Prevent unexpected behavior when using both full-page and sub-request caching

[jplhomer]

I think we have a bit of a footgun today:

• It is unclear how full-page caching and sub-request caching are meant to work together
• It is easy to set a short sub-request cache value (or even noStore) but have full-page caching still set to 4 hours (the default) without the developer being warned

Let's revisit the purpose of full-page cache vs sub-request cache

Benefits of sub-request cache

This is really the bread-and-butter of caching in Hydrogen. It's critical to have granular control over the freshness of sub-requests made to both SFAPI and 3P sources.

I'm not too worried about this, and I don't think we require a ton of changes here.

Benefits of full-page cache

I think full-page cache really has two concrete use cases:

• Long-lived cache for "static" pages, like marketing pages
• Basic thundering herd protection for dynamic pages

Given these benefits:

• Can we modify the full-page cache API to be a simple toggle between static and dynamic mode?
• Can we add heuristics in Hydrogen's rendering pipeline to determine which sub-requests were made, what their cache values were, and adjust FP cache values accordingly?

[wizardlyhel]

I feel that this should be a developer tool to help developer to identify potentially broken caching strategy.

Thinking somewhere along this line ...

• A developer toggle to display logging about caching strategy used in a given render

When enabled, developer is able to see server logging about the caching strategy used in a given render

GET str 200 13.06 ms   http://localhost:3000/
⎾ Caching strategy for http://localhost:3000/ 
⎢ public, max-age=10, stale-while-revalidate=10
⎢ query shopName              max-age=1, stale-while-revalidate=10
⎢ query indexContent          max-age=1, stale-while-revalidate=10
⎣___________________

We can create warning/error on detected unexpected usage like:

Warning: Detect page caching max-age duration (10s) longer than query shopName max-age duration (1s)