[whatwg] HTML resource packages

[Justin Lebar]

On Mon, Aug 9, 2010 at 9:47 AM, Aryeh Gregor <Simetrical+w3c at gmail.com> wrote:
> If UAs can assume that files with the same path
> are the same regardless of whether they came from a resource package
> or which, and they have all but a couple of the files cached, they
> could request those directly instead of from the resource package,
> even if a resource package is specified.

These kinds of heuristics are far beyond the scope of resource
packages as we're planning to implement them.  Again, I think this
type of behavior is the domain of a large change to the networking
stack, such as SPDY, not a small hack like resource packages.

-Justin

On Mon, Aug 9, 2010 at 9:47 AM, Aryeh Gregor <Simetrical+w3c at gmail.com> wrote:
> On Fri, Aug 6, 2010 at 7:40 PM, Justin Lebar <justin.lebar at gmail.com> wrote:
>> I think this is a fair point.  But I'd suggest we consider the following:
>>
>> * It might be confusing for resources from a resource package to show
>> up on a page which doesn't "opt-in" to resource packages in general or
>> to that specific resource package.
>
> Only if the resource package contains a different file from the real
> one.  I suggest we treat this as a pathological case and accept that
> it will be broken and confusing -- or at least we consider how many
> extra optimizations we could make if we did accept that, before
> deciding whether the extra performance is worth the confusion.
>
>> * There's no easy way to opt out of this behavior.  That is, if I
>> explicitly *don't* want to load content cached from a resource
>> package, I have to name that content differently.
>
> Why would you want that, if the files are the same anyway?
>
>> * The avatars-on-a-forum use case is less convincing the more I think
>> about it.  Certainly you'd want each page which displays many avatars
>> to package up all the avatars into a single package.  So you wouldn't
>> benefit from the suggested caching changes on those pages.
>
> I don't see why not.  If UAs can assume that files with the same path
> are the same regardless of whether they came from a resource package
> or which, and they have all but a couple of the files cached, they
> could request those directly instead of from the resource package,
> even if a resource package is specified.  So if twenty different
> people post on the page, and you've been browsing for a while and have
> eighteen of their avatars (this will be common, a handful of people
> tend to account for most posts in a given forum):
>
> 1) With no resource packages, you fetch two separate avatars (but on
> earlier page views you suffered).
>
> 2) With resource packages as you suggest, you fetch a whole resource
> package, 90% of which you don't need.  In fact, you have to fetch a
> resource package even if you have 100% of the avatars on the page!  No
> two pages will be likely to have the same resource package, so you
> can't share cache at all.
>
> 3) With resource packages as I suggest, you fetch only two separate
> avatars, *and* you got the benefits of resource packages on earlier
> pages.  The UA gets to guess whether using resource packages would be
> a win on a case-by-case basis, so in particular, it should be able to
> perform strictly better than either (1) or (2), given decent
> heuristics.  E.g., the heuristic "fetch the resource package if I need
> at least two files, fetch the file if I only need one" will perform
> better than either (1) or (2) in any reasonable circumstance.
>
> I think this sort of situation will be fairly common.  Has anyone
> looked at a bunch of different types of web pages and done a breakdown
> of how many assets they have, and how they're reused across pages?  If
> we're talking about assets that are used only on one page (image
> search) or all pages (logos, shared scripts), your approach works
> fine, but not if they're used on a random mix of pages.  I think a lot
> of files will wind up being used on only particular subsets of pages.
>
>> In general, I think we need something like SPDY to really address the
>> problem of duplicated downloads.  I don't think resource packages can
>> fix it with any caching policy.
>
> Certainly there are limits to what resource packages can do, but we
> can wind up closer to the limits or farther from them depending on the
> implementation details.
>