The Cache employs TTL-based cache consistency, patterned after the
Internet's Domain Naming System [19]. Because of this, the
Cache can return stale data. Unfortunately, HTTP, Gopher, and FTP
provide neither a means for owners to specify TTLs, nor a protocol to
pass TTLs from servers to caches and clients . Hence, when the Harvest cache fetches an object from the
object's home, it is forced to assign a default TTL. When the TTL
expires, the cache discards the object. When a cache fetches an object
from a parent or neighbor, it inherits the parent's remaining TTL. In a
future release of Harvest we will implement a heuristic mechanism to set
TTLs based on objects' last modification dates.
For measurements about the Cache's consistency mechanism (and other mechanisms with which we have experimented), see [9].