Document Cache Clearing and Revalidating

[dficker]

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