Trước hết, trong một chính sách duy nhất có thường là một tập hợp các lớp hoặc gây nên quan hệ với nhau bởi những quan hệ ưu tiên. Những mối quan hệ này hạn chế các thứ tự mà trong đó các chính sách có thể được áp dụng, và các đồ thị này phải được phân tích cú pháp. | 246 CHAPTER 7. CONFIGURATION AND MAINTENANCE First of all within a single policy there is often a set of classes or triggers which are interrelated by precedence relations. These relations constrain the order in which policies can be applied and these graphs have to be parsed. A second way in which scheduling enters is through the response of the configuration system to arriving events. Should the agents activate once every hour in order to check for policy violations or immediately should they start at random times or at predictable times Should the policies scheduled for specific times of day occur always at the same times of day or at variable times perhaps random. This decision affects the predictability of the system and thus possibly its security in a hostile encounter. Finally although scheduling is normally regarded as referring to extent over time a distributed system also has two other degrees of spatial extent h and c. Scheduling tasks over different hosts or changing the details of software components is also a possibility. It is possible to confound the predictability of software component configuration to present a moving target to would-be attackers. The challenge is to accomplish this without making the system nonsensical to legitimate users. These are the issues we wish to discuss below. A set of precedence relations can be represented by a directed graph G V E containing a finite nonempty set of vertices V and a finite set of directed edges E connecting the vertices. The collection of vertices V V1 v2 . vn represents the set of n policies to be applied and the directed edges E eij define the precedence relations that exist between these policies eij denotes a directed edge from policy vi to Vj . This graph can be cyclic or acyclic. Cyclic graphs consist of inter-cycle and intra-cycle edges where the inter-cycle edges are dependencies within a cycle and intra-cycle edges represent dependencies across cycles. When confronted with a cyclic graph then
