Memcached Expiration: Transitioning from Relative TTL to Absolute Unix Timestamps

Memcached manages item expiration using a 32-bit unsigned integer. The system distinguishes between a relative offset and an absolute timestamp based on a specific threshold: values under 2,592,000 seconds (30 days) are treated as relative TTLs, while values exceeding this limit are interpreted as Unix epoch timestamps.

When migrating data structures or adjusting cache durations, there is uncertainty regarding how the slab allocator and lazy expiration process handle items that cross this 30-day boundary. Because Memcached treats stored values as opaque blobs, the internal expiration logic relies entirely on the integer passed during the set or add command.

If a client application provides a timestamp that is slightly off due to clock drift across a distributed cluster, the behavior of the item's lifecycle becomes unpredictable.

• Does the lazy expiration mechanism trigger differently for absolute timestamps compared to relative offsets?
• How does the system resolve conflicts when distributed clients provide inconsistent Unix timestamps for the same key?