Cache Invalidation at Scale: Versioned Keys, Purges, and Bounded Staleness

• Panel 1 - Bound the promise:
• Name the stale-data window
• Name the failures you tolerate
• TTL limits age; it is not instant
• Freshness is a stated guarantee

• Panel 2 - Fast purge, fragile path:
• Purge is fast when events arrive
• One lost event leaves stale data
• Write then delete can race
• Delete then write can race too

• Panel 3 - Version the namespace:
• Publish v2; readers stop asking for v1
• Old v1 keys become unreachable
• Distribute the version reliably
• Garbage-collect old keys later
• Coalescing stops load, not staleness

• Panel 4 - Falsify the guarantee:
• Drop one invalidation event
• Reorder the DB write and delete
• Read through every cache layer
• Measure age of the stale response
• Fail if it exceeds the promised window