Hi,
this seems like a pretty useful, yet reasonable easily implementable
feature.
I'd add 5th value "NORMAL", which would be the default value.
const unsigned short CRITICAL = 0;
const unsigned short HIGH = 1;
const unsigned short NORMAL = 2
const unsigned short LOW = 3;
const unsigned short LOWEST = 4;
Not sure if we need all the values, or would
HIGH, NORMAL, LOW be enough?
-Olli
On 4/13/10 7:13 PM, Mike Belshe wrote:
Hi,
I'm a developer on the chrome team, and also working on SPDY.
Others here at Google have requested that we expose some of the
priority-based resource loading mechanics to applications so that
applications can hint to the browser more information about which
resources are critical and which are not. Some of the Google Apps teams
have already implemented their own, manual priority-based resource
fetchers, and our maps team saw a huge latency reduction as a result of
doing so. Internally to chromium and webkit, resource loading is also
priority-aware today. Finally, in SPDY, we've observed good
improvements by exposing priorities all the way across the protocol. We
believe exposing priority on the XHR object may benefit many
applications manage their resource loads.
Here is a quick writeup of one proposal which we think would work in
browsers. We believe it is backward compatible with existing XHR, and
can be optionally implemented. It also leaves a fair amount of the
tuning at the discretion of the browser, so it does not create a
long-term liability in the browser. We hope that these considerations
make it an easy choice to approve.
I'm wondering if the XMLHttpRequest group would be interested in taking
this on?
Thanks,
Mike
XMLHttpRequest Priority Fetching
Every performant web browser implementation today implements various
heuristics for resource loading prioritization internally. The notion
is simple, that loading some resources, such as images, are less
performance critical than loading other resources, such as external
style sheets. By implementing basic priorities, browsers achieve
substantially better performance loading web pages. Today, however, web
applications have no way of giving hints to the browser about what may
be high or low priority.
Because complex applications heavily rely on resource loading by way of
XmlHttpRequest, we propose a simple, backward compatible, and optional
mechanism whereby application developers can hint to a browser how to
load a XmlHttpRequest.
Proposed API:
interface XMLHttpRequest {
// XMLHttpRequest Priorities.
const unsigned short CRITICAL = 0;
const unsigned short HIGH = 1;
const unsigned short LOW = 2;
const unsigned short LOWEST = 3;
// Set the load priority for this request.
void setPriority(unsigned short priority);
}
Example Usage:
var client = new XMLHttprequest;
client.setPriority(HIGH);
client.open(’GET’, ‘demo.cgi’);
client.send();
Description:
When a new XMLHttpRequest object is created, it contains a notion of
priority. Browsers which schedule resource fetches may optionally use
this priority to determine in which order resources are fetched.
4 priorities are provided. By keeping the number of different
priorities small, we keep browser and XMLHttpRequest priority
implementations simple.
By default, all XMLHttpRequest objects have a priority ‘LOW’.
Applications may alter the priority by calling the setPriority() method
on the XMLHttpRequest object. The priority set on the object at the
time the applicaiton calls the XMLHttpRequest.send() method determines
the priority the browser should use when fetching this resource.
Calling setPriority() after the send() method will have no effect on
the priority of the resource load.
Browsers are not required to support the priority requested by
applications, and may ignore it altogether. However, browsers are
encouraged to support the requested priority order. The following is a
description of one possible prioritization policy:
CRITICAL resources are loaded first. When CRITICAL resources are in
progress, requests for HIGH/MEDIUM/LOW resources are deferred until all
CRITICAL resources have finished.
HIGH/MEDIUM/LOW resources are loaded in that order. When no CRITICAL
resources are in progress, HIGH/MEDIUM/LOW resources will be loaded with
HIGH priority first. The browser does not need to wait until higher
priority resources have finished fetching before it starts a request for
a lower priority resource, although it may chose to do so.
Existing Implementations:
Google is currently using resource prioritization techniques in its
Google Maps application, internally to the Google Chrome browser, and
also as a part of the SPDY protocol.
