Replicated directory service for weakly consistent distributed caches

Relais is a replicated directory service that improves Web caching within an organization. Relais connects a distributed set of caches and mirrors, providing the abstraction of a single consistent, shared cache. Relais is based on a replication protocol that exploits the semantics of user requests to guarantee cache coherence. The protocol also exploits the semantics of origin servers to optimize bandwidth requirements. At the entry point for a client, Relais maintains a directory of the contents of each Relais data provider; this minimizes look-up latency, by making it a local operation. The cost of maintaining the directories consistent is minimized thanks to a weak consistency protocol. Relais has been prototyped on top of Squid; it has been in daily use for over a year.