Caching

A Bloom filter can cheaply rule out membership in the set it represents, while a positive answer means only “possibly present.” That asymmetry depends on correct updates and unchanged hash rules. A stale or corrupted filter can reject a key that exists in the underlying database, even though the ordinary append-only algorithm has no false negatives for inserted items. ...

A cache does not receive a new freshness allowance every time another cache forwards a response. It must account for age already accumulated along the delivery path, then add its own residence time. Otherwise a chain of individually plausible sixty-second caches can keep old content reusable much longer than the origin intended. ...

A cache stampede occurs when concurrent misses cause redundant work against the same dependency. Synchronized expiration is one trigger, but eviction, a cold deployment, or cache unavailability can create the same burst. A single popular key can be enough. The useful question is how many rebuilds reach the source while one result is still being computed. ...

LRU chooses a victim by recency, not by popularity, age, or business importance. When a bounded cache needs space, an exact least-recently-used policy removes the entry whose last qualifying access occurred longest ago. That rule can exploit temporal locality, but it neither guarantees useful hits nor keeps cached data fresh. ...

Consistent hashing limits placement changes when membership changes; it does not itself move data, make replicas agree, or distribute a hot key’s requests. In the classic clockwise ring, a key belongs to the first node point at or after its hash position, wrapping to the smallest point when necessary. ...

Changing an origin does not notify every cache that already holds its response. Invalidation is about controlling which stored copies remain eligible for reuse, not merely updating a file. A reliable release distinguishes expiration, validation, targeted removal, and choosing a new URL. These mechanisms have different scopes, and none remotely erases every copy a reader has downloaded. ...

Read-through caching gives callers one reading interface: a hit returns a stored value, while a miss invokes a loader and stores its result. The important boundary is who owns that loader. Moving it behind the cache interface can remove duplicated lookup logic, but does not automatically solve concurrency, writes, authorization, or consistency between independent copies. ...

A stale response can preserve access to a public manual during an origin outage. It can also preserve an obsolete price, revoked permission, or misleading status. Availability is not automatically more important than correctness. Choose the information that may remain usable, the conditions that permit reuse, and the maximum tolerable age before enabling this behavior. ...