Re: Partial loading of fonts

On Thu, 30 Apr 2020 at 00:05, Myles C. Maxfield <mmaxfield@apple.com> wrote:

>
> The request works like this:
>
>    1. We request the font from the webserver as normal, but including
>    "Range: bytes=0-" as a request header.
>    2. If the response (includes "Accept-Ranges: bytes" AND if it has a
>    valid Content-Length header AND if it has no "Content-Encoding" header),
>    then close the connection after reading a respectable amount of data, but
>    at least enough to include the initial lookup table.
>    3. We then lazily load the blocks we need from the font depending on
>    the tables (or parts of tables) we need, by submitting a request with an
>    appropriate "Range" header. We break the file up into chunks (we chose 32KB)
>
> This is interesting - I hadn’t considered using blocks. What was the
> motivation for using blocks?
>

Reducing overhead really. There's no point in bolting 1KB of headers onto a
request for 100 bytes, and many individual glyphs are not much more than
that. We did no analysis on block size though - I'm sure this could be
tuned quite a bit, and the balance is probably quite different for HTTP/2.

Oh and, for completion, I now recall that the reason we send "Range: bytes
0-" is not for any HTTP reason. It's to work around a Java bug - web
servers SHOULD send "Accept-Range: bytes" unprompted.

Overall, this is super interesting, thanks for posting it!
>

Glad to help! While I can't offer you web headers and timing details from
the client, it's still possible to log the request headers received and
data sent on the server end if you're hosting the font.