1. A method of operating a cache, including steps of: automatically refreshing a set of objects maintained in said cache in response to an estimate for each particular object in said set that said object is stale wherein aid step of automatically refreshing includes the steps of: for each said object, estimating a probability that said object will be next requested and will also be stale; automatically refreshing a first object in response to said probability that said object will be next requested and will also be stale; wherein said step of estimating includes the steps of: estimating a first probability distribution of client requests for said object; estimating a second probability distribution of server changes to said object; and estimating said probability that said object will be next requested and will also be stale in response to said first probability distribution and said second probability distribution.