Caching invalidation: just say no

Caching is a deal you make with future you. You get faster reads now, in exchange for owning every cache invalidation bug for the rest of the system's life. It's a worse deal than it looks, especially the first time you reach for it.

Try these first

• Index the database properly.
• Optimize the query that's actually slow.
• Move computation to write-time, not read-time.
• Denormalize where it actually matters.

When caching is actually right

Read-heavy data that changes infrequently. Expensive computations that have a clear key. Cases where stale data is acceptable for a known window. If any of those don't hold, you're signing up for invalidation bugs for an unclear payoff.

Add caches like you'd add diet pills — only when the simpler thing has failed.