Memcached handles cache expiration through the use of expiration times set by the client application when storing data in the cache. Here's how it works:

Expiration Time : When storing data in Memcached, the client application can specify an expiration time for each key-value pair. This expiration time is a duration in seconds after which the data will be considered expired and automatically evicted from the cache.

TTL (Time-to-Live) : The expiration time, also known as the Time-to-Live (TTL), starts counting down from the moment the data is stored in the cache. Once the TTL expires, the data becomes stale, and subsequent attempts to retrieve it will result in a cache miss.

Automatic Eviction : Memcached periodically checks the expiration time of cached items to determine if they have expired. When an item's expiration time is reached, Memcached automatically evicts the item from the cache to reclaim memory space for new data.

Lazy Expiration : Memcached employs a lazy expiration mechanism, meaning it does not actively check every item's expiration time at all times. Instead, it relies on the access pattern of the data. When a client attempts to access an item, Memcached checks if it has expired. If the item has expired, it is evicted from the cache before returning a cache miss to the client.

Eviction Policy : In addition to expiration-based eviction, Memcached may also employ an eviction policy, such as Least Recently Used (LRU), to handle situations where the cache is full and needs to make room for new data. This policy determines which items should be evicted when additional space is required in the cache.

Renewal : If the client application still needs to keep data in the cache beyond its original expiration time, it can refresh the data by storing it again with a new expiration time. This effectively extends the TTL for the data and prevents it from being evicted prematurely.