After lots of experimenting with Wireshark and
current Chrome + Firefox on Ubuntu 13.10, this is my
current understanding of the caching when preloading images
with AJAX requests:
* on Chrome, the image request always comes from browser cache
* Firefox makes two separate requests by default
* Firefox with img.crossOrigin = 'anonymous' makes two separate
requests, but the second one is a 304 (does not load the
image twice)
* when the image has already been cached by the browser (but not in
this session), Chrome skips both requests; Firefox skips the AJAX
request, but sends the normal one, and it returns with 304.
"wish I knew this when I started" things:
* the Chrome DevTools has an option to disable cache. When this is
enabled, requests in the same document context still come from
cache (so if I load the page, fire an AJAX request, then without
reloading the page, fire an AJAX request to the same URL, then the
second request will be cached), but an AJAX request - image request
pair is an exception from this.
* when using Ctrl-F5 in Firefox, requests on that page will never hit
the cache (even AJAX request fired after user activity; even if
two identical requests follow each other). When using clear cache
+ normal reload, this is not the case.
* if the image does not have an Allow-Origin header and is loaded
with crossOrigin=true, Firefox will refuse to load it. Chrome will
log an error in the console saying it refused to load it, but will
actually load it.
* Wireshark rocks.
Pushed some tech debt (browser + domain whitelist) into other tickets:
https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/232https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/233
Reverted commits:
8a8d74f01d.
63021d0b0e.
Change-Id: I84ab2f3ac0a9706926adf7fe8726ecd9e9f843e0
Bug: 61542
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/207
* more robust method of obtaining URL
* decouple performance logging from providers (mostly)
* ignore fake XHR object which jQuery returns for JSONP requests
* guard for CORS requests - apparently Chrome refuses to return
certain information even with an Allow-Origin: * response header.
* Resource Timing is limited to 150 results, which causes fake
misses in debug mode. There is an API to increase the limit
but it is not implemented in Chrome. I am calling it nevertheless,
maybe IE understands it (it is present in the MSDN docs at least).
This seems to work for AJAX, CORS, JSONP, image AJAX; CORS requests
return 0 for a lot of values, per spec a Timing-Allow-Origin: *
header might help that.
Change-Id: I8353858022f51a7e70774e65513d0fa2554a5064
