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Policy Service for Distributed Systems

Damian Marriott

PhD Thesis
Imperial College London
October, 1997

The ever-increasing size and complexity of large distributed systems makes management of the system very difficult. This thesis describes a novel policy notation supported by a policy service which permits flexible evolution of the management system. The policies are interpreted by automated managers and so can be easily modified or changed without shutting down or reprogramming the managers. Policies define the overall strategy of the management system and hence influence its behaviour. Obligation policies specify what activities a manager must perform, and authorisation policies specify what activities a manager is permitted to do. The policy service enables policies to be specified independent of the distributed agents which interpret them, thus enabling dynamic change of policies and reuse of these agents with different policies. Graphical tools are provided for specifying and interactively manipulating (distributing, enabling, disabling and removing) policies. The policy notation can be used to express both high-level (abstract) and more refined low-level (concrete) policies, and support is provided for specifying and maintaining a hierarchical refinement relationship between policies. Examples given in the thesis will show that the notation is widely applicable, to areas such as network management (e.g. traffic control), application management (e.g. licensing) and security (access control). Policies are represented as objects which specify relationships between subjects (managers) and targets (managed objects). Domains are used to group objects to which a policy applies. A policy is specified in terms of subject and target domain scope expressions, with the policy applying between all objects in the resulting sets, thus obviating the need to specify separate policies for individual objects. Changes in domain membership dynamically affect the set of objects to which the policies apply. Policies can have constraints limiting their applicability. Policy objects themselves can have policies specified about them, so that, for instance, authorisation policies can be used to control access to policy objects. The policy service, tools and interpreters are implemented using a CORBA-compliant distributed platform and Tcl/Tk which provides the interpreted and graphical environments.

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