feat(browser): Add bfcacheMetricsIntegration#22397
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Add an opt-in browser integration that tracks back/forward cache health as metrics: - browser.bfcache.navigation: hit/miss counter by navigation type - browser.bfcache.not_restored: Chromium notRestoredReasons per miss - browser.bfcache.miss.reload.duration: fallback reload cost on a miss Hits are detected via pageshow.persisted; misses via a back_forward navigation entry. Includes unit tests for the notRestoredReasons parser and a Playwright suite for hit/miss detection.
Adds a dedicated `browser-bfcache` e2e app that exercises `bfcacheMetricsIntegration` against a genuine back/forward cache: real cross-document navigation served by `vite preview`, restored via renderer-initiated `history.back()`, under the full Chrome-for-Testing binary (`channel: 'chromium'`) with the disable flag stripped and `BackForwardCache` enabled. Covers hits, misses with real `notRestoredReasons`, and deliberately bfcache-ineligible pages via `?botch=`: an `unload` listener (stable blocker) and an open WebSocket (blocks only before Chrome 149, so that assertion is version-gated). `Cache-Control: no-store` no longer blocks. Removes the simulated browser-integration bfcache suite, which faked the restore with a synthetic `pageshow` and a manual flush; it is superseded by the real assertions here. The reason-parser unit test stays.
The unload miss deterministically also yields a privacy-masked reason. Assert the integration reports it as a `masked` frame, giving real-browser coverage of the masked-frame classification path (previously only unit-tested).
A plain open IndexedDB connection (and even an in-flight transaction) no longer blocks bfcache in current Chrome, contrary to web.dev's list. The condition that still blocks is a connection holding up a version upgrade, which yields an `idbversionchangeevent` reason. Add a deterministic e2e case for it and document how real Chrome diverges from the article.
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A no-store (CCNS) page is only evicted from bfcache once a cookie changes; neither the header nor a cookie change blocks on its own. Add a `?botch=nostore` case that serves the document with `Cache-Control: no-store` (via a preview middleware, since a static server can't set per-request headers) and mutates a cookie, asserting the `response-cache-control-no-store` reason. Also trim the version-specific blocker details from the README so they don't drift; the botch cases and their assertions are the source of truth.
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`browser.bfcache.navigation_type` was 1:1 with `browser.bfcache.outcome` (hit <-> back-forward-cache, miss <-> back-forward), so it added a second attribute encoding the same bit. Keep only `outcome`; navigation-type vocabulary belongs on web-vital samples (future soft-navigation work), not on bfcache counters. Also drops an unnecessary PageTransitionEvent cast.
Adds two real-browser e2e cases: an embedded bfcache-eligible iframe keeps the top page restorable (hit), and an iframe with an unload listener makes the top page ineligible with the reason reported from the child frame. This gives the `frame: 'child'` classification real coverage (previously unit-only).
`frame` no longer special-cases the `masked` reason value; it now reflects only the frame's position in the notRestoredReasons tree (`top`/`child`). The `masked` value already lives on the `reason` attribute, so tagging the frame as `masked` too was redundant. The integration no longer inspects any reason string - reasons pass through verbatim.
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Adds an opt-in
bfcacheMetricsIntegrationthat reports back/forward-cache health as metrics: a hit/miss navigation counter, the (Chromium-only)notRestoredReasonsbehind a miss, and the fallback reload cost on a miss.Metrics are the source of truth rather than spans, because a bfcache hit restores an existing page and never creates a pageload span, so span-only instrumentation would drop every successful restore from the dataset.
New Metrics
Three metrics, all under the
browser.bfcache.*namespace.browser.bfcache.navigationbrowser.bfcache.outcome(hit/miss),browser.bfcache.not_restored_reason_count(miss only)outcome:hit/ total)browser.bfcache.not_restoredbrowser.bfcache.reason(verbatim),browser.bfcache.frame(top/child)browser.bfcache.reload.durationConventions PR is here getsentry/sentry-conventions#513
An example of a dashboard that uses bfcache metrics would look like this:
We can pre-ship a sentry-provided dashboard to onboard users to this feature.
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