[whatwg] HTML resource packages

Tue Aug 3 17:31:43 PDT 2010

We at Mozilla are hoping to ship HTML resource packages in Firefox 4,
and we wanted to get the WhatWG's feedback on the feature.

For the impatient, the spec is here:

    http://people.mozilla.org/~jlebar/respkg/

and the bug (complete with builds you can try and some preliminary
performance numbers) is here:

    https://bugzilla.mozilla.org/show_bug.cgi?id=529208

You can think of resource packages as image spriting 2.0.  A page
indicates in its <html> element that it uses one or more resource
packages (which are just zip files).  Then when that page requests a
resource (be it an image, a css file, a script, or whatever), the
browser first checks whether one of the packages contains the
requested resource.  If so, the browser uses the resource out of the
package instead of making a separate HTTP request for the resource.

There's of course more detail than that, of course.  Hopefully it's
(mostly) clear in the spec.

I envision two classes of users of resource packages.  I'll call the
first "resource-constrained developers".  These developers care about
how fast their page is (who doesn't?), but can't spend weeks speeding
up their page.  For these developers, resource packages are an easy
way to make their pages faster without going through the pain of
spriting their images and packaging their js/css.

The other class of users are the resource-unconstrained developers;
think Google or Facebook.  These developers have already put a huge
amount of effort into making their pages fast, and a naive application
of resource packages is unlikely to make them any faster.  But these
developers may be able to use resource packages cleverly to gain
speedups.  In particular, nobody (to my knowledge) currently sprites
content images, such as the results of an image search.  A determined
set of developers should be able to construct resource packages for
image search results on the fly and save some HTTP requests.

So we can avoid rehashing here the common objections to resource
packages, here's a brief overview of the arguments I've heard against
the feature and my responses.

* Argument: Packaging isn't the way forward.  When you change one
resource in a package you have to change the whole package and so the
user has to re-download all the bits when most of what was in their
cache would have been fine.

This is of course correct, but we don't think it eliminates the
utility of resource packages.  The resource-constrained developer is
probably happy with anything which speeds up page loads, even if it's
not optimal when one part of the page changes.  And the
resource-unconstrained developer probably won't find resource packages
too useful for non-dynamic content, so caching isn't an issue in that
case.

* Argument: We can already package things pretty well.  Mozilla should
instead be focusing on improving caching (or something else).

I'd contend that we don't package particularly well in general.  The
Facebook homepage loads 100 separate resources on a cold cache, and
they certainly care about speed.  But anyway, this is just one
project.  We're also looking at caching.  :)

* Argument: Isn't this subsumed by HTTP pipelining?

Mostly.  But we can't turn on HTTP pipelining because transparent
proxies break it.

Resource packages have the further benefit that they allow page
authors to explicitly set the order in which the UA will download the
resources -- with pipelining, an important resource might get stuck
behind a large, unimportant resource, while with resource packages,
the UA always downloads resources in the order they appear in the zip
file.

Last, my understanding is that the HTTP pipeline isn't particularly
deep, so perhaps resource packages fill the TCP pipe better on
high-latency connections.  I haven't looked into this, though.

* Argument: What about SPDY?

I think SPDY should subsume resource packages.  But its deployment
will require changes to both web clients and servers, so it will
probably take a while after it's released before it's available on all
web servers.  And we have no idea when to expect SPDY to be ready for
production.  Resource packages, in contrast, are something we can have
Right Now.

Additionally, since resource packages are backwards-compatible -- a
page which specifies resource packages should display just fine in a
browser which doesn't support them -- we should be able to turn off
resource packages in the future if we decide we don't want them
anymore.

We'd love to hear what you think of the specification and our implementation.

-Justin