> Do you have a sense of how often the failure cases happen? Also what is the cost of this failure? The discussion is limited to in-progress prefetches that have not yet received headers when the navigation to a non-exact match URL happens.
- With our partner, both hint and header are the same in a same site navigation use case. It would be possible to have hint and header be different and prefetch and navigation URLs still match by No-Vary-Search header variance but the easiest way is to use the same hint and header. So in this sense, waiting for headers is always successful in our experiment. If the navigation URL doesn't match the prefetch URL by No-Vary-Search header variance the cost is the time we blocked navigation waiting for headers, so the portion of time to first byte of the prefetch between navigation time and the prefetch receiving headers. This cost decreases with how close to the prefetch receiving headers the navigation happens. - Not having the hint, the cost is the fact that we could have used the in-progress prefetch serving No-Vary-Search header but we didn't. This cost is the portion of time to first byte from the beginning of the prefetch request to navigation time. This cost increases with how close to the prefetch receiving headers the navigation happens. For a sense of scale for the above costs, time to first byte (TTFB <https://web.dev/articles/ttfb>) is defined as "good" below 0.8s and "needs improvement" up to 1.8s. The hint is useful for: - prefetch triggered independently from navigation. As an example, at page load the page could prefetch most likely next links. In this case, the prefetch might still be in progress when a user clicks one of these next links immediately after page load. - prefetch triggered based on user interaction leading to a navigation (e.g. at hover or pointerdown) - these prefetches will very likely be in progress when navigation happens (pointerdown to navigation is less than one hundred ms and from hover to navigation a few hundred ms - see https://instant.page/). The hint helps to trigger such prefetches based on user interaction and thus deliver these gains to the navigation. - having parity with navigation to a URL that exactly matches an in-progress prefetch URL The above discussion about No-Vary-Search hint applies to prerender as well. On Thursday, June 6, 2024 at 1:26:34 PM UTC-4 Vladimir Levin wrote: > From the explainer, expects_no_vary_search helps because when we're > prefetching but have not yet received headers, we don't know if that > request would match what the user is now navigating to. Do you have a sense > of how often the failure cases happen? (ie we waited for that navigation > request to finish but we shouldn't have, or vice versa). Also what is the > cost of this failure? It seems like this is a good addition but it seems > like a very fine-grained control for cases that may not be problematic or > costly. > > On Thu, Jun 6, 2024 at 12:45 PM Chris Harrelson <chris...@chromium.org> > wrote: > >> LGTM2 >> >> On Wed, Jun 5, 2024 at 9:58 PM Mike Taylor <miketa...@chromium.org> >> wrote: >> >>> LGTM1 >>> On 6/5/24 11:21 PM, Liviu Tinta wrote: >>> >>> Contact emails >>> >>> dome...@chromium.org, jbro...@chromium.org, liviuti...@chromium.org >>> >>> Explainer >>> >>> >>> https://github.com/WICG/nav-speculation/blob/main/triggers.md#no-vary-search-hint >>> >>> Specification >>> >>> https://wicg.github.io/nav-speculation/prerendering.html >>> >>> Summary >>> >>> Adds a hint to speculation rules that informs the navigation prerender >>> cache that the URL to be prerendered expects to receive the same >>> No-Vary-Search header in the response. >>> >>> The hint is useful because prerenders that depend on No-Vary-Search to >>> match to navigations do not benefit the user if the navigation happens >>> before prerender headers return from the server. Using the hint, the web >>> browser will expect, but verify, that the No-Vary-Search hint matches with >>> the No-Vary-Search header. If the No-Vary-Search hint does not match the >>> No-Vary-Search header received then the web browser will send a new request. >>> >>> We would like to ship "No-Vary-Search Hint for Prerender Speculation >>> Rules" together with "No-Vary-Search support for prerender >>> <https://chromestatus.com/feature/5099218903760896>". >>> >>> Blink component >>> >>> Internals>Preload>Prerender >>> <https://bugs.chromium.org/p/chromium/issues/list?q=component:Internals%3EPreload%3EPrerender> >>> >>> TAG review >>> >>> Not applicable. The TAG has already given a negative opinion on the >>> overall complexity of speculation rules ( >>> https://github.com/w3ctag/design-reviews/issues/721 ), so we anticipate >>> it would not be a good use of their time to review additions to the syntax. >>> The addition itself is small and any architectural questions about it would >>> be covered under the general closed speculation rules review. >>> >>> TAG review status >>> >>> Not applicable >>> >>> Risks >>> >>> Interoperability and Compatibility >>> >>> None >>> >>> >>> Gecko: No signal ( >>> https://github.com/mozilla/standards-positions/issues/620#issuecomment-1608195274 >>> ) >>> >>> WebKit: No signal ( >>> https://github.com/WebKit/standards-positions/issues/54#issuecomment-1608197504 >>> ) >>> >>> Web developers: Below is the text from the I2S of the No-Vary-Search on >>> navigational prefetch, and we believe the same applies to Prerendering. >>> Google Search has been experimenting with No-Vary-Search header / >>> Speculation Rules "expects_no_vary_search". This functionality helps Google >>> Search to