[whatwg] Global Script proposal.

[Mike Wilson]

This is an interesting suggestion as it isolates the 
stateful parts from the rest of the previous suggestions. 
I like state.

Here's how I see how it fits inside the big picture:
 
Scope              Serialized state     Live state
-----              ----------------     ----------
user agent         WS_localStorage,     GlobalScript [2]
                   SharedWorker [1]
                   cookie

browsing context   WS_sessionStorage    - [3]
                   window.name

document           -                    plain JS objs [4]

history entry      History.pushState    plain JS objs [4]
 
[1] Global state can be coordinated by a SharedWorker but
it would need to be serialized in postMessage on transfer
so that's why I've put it in the "serialized" column.

[2] As I understand it, the new GlobalScript construct is
a context that can be shared by all browsing contexts in
the user agent.

[3] You also mention that the feature could be usable for
page-to-page navigation within the same browsing context.
It hasn't been suggested yet, but it would be possible to 
have a variation of GlobalScript that binds to a specific
browsing context, analogous to sessionStorage.

[4] These plain JavaScript objects indeed live throughout
their Document's life, but this lifetime is usually 
shorter than what the user's perception tells him. Ie, 
when the user returns to a previous page through the Back 
button he regards that as the "same" document, while 
technically it's usually a "new" Document, with a freshly 
created document tree and JavaScript context.
 
Questions
---------

Threading:
This is the unavoidable question ;-) How do you envision
multiple threads accessing this shared context to be 
coordinated?

Process boundaries:
In this past discussion there have been several mentions
about having to cluster "pages" inside the same process 
if they are to share data.
Why is this so, and why can't shared memory or proxied
objects be an option for browsers doing process 
separation?

Best regards
Mike Wilson


________________________________

From: whatwg-bounces at lists.whatwg.org
[mailto:whatwg-bounces at lists.whatwg.org] On Behalf Of Dmitry Titov
Sent: den 17 augusti 2009 23:38
To: whatwg at whatwg.org
Subject: [whatwg] Global Script proposal.
	
	
	Dear whatwg,

	The previous discussion about shared page and persistence has sent
us back 'to the drawing board', to think again what is the essence of the
feature and what's not important. Talking with web apps developers indicates
the most of benefits can be achieved without dangerous background
persistence or the difficulty to specify visual aspects of the invisible
page.
	 
	Here is the new proposal. Your feedback is very appreciated. We are
thinking about feasibility of doing experimental implementation in
WebKit/Chrome. Thanks!

	-----

	SUMMARY

	Currently there is no mechanism to directly share DOM, code and data
on the same ui thread across several pages of the web application.
Multi-page applications and the sites that navigate from page to page would
benefit from having access to a shared "global script context" (naming?)
with direct synchronous script access and ability to manipulate DOM. This
would compliment "Shared Workers"
(http://www.whatwg.org/specs/web-workers/current-work/) by providing a
shared script-based context which does not run on a separate thread and can
be used directly from the application's pages.

	USE CASES

	Chat application opens separate window for each conversation. Any
opened window may be closed and user expectation is that remaining windows
continue to work fine. Loading essentially whole chat application and
maintaining data structures (roster) in each window takes a lot of resources
and cpu.

	Finance site could open multiple windows to show information about
particular stocks. At the same time, each page often includes data-bound UI
components reflecting real-time market data, breaking news etc. It is very
natural to have a shared context which can be directly accessed by UI on
those pages, so only one set of info is maintained.

	A game may open multiple windows sharing the same model to provide
different views at the game objects (as in flight simulator).

	In an email application, a user may want to open a separate
"compose" window for a new email, often after she started to "answer in
place" but realized she'd like to look up something else in the mailbox for
the answer. This could be an instantaneous operation if the whole html tree
and the compose editor script were shared.

	Such multiple-window use cases could be simpler and use much less
resources if they had access to a shared Global Script Context so there is
no need to re-initialize and maintain the same state in all the pages.
Having direct, same-thread DOM/JS access to this context makes it possible
to avoid loading and initialization of repetitive code and data, makes
separate 'UI windows' simpler and independent.

