Caching Strategies: How Misconfigured TTLs Cause Stale Data and Six-Figure Losses

Caching is a critical performance tool that can make systems significantly faster and reduce database load, but it introduces the persistent risk of serving outdated data. A 2017 cloud provider outage illustrated this danger when a 24-hour TTL caused customers to see stale pricing for half a day, resulting in six-figure losses. Developers must decide what data to cache and how long to retain it, balancing performance gains against data freshness based on business needs. Key strategies include HTTP-level caching using headers like Cache-Control and ETags, as well as application-layer patterns such as cache-aside and write-through. The cache hit ratio remains the most important metric for measuring effectiveness, and no engineering approach can fully eliminate the fundamental challenge of keeping cached and live data in sync.