Definition
Caching ( Distributed System)
Caching retains copies of remotely obtained data or computation results near their point of use, so that subsequent requests can reuse them without repeating the remote operation.
It is a form of replication motivated by proximity and reuse. Cache contents are often selected by client requests or local policy, rather than by maintaining a predetermined copy of an entire data set. A cache may reside at the client or at a shared intermediary.
Freshness
If the original data changes, a cached copy may become stale. Expiry, validation or invalidation rules determine when it may still be used and when it must be refreshed. Maintaining freshness can require communication that reduces the scalability benefit.