[whatwg] Proposal: Loading and executing script as quickly as possible using multipart/mixed

Adam Barth w3c at adambarth.com
Mon Dec 3 23:19:06 PST 2012

On Mon, Dec 3, 2012 at 9:57 PM, Maciej Stachowiak <mjs at apple.com> wrote:
> On Dec 3, 2012, at 2:11 PM, William Chan (陈智昌) <willchan at chromium.org> wrote:
>> Unless I am misunderstanding, SPDY will not solve this problem. SPDY uses
>> prioritized multiplexing of streams.
> It seems to me like SPDY could make this case work better:
> <script async src="path/to/script-part1.js"></script>
> <script async src="path/to/script-part2.js"></script>
> <script async src="path/to/script-part3.js"></script>
> Specifically the individual script chunks could be ordered and prioritized such that all of script-part1.js transfers before any of script-part3.js. That's harder to do with HTTP because the scripts could be loading on wholly separate HTTP connections, while SPDY will use one connection to the server.
> That being said, I do not know if SPDY will actually achieve this. Presumably it makes sense for it to serialize within a given priority level, at least a priority level that's likely to correspond to resources that are only atomically consumable, like scripts. But I don't know if SPDY implementations really do that.

It also has disadvantage (3):

(3) This approach requires the author who loads the script to use
different syntax than normally used for loading script.  For example,
this prevents this technique from being applied to the JavaScript
libraries that Google hosts (as described by


>> Generally speaking, a browser will map
>> a single resource request to a single stream, which would prevent chunked
>> processing by the browser without multipart/mixed. One could imagine
>> working around this by splitting the single resource into multiple
>> resources, and then relying on SPDY priorities to ensure sequential
>> delivery, but that is suboptimal due to having limited priority levels (4
>> in SPDY/2, 8 in SPDY/3), and many of them are already used to indicate
>> relative priority amongst resource types (
>> https://code.google.com/p/chromium/source/search?q=DetermineRequestPriority&origq=DetermineRequestPriority&btnG=Search+Trunk
>> ).
>> On Mon, Dec 3, 2012 at 1:40 PM, Anne van Kesteren <annevk at annevk.nl> wrote:
>>> On Mon, Dec 3, 2012 at 10:14 PM, Adam Barth <w3c at adambarth.com> wrote:
>>>> The HTTP server would then break script.js into chunks that are safe
>>>> to execute sequentially and provide each chunk as a separate MIME part
>>>> in a multipart/mixed response.
>>> Is it expected that SPDY will take much longer than getting this
>>> supported in all browsers? Or am I missing how SPDY will not address
>>> this problem?
>>> --
>>> http://annevankesteren.nl/

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