Caching
System Design: lesson 4 of 15
Keep the answer near whoever keeps asking.
Lesson 4 of 15 · 5 min
Caching
Step 1 of 11
A cache keeps the result of expensive work near whoever keeps asking.
The Idea
A cache stores the result of expensive work close to whoever needs it. A hit answers immediately; a miss does the real work and usually stores the result on the way back. Caching pays off only when a small set of items is requested far more often than everything else.
Real-World Example
A scrub nurse lays out the ten instruments this operation actually uses. Anything else means a runner walking to the store room. The tray is tiny next to the store room, yet it answers nearly every request — because surgical demand is nowhere close to uniform.
The Tradeoff
A cache is a second copy of the truth, so it will be wrong, briefly. Caching requests that rarely repeat buys nothing but memory cost and a stale-data bug. Measure the hit ratio before believing the cache earns its place: ninety hits in a hundred is a cache, nine is a liability.
Your turn
Put the steps in the right order.
- Store the fetched value in the cache under the request's key
- Look the key up in the cache
- Return the value to the caller
- On a miss, fetch the value from the database
Mini quiz
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