Definition
Replication ( Distributed System)
Replication maintains multiple copies of data or multiple instances of a service on different computers. Replicas represent the same logical resource or service, although their states may temporarily differ under the chosen consistency rules.
Requests can be directed to different replicas to distribute load, use a nearby copy, or continue service after some replicas fail. These benefits depend on suitable request routing and recovery mechanisms, not merely on having copies.
Consistency Cost
Updates must be propagated or coordinated according to the required consistency guarantees. Stronger guarantees may require more communication and waiting, reducing the benefit of serving requests at different replicas.
Replication transparency concerns hiding the copies from clients; replication concerns maintaining them. Caching retains copies near consumers for reuse.