feat: make CDN SWR background revalidation discard stale cache content in order to produce fresh responses #2765
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Description
We are currently having "double SWR":
This leads to us having to instruct edge not to cache stale responses served by Next serverless handler as otherwise it would lead to stale content lingering for longer than it should.
This is optimistic change looking to explore option of disabling SWR behavior in serverless handler for the background refresh request coming from the edge. Today we can't yet distinguish those, so this is using not actually existing
x-background-revalidationheader that can be manually used on requests for testing purposesDocumentation
Tests
For now testing was done manually for various scenarios comparing those changes both against the way things work today and when deployed on Vercel - from those manual tests things seem to be working correctly:
a) served content remain the same as with current setup (just it leads to less invocations and ability to serve content directly from edge caches without blocking on serverless responses)
b) comparing against behavior when deployed on Vercel and reported status (
x-vercel-cache)For our automated tests - right now no changes yet, as behavior should only be different when
x-background-revalidationis provided. E2E can't really be changed yet ... unless we manually provide that header in our fetches/playwright navigations. Integration tests need some thought, as this doesn't entirely elimantate possibility of serving stale content by lambdas (which we can't just do as there are scenarios that still need it)Relevant links (GitHub issues, etc.) or a picture of cute animal
resolves https://linear.app/netlify/issue/FRB-1639/investigate-disabling-double-swr-behavior-in-next-runtime