TTL, explicit purge, event-driven invalidation, and hybrid strategies are the main design families for keeping cached answers trustworthy.
Time- and event-based invalidation turn the invalidation fundamentals from Chapter 9 into concrete design families. Most cache systems rely on one or more of these approaches: entries expire because time passes, the system explicitly purges them, events tell the system what changed, or several techniques are layered together to balance correctness and operational resilience.
The four lessons move through those main options directly. The first covers TTL and time-based expiration. The second covers explicit delete and purge strategies. The third covers event-driven invalidation based on source changes. The fourth explains why most production systems eventually combine these approaches instead of depending on only one of them.
Use this chapter when the question is not “what should this cache store?” but “how will the system know when cached answers stop being trustworthy?” The goal is to leave the child lessons with a clearer sense of which invalidation family best fits a workload and why hybrid strategies often win in practice.