On Mon, 17 Aug 2020 07:18:31 +0100
Andy Burns  wrote:

> Eli the Bearded wrote:
>
> > Rendering images with JS as you scroll to them serves two purposes. One
> > benign, one not.
> >
> > 1. It makes the page in view to load before the content that is
> >     scrolled off the bottom. Other wise it could be more random what
> >     loads when. This allows you to start reading the page faster, and
> >     reduces the memory footprint, and bandwidth if you are not going to
> >     scroll at all.
>
> It's a newish attribute but  avoids using
> js for that.

 Good enough for images but not for some of the more extreme uses of
AJAX. Enough time has passed that I can give a simplified overview of how
the Yahoo! front page used to work (it clearly doesn't work that way now)
when it was a complex mass of widgets.

 Page request comes in and is recognised by the front end proxy as
such and sent to the VIP for the 'fast' server farm. That server decides
which page layout is needed and fires off all the internal service requests
for outline content and status for each widget in the layout. The SLAs on
the internal services are *really* tight so they serve from cached data.
When all of those are back the page is assembled with ad-server hooks and
sent to the user. As the page is served the hooks are replaced by advert
content. At this point the widgets have really minimal content usually just
enough to show the status lines and load the right images and fix the page
layout. Once the page hits the browser they all send requests for fill in
content so that the mouseovers and popup links start to work. Those
requests hit the front end proxy and get sent to the 'slow' server farm,
which also updates the cached data used by the internal services.

 Simplified because I've left out a whole pile of proprietary detail
on scaling, geographic distribution, CDN usage, tracking, failure handling,
DDOS handling etc. etc. What remains pretty much applies to any "Web 2.0"
AJAX based page.

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