Before we explore where to cache, let's understand when caching can be dangerous and why some data should never be cached.
Imagine you just sent $100 to your friend through a banking app. Your friend calls excitedly: "Thanks for the money!" But when you check your account balance, it still shows the old amount. You panic: "Did the transfer fail?"
This nightmare scenario happens when banks cache account balances. Some data is too important to be stale.
The Staleness Problem: means serving potentially outdated information. While this is fine for a website logo (who cares if it's 5 minutes old?), it's catastrophic for:
Key Insight: "If being wrong for even a few seconds could cause problems, don't cache it. Consistency trumps speed for critical data."
Caching is like having a smart assistant who remembers things for you - incredibly helpful, but you wouldn't want them giving outdated information about your bank balance!
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