Contact emails
bfcac...@chromium.org, alt...@chromium.org, har...@chromium.org, kenji...@chromium.org, rak...@chromium.org
Spec
N/A, but see I2S of back-forward cache linked below for information related to the back-forward cache.
Summary
We’re running an experiment that enables the back-forward cache for same-site navigations. Websites might be interested to know whether a page they’re serving is included in the experiment (which means the page might get stored into the back-forward cache and later restored) for analytics purposes (e.g. analyzing user behavior with back-forward cache enabled vs not).
When a page opts-in into the Origin Trial, we will send a “Sec-bfcache-experiment” HTTP header on resource requests made by the page. The value will be set to the back-forward cache experiment group that the page is currently in (e.g. "experiment1-enabled", "experiment1-control", "experiment1-default")
Note that this is different from the ability to tell if a page got stored into/restored from the back-forward cache. It is already possible to do so by checking the pagehide/pageshow event’s persisted property. Knowing only this information is not enough to determine the bfcache “hit rate”, as the site doesn't have a way to distinguish between cases when back-forward cache wasn't enabled for the user and when it was enabled, but the page couldn't have been stored in the back-forward cache.
Link to “Intent to Prototype” blink-dev discussion
No I2P for the header, but we have an I2S for cross-site back-forward cache: https://groups.google.com/a/chromium.org/d/msg/blink-dev/S9qRFx4ozXk/DNT8tiR3BAAJ
Goals for experimentation
As the HTTP header should only be used to determine whether a page is included in the same-site back-forward cache experiment, it is intended to be temporary and its lifetime will be bounded by the same-site back-forward cache experiment. We will not ship this HTTP header.
Experimental timeline
M86-M88
Any risks when the experiment finishes?
None - this HTTP header is intended only to help websites know whether they have same-site bfcache enabled or not for analyzing benefits or potential issues.
Ongoing technical constraints
None.
Will this feature be supported on all five Blink platforms supported by Origin Trials (Windows, Mac, Linux, Chrome OS, and Android)?
No - since the same-site bfcache experiment is only enabled on Android, the HTTP header will only be supported on Android.
Link to entry on the feature dashboard
Back-forward cache: https://chromestatus.com/feature/5815270035685376
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