Maven Virtual Registry: Cache logic

🔭 Issues plan

1. Maven Virtual Registry: Database models (#467972 - closed).
2. Maven Virtual Registry: Permissions policy (#467977 - closed).
3. Maven Virtual Registry: Registry models API (#467979 - closed).
4. Maven Virtual Registry: Maven API endpoint (#467982 - closed).
5. Maven Virtual Registry: Cache logic (#467983 - closed). 👈 You're here.
6. Maven Registry: Cached responses destruction (#468113 - closed).
7. Improve workhorse dependencyproxy logic (#461561 - closed).
8. Maven Virtual Registry: Documentation (#468115).
9. Maven Virtual Registry: Performance review (#468116).
10. Maven Virtual Registry: feature flag cleanup (#468117).

🗒 Description

Cold cache or when the related cached response doesn't exist:

• Use workhorse senddependency logic to return and upload the response from upstream.
  • At this time, record the ETag sent by upstream.
  • Set the upstream_etag_checked_at to now.
  • Set the downloaded_at to now.
  • Set the download_count to 1.

Warm cache or when the related cached response exist:

• Check if now is within [upstream_etag_checked_at, upstream_etag_checked_at + cache_validity_period_hours].
  • If that's the case, use workhorse sendurl to send the cached_response.
    • Set the downloaded_at to now.
    • Set the download_count to +1.
  • If that's not the case, ping the related upstream to get the ETag header (HEAD request).
    • If the ETag is the same, use workhorse sendurl to send the cached_response.
      • Set the upstream_etag_checked_at to now.
      • Set the downloaded_at to now.
      • Set the download_count to +1.
    • If the ETag is not the same (stale cached response), destroy the cached_response and use workhorse senddependency logic to return and upload the response from upstream.
      • At this time, record the ETag sent by upstream.
      • Set the upstream_etag_checked_at to now.
      • Set the downloaded_at to now.
      • Set the download_count to download_count of the destroyed cached_response + 1. (the download_count carries over the cached response destruction.)
  • Special case: ETag header is missing from upstream. Execute the "ETag is not the same" branch.