ExpiryTimers

ExpiryTimers is a mechanism used in software systems to automatically remove or invalidate items after a defined time or condition. They help manage resources, enforce security policies, and keep data current by ensuring that stale entries do not persist indefinitely. Expiry timers are commonly embedded in caches, session stores, message queues, and token validation systems.

Expiry timers can be implemented in several ways. They may rely on explicit timers that fire at a future moment, periodic sweeps that check for expired entries, or lazy expiration where items are considered expired when they are next accessed. Expiration can be relative (expires after a duration) or absolute (expires at a specific timestamp). Some systems support recurring expirations or per-item timers with individual durations.

Key implementation considerations include clock choice and synchronization. Monotonic clocks avoid problems caused by wall clock adjustments but do not reflect real-world time, while wall clocks enable absolute expirations but can be skewed by synchronization issues. In distributed environments, clock skew and clock drift require strategies such as synchronized time sources, logical clocks, or consensus-based expiry handling to maintain consistency. Durability concerns may necessitate persisting expiration data across restarts or using external services to coordinate expirations.

Common use cases include caching with time-to-live values, expiring user sessions or authentication tokens, deadlines in message delivery, and maintenance windows in rate-limiting schemes. The choice of expiry semantics affects correctness and performance, influencing how promptly expired data is purged and how resources are reclaimed in high-volume or distributed systems.