Should I use cache-aside or read-through?

Default to cache-aside. It keeps the cache off the critical path, degrades to database reads if the cache dies, and lets you cache computed values with any key shape. Choose read-through when a managed product like DAX offers it on a store you already use, because then the consistency of a single fetch path comes nearly free.

What is a good cache hit rate?

It depends on the traffic pattern, but for read-heavy workloads with hot keys, 90 to 99 percent is common and achievable. More useful than the absolute number is the trend: alert when the hit rate drops several points below its baseline, because your database is sized assuming the miss traffic stays small.

Why delete the cache key on writes instead of updating it with the new value?

Updating races with concurrent cache fills. A reader that fetched the old database value can write it into the cache just after your update, pinning stale data until the TTL expires. A delete cannot lose that race the same way; the worst case is an extra miss, and the next read fetches fresh data.

Cachingchapter 3 of 8 · 4 lessons

Cache Read Strategies

How the application reads data, from cache first, or transparently.

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Your product page takes 38 milliseconds, and 30 of those are three database queries that return the same rows for every visitor.

So you cache it. Ask the cache first, and when it comes back empty, read the database and put a copy back. The page drops to 2 milliseconds and your database load falls by 95 percent.

You have just written the pattern almost everybody writes first, and there is a second arrangement where the cache does that fallback for you and your code never knows whether it was a hit.

The difference sounds cosmetic. It decides where your logic lives, and it decides what happens to your site the day the cache goes down, which is the question worth asking before you pick.

Lessons

4 in this chapter

Cache-AsideThe app checks the cache first and takes responsibility for filling it on a miss.3 min
Read-ThroughThe cache fetches from the database itself, so the app only ever talks to the cache.3 min
What a Hit Rate Actually Buys YouHit rate is a multiplier on database load, and the last few percent matter most.2 min
Staleness and TTLsEvery cached read might be a lie, and the TTL is how long you agree to be lied to.2 min