-balancing the async frontend.

----

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."

An example could be
``longpoll.subscribe.a966e051-8817-4897-840f-9de18404c8f0``.

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())

Or, when ``some_object`` can be adapted to ``ILongPollEvent``::

  subscribe(some_object)

For example, if ``some_object`` is a Storm model object (i.e. inherits
from Storm, or indirectly via SQLBase) then this single call arranges
the following:

1. Declares a new subscribe queue with an unguessable name (see
   ``subscribe_key``).

2. Binds the subscribe queue to the ``event_key`` defined by the
   ``ILongPollEvent`` adapter of ``some_object``.

3. Puts the ``subscribe_key`` in the request's JSON cache as
   ``longpoll.key``, sets ``longpoll.uri`` so that the in-page
   JavaScript knows where to connect, and adds the ``subscribe_key``
   to the ``longpoll.subscriptions`` list.

That's the back-end plumbing, now it's up to the in-page JavaScript to
know what to do with the events that it'll be able to long-poll for...

Next: `/JavaScript`_

LongPoll (last edited 2011-10-04 20:06:01 by allenap)