Caching: hits, misses, invalidation, and Redis
By the end of today you can decide what to cache and for how long, name the failure modes caching introduces, and explain why invalidation is genuinely one of the hard problems.
YesterdayOn Day 15 you learned a cache is a hash map allowed to forget. On Day 43 an index traded write cost for read speed. Today that trade happens across machines.
TomorrowTomorrow, scaling the database itself, when caching is no longer enough.
Why this matters
Caching is the highest-leverage performance tool and the most common source of confusing bugs, because a cache means the truth now exists in two places.
- Cache patterns
- Invalidation
- Cache failure modes
- Redis
- CDNs
- Object storage
Learn it
80 minCopy this into Claude or ChatGPT. It quizzes you before it explains anything, which is deliberate. The resources under it are how you check what it told you.
Today's Master Prompt
Free · sign inA prompt written for this day alone: your level, the exact scope, what to leave out, and an instruction to quiz you before it explains anything. Paste it into Claude or ChatGPT and it teaches you today's material.
Check it against something that is not a model
An assistant can be fluent and wrong, and on a topic you met today you will not catch it. These cover the same ground and were made by people who do this for a living, so they are what you hold the explanation up against. They are other people's work and we only link to them, so judge them for yourself.
3 hand-picked resources
Free · sign inVideos, official docs and articles covering the same ground, each opened and annotated by hand. They are what you check the assistant against on a day you cannot yet catch it being wrong.
Build it
50 minAdd a cache-aside layer to your slowest endpoint using Redis. Measure latency and database query count before and after. Then make the underlying data change without invalidating, and observe stale data being served. Add invalidation, then deliberately expire a hot key under load and watch what reaches the database.
Recall it
20 minAnswer out loud, reveal, then mark honestly whether you had it. That score is the only thing on this page you do not get to choose.
5 recall questions
Free · sign inQuestions you answer from memory, then grade yourself against the real answer. The score is carried into the mastery rating below it, so an honest miss cannot quietly become a tick.
Rate it
Completion and mastery are tracked separately. Be honest, because an inflated rating only means the concept resurfaces sooner.
Mastery tracking
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Recap
- 01Caching buys speed with memory and correctness
- 02A key expiring under load sends the herd straight to the database
- 03A load-bearing cache is a hidden single point of failure
- 04Invalidation is hard because you must know every place a fact lives
Your progress
Free · sign inMark days complete, pick up where you left off across devices, and watch completion and mastery diverge. Free, and the account exists only so ninety days of work cannot vanish with a cleared browser.