Cache invalidation is hard because the moment you copy data to make a system faster, you create two truths that can drift apart: - One is the real data, - one is the saved fast version. "Just update the cache when data changes" is practically useless advice. That only sounds clean if you ignore delays, failures, race conditions, multiple services, retries, out-of-order events, and users reading data while writes are still moving through the system. In general, you need rules for - when the fast version is still safe, - when it must be replaced, - who decides that, - what happens when that decision is late, lost, or wrong. Cache invalidation is a constant trade between speed and correctness, and people often pretend they can fully have both.
