The most important consideration in designing the supervisory structure in an ACS is the degree of centralization needed or desired to perform requisite actions. In a fully centralized approach, a single system supervisor continuously assesses the state of every component whose reliability may vary, and and manages the state of every component whose behavior may be altered. This leads to simple control mechanics, but can generate bottlenecks and unacceptable performance. In a fully decentralized approach, each component assesses and manages its own control state, as well as that of other Connected components in the course of its primary operations. This avoids the need for supervision, but requires often-unattainably accurate and complex synchronization and propagation operations to nearly every component in the system.
Hierarchical approaches represent the best and most common middle ground.
Here, each component manages strictly local concerns. Intermediate supervisory components manage groups of components, but are in turn managed by more centralized supervisors. Taking a hierarchical approach hits the worst aspects of full centralization or full decentralization only when they unavoidable due to the nature of the particular supervisory problem at hand. Design Steps Organize monitoring and control facilities into hierarchically partitioned domains. When possible, match this partitioning to Configuration strategies so that control domains mirror the implementation-level decomposition of the system.