In a similar sense to QoS terminology used in telecommunications, we may assign a set of values to various parameters of Open Distributed System services, such as availability and performance. A set of prescriptive requirements may be imposed on the system excluding certain behaviors merely for policy reasons. For instance some particular set of users may be disallowed from access to some services at certain times. Constraints are similar to policies but apply to the behavior of particular objects in the system, rather than to behavior of the system as a whole. These obligations are not implemented as objects themselves, but may be imposed by some objects within the system.
Persistence is the notion of activity versus passivity.
In conventional systems, the idea of process and data express activity versus passivity. The reader may ask what freedom is left for the systems or applications programmer, when there are so many descriptive and prescriptive properties. Prescriptive properties rule out those not available. Descriptive properties give choice - for example between failure transparency and access transparency constrained by a cost policy. The first seven chapters of this book discuss the design of the mechanisms needed to support these transparencies and describe some of the engineering costs involved in implementations of these mechanisms.