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Document Cache Clearing and Revalidating #9187

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dficker opened this issue Aug 19, 2024 · 3 comments
Open

Document Cache Clearing and Revalidating #9187

dficker opened this issue Aug 19, 2024 · 3 comments
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Process: SME Wanted Issues/PRs that benefit SME. Set a date to proceed if SME input does not come Topic: Caching Caching related issues at the CDN or CMS level Topic: CDN Issue or PR relates to the Global CDN

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@dficker
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dficker commented Aug 19, 2024

Re: <Add Doc title and/or link>

Priority: Low

Issue Description

  • I'm not sure where in the Docs this should be.
  • A customer found that if they have the "Last-Modified" header set and the header does not change over time, then the Pantheon Global CDN effectively does not give you a way to clear these from caches.

How will this impact users?

  • Maybe it could be helpful to document some more of the details of how the Global CDN works.
  • Maybe include that stale/invalidated pages are kept around for Experience Protection as well?

Context

  • When the "Clear Caches" button is pressed, all entries are "invalidated" from the Pantheon Global CDN cache. But when it tries to get the new page from the origin, if the "Last-Modified" header is the same, it will "revalidate" the invalidated cache entry that hasn't yet been fully purged.
  • If customers want CDN cache clearing to work effectively, they should not set a "Last-Modified" header and then modify the page's content without updating the "Last-Modified" header's contents to show it's new.
  • A customer thought this behavior was confusing and unexpected and thinks this should be documented publicly. (Though to my knowledge, this has always been the case and only this one of many thousands of Pantheon customers has run into this issue in the 7+ years Pantheon Global CDN has existed.)
  • Let me know if you have any questions.

Additional Information

@rachelwhitton rachelwhitton added Process: SME Wanted Issues/PRs that benefit SME. Set a date to proceed if SME input does not come Topic: CDN Issue or PR relates to the Global CDN Topic: Caching Caching related issues at the CDN or CMS level labels Sep 11, 2024
@rachelwhitton rachelwhitton self-assigned this Nov 6, 2024
@rachelwhitton
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Todo connect with PDE SME - @conorbauer23 is there someone you can assign to review this issue?

@stevector
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And the review should probably start with getting a reliable reproduction of the described behavior.

@rachelwhitton
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@dficker could you provide steps to replicate this issue in a vanilla WP or Drupal site?

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Labels
Process: SME Wanted Issues/PRs that benefit SME. Set a date to proceed if SME input does not come Topic: Caching Caching related issues at the CDN or CMS level Topic: CDN Issue or PR relates to the Global CDN
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