You cached more. Your hit ratio went down.

transcript

You start caching everything, for an hour. More cache, more speed? Your product page hit ratio drops. More reads reach the database.

You added order history and audit logs, cached for an hour. They're rarely read again. But each one still holds memory, for an hour. The cache fills up, so it has to evict something. Out go the product entries that were actually earning hits.

So cache strategically. Product details are hot and expensive. Five minutes. User profiles are hot and stable. Ten minutes. Order history is rarely re-read. Don't cache it. Audit logs are written once, read rarely. Skip them too.

And watch it. Alert when the hit ratio drops below 80 percent. Or p99 latency passes 50 milliseconds. Or errors pass 10 a minute. Caching isn't deploy and forget.

Rarely read data wastes memory. And the evictions hurt your hot data. Cache hot, expensive data. Skip the rest.

Full breakdown in the lecture.