Adding a cache takes ten minutes. Debugging the bug it causes six months later takes a week.

Caching trades correctness for speed . The senior move is deciding how much correctness you're willing to give up, and making that decision explicit.

Start with the question, not the tool

Before you reach for Redis, answer three things:

How stale can this data be? (seconds, minutes, never)

What does a wrong answer cost? (a flicker, or a wrong invoice)

What's the read-to-write ratio? (caches pay off on read-heavy data)

If you can't answer #1, you're not ready to cache it.

The four patterns

CACHE-ASIDE (lazy) WRITE-THROUGH App -> Cache: hit?…