	Another case is an application that uses navigation from page to
page using menu or some site navigation mechanism. Global Script Context
could keep the application state so it doesn't have to be round-tripped via
server in a cookie or URL. For example, wizard-like file upload web
application could be implemented as a simple sequence of static pages
connecting to the local Global Script. It also makes browser's history
feature 'just work' so there is no need for complicated history managers
like this: http://developer.yahoo.com/yui/history/. Note that Global Script
should be able to live through a page-to-page navigation for this to work
well.

	Yet another use case is provided by JS frameworks like SproutCore
(http://wiki.sproutcore.com/Basics-Introducing+SproutCore+MVC) which try to
bring to the Web the traditional Model-View-Controller model which is based
around having a single data model which can be bound to various 'views'. The
binding usually happens via Controller that keeps specific mapping between
the application's data structure and the structures needed to support UI
views. Controller is also the one responding to UI events and figuring out
what change should be applied to the model. This means that controller is
best shared across 'UI pages' and run on the same thread. The Model part
could be implemented in a Global Script or as a Shared Worker though.

	WORKAROUNDS

	HTML5 provides several mechanisms that can be used to workaround the
absence of Global Script. The Application Cache may be used to avoid server
roundtrips for megabyte-sized JS libraries. Local Storage may help to pass
data from one page to another. Shared Workers seem like a great place to put
a shared server connection for a chat application. However, even loading JS
library from the local cache takes time, which makes sub-100ms typical UI
response times difficult to achieve sometimes and increases memory
footprint. Storing transient application data in local storage requires
serialization of it and is difficult for some types of data (like a current
selection in a document). Shared Workers are actually separate threads and
it's impossible to directly call JS in them or make them operate on DOM or
receive input events.

	As a workaround, today many sites try to maintain a single "main
page" and avoid navigating from it. This page provides a shared context,
while 'UI panels' are built into that page as iframes or generated
dynamically. Many applications use dynamic content creation to simulate
'page navigation' (as in Facebook). To support back-forward history in
browser they use fragment URLs which requires more client-side code, extra
network request (for example, when navigating to a bookmark) and are not
reliable (may loose history on refresh for example). Because of the
additional code required to 'wrap' almost every regular browser feature like
refresh or a click on a link, these solutions tend to be buggy. Hotmail.com
viewed in Safari is a vivid example of it.

	PROPOSAL

	A web page will be able to create a Global Script and connect to it,
as in this example:

	var context = new GlobalScript();  // perhaps 'webkitGlobalScript'
as experimental feature?
	context.onload = function () {...}
	context.onerror = function () {...}
	context.load('foo.js');

	All pages connected to the same Global Script should run on the same
thread, in the same process.  Since this is not always technically possible,
it should be legal (and not break the applications) for there to be
duplicate global script contexts within a UA.  For example, in a
multi-process browser,  two pages cannot share a context if they're loaded
in separate processes.  That said, there are many heuristics that UAs could
use to alleviate this problem.  For example, if one page uses a global
script, subsequent pages from the same origin could be loaded in that same
process. It is possible to structure the web application in a way to take
advantage of the shared Global Script.

	The Global Script is terminated soon after last page that is
connected to it closes (just like Shared Workers). A UA should use
navigation's target url to keep Global Script alive across navigations from
page to page of the same application.

	The return value from a constructor is the Global Script's "global
scope object". It can be used to directly access functions and variables
defined in global scope of the Global Script. While this global scope does
not have 'window' or 'document' and does not have visual page associated
with it, the local storage, database, timers and XHR are exposed to it, and
it can build up DOM for the connected pages using their 'document' object.
The list of interfaces exposed in the global scope of the Global Script is
similar to that of Shared Worker, except message-passing interface. It could
also include events fired when a page connects/disconnects to it and before
it is terminated.

	For security reasons, the Global Script falls under the limits of
the same origin policy. 

	Using Global Script is better for certain tasks then using Shared
Worker since it is not necessary to serialize or deal with concurrency, and
it can access DOM directly.