match prefetched content to the next user navigation. Developers >>> can use parameters in the prefetched URL that are not needed when >>> navigating to the actual link (e.g. the source of the link click). The >>> server can customize behavior using these parameters without causing a >>> cache miss in the browser. "expects_no_vary_search" addition to Speculation >>> Rules allows the browser to completely handle the case where the user >>> navigates to a URL that is currently prefetched by waiting for the ongoing >>> prefetch instead of directly requesting the page from the server. Google >>> Search conducted experiments prefetching Search results pages from the >>> search box and other links that lead to another Search results page. There >>> was significant latency improvement for navigating to Search result pages >>> prefetched using No-Vary-Search header and "expects_no_vary_search". >>> >>> Other signals: No-Vary-Search header has been discussed, together with >>> No-Vary-Search Hint for Prefetch Speculation Rules at Web Perf WG meeting >>> at TPAC 2023. >>> https://docs.google.com/presentation/u/1/d/1GK92nCORW5vKd7LgGtTsgy35eqTV7P71l05pHsni8ok/edit#slide=id.g240fd6541f7_0_31 >>> >>> Ergonomics >>> >>> (Text taken from Prefetch NVS I2S) >>> >>> No-Vary-Search will be used in tandem with Speculation Rules ( >>> https://wicg.github.io/nav-speculation/speculation-rules.html). The >>> default usage of No-Vary-Search will not make it hard for Chrome to >>> maintain good performance. >>> >>> >>> Security >>> >>> See: >>> https://github.com/WICG/nav-speculation/blob/main/no-vary-search-security-privacy-questionnaire.md >>> >>> >>> WebView application risks >>> >>> Does this intent deprecate or change behavior of existing APIs, such >>> that it has potentially high risk for Android WebView-based applications? >>> >>> We are closely working with Android WebView team, and the broader >>> prerender feature is gated through new AwSettings API (currently >>> @RequiresOptIn) so there's zero risk that it will break existing WebView >>> apps. >>> >>> >>> >>> >>> Debuggability >>> >>> None >>> >>> >>> Will this feature be supported on all six Blink platforms (Windows, Mac, >>> Linux, ChromeOS, Android, and Android WebView)? >>> >>> Yes >>> >>> Is this feature fully tested by web-platform-tests >>> <https://chromium.googlesource.com/chromium/src/+/main/docs/testing/web_platform_tests.md> >>> ? >>> >>> Yes >>> >>> >>> https://wpt.fyi/results/speculation-rules/prerender?label=experimental&label=master&aligned >>> >>> (The test files starting with no-vary-search) >>> >>> >>> Flag name on chrome://flags >>> >>> None >>> >>> Finch feature name >>> >>> Prerender2NoVarySearch >>> >>> Requires code in //chrome? >>> >>> False >>> >>> Estimated milestones >>> >>> Shipping on desktop >>> >>> 127 >>> >>> Shipping on Android >>> >>> 127 >>> >>> Shipping on WebView >>> >>> 127 >>> >>> >>> >>> Anticipated spec changes >>> >>> Open questions about a feature may be a source of future web compat or >>> interop issues. Please list open issues (e.g. links to known github issues >>> in the project for the feature specification) whose resolution may >>> introduce web compat/interop risk (e.g., changing to naming or structure of >>> the API in a non-backward-compatible way). >>> >>> None >>> >>> Link to entry on the Chrome Platform Status >>> >>> https://chromestatus.com/feature/5190922953555968?gate=5148879099265024 >>> >>> Links to previous Intent discussions >>> >>> Intent to prototype: >>> https://groups.google.com/a/chromium.org/d/msgid/blink-dev/CA%2B7y87rgssbBecLSY5fz03ATcAAjgy%3DjY322w_2roQdsehP4Dg%40mail.gmail.com >>> >>> This intent message was generated by Chrome Platform Status >>> <https://chromestatus.com/>. >>> >>> -- >>> You received this message because you are subscribed to the Google >>> Groups "blink-dev" group. >>> To unsubscribe from this group and stop receiving emails from it, send >>> an email to blink-dev+unsubscr...@chromium.org. >>> To view this discussion on the web visit >>> https://groups.google.com/a/chromium.org/d/msgid/blink-dev/CAHaAqYJphk8wg8yLr-VFe2QJDZ4N%3D8Z%2BBAs9VL5scY-brTmRTg%40mail.gmail.com >>> >>> <https://groups.google.com/a/chromium.org/d/msgid/blink-dev/CAHaAqYJphk8wg8yLr-VFe2QJDZ4N%3D8Z%2BBAs9VL5scY-brTmRTg%40mail.gmail.com?utm_medium=email&utm_source=footer> >>> . >>> >>> -- >>> You received this message because you are subscribed to the Google >>> Groups "blink-dev" group. >>> To unsubscribe from this group and stop receiving emails from it, send >>> an email to blink-dev+unsubscr...@chromium.org. >>> To view this discussion on the web visit >>> https://groups.google.com/a/chromium.org/d/msgid/blink-dev/457ce01c-56ea-42a9-b9ef-1d84216ccd90%40chromium.org >>> >>> <https://groups.google.com/a/chromium.org/d/msgid/blink-dev/457ce01c-56ea-42a9-b9ef-1d84216ccd90%40chromium.org?utm_medium=email&utm_source=footer> >>> . >>> >> -- >> You received this message because you are subscribed to the Google Groups >> "blink-dev" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to blink-dev+unsubscr...@chromium.org. >> > To view this discussion on the web visit >> https://groups.google.com/a/chromium.org/d/msgid/blink-dev/CAOMQ%2Bw9p78D28sGy7zBHQk1cij7PwDs7EE8GnDZ62hpudydOtg%40mail.gmail.com >> >> <https://groups.google.com/a/chromium.org/d/msgid/blink-dev/CAOMQ%2Bw9p78D28sGy7zBHQk1cij7PwDs7EE8GnDZ62hpudydOtg%40mail.gmail.com?utm_medium=email&utm_source=footer> >> . >> > -- You received this message because you are subscribed to the Google Groups "blink-dev" group. 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