[NovaX]

Yes, that would be fun. :)

For fixed expiration (e.g. 5 min TTL), I use time-bounded LRU queues. Then if the head item has not expired, its successors have not either. If it has, it is merely a poll.

For custom expiration (varies per-entry), I use a hierarchical timer wheel. This replaces sorting with hashing, with an amortized O(1) cost.

For refresh, that's when I do it on access since its being used. I keep a refresh timestamp and an expiration timestamp, so that a refresh can occur while the data is valid but aging, but a blocking load is required when its too stale (expired).

You might enjoy this slide deck of the ideas that I use: https://docs.google.com/presentation/d/1NlDxyXsUG1qlVHMl4vsU...