If I build edge caching with Cloudflare Workers + KV, how do I do invalidation?

Short answer: don’t rely on TTL alone — TTL is eventual and serves the stale price until it expires. Make invalidation push-based and event-driven.

In your scenario the unacceptable thing is the “wrong price” window. The fix is to push the change to the edge, not leave it waiting:

1. Write-through (or purge) on change. The moment a price changes at origin, call Cloudflare’s API in the same transaction/outbox and write the new value into KV, or delete the key. That makes the update part of the write itself, not a separate “hope it runs” step.
2. Treat KV as a cache, not the source of truth. Your source of truth is the origin DB; KV is a fast copy of it. This mindset lets you re-seed from origin whenever KV gets corrupted or inconsistent.
3. Key by entity+version. Stamp each value with a version (like price:42:v7). If the Worker sees an old version it can serve-stale-while-revalidate and refresh in the background; the user doesn’t wait, but doesn’t get the wrong value either.
4. Keep a short TTL as a safety net. If a push invalidation misses for some reason, a short TTL caps staleness to a few seconds at worst. Let TTL be the last line of defense, not the only one.
5. Accept KV’s global eventual consistency. KV writes can take a few seconds to propagate worldwide; a small inconsistency window is unavoidable. If you need instant global correctness, KV is the wrong layer — go to origin or choose a different consistency model.

Bottom line: I’d set up the trio of write-through (or purge) on change + a short TTL safety net + versioned keys. A TTL-only solution is wrong for money-affecting data like prices; with push-based invalidation, the edge updates the instant origin changes. Design knowing KV’s seconds-long propagation window — use it as a fast, refreshable cache, not as the source of truth.