Separation of mechanism and policy

The separation of mechanism and policy is a design principle in computer science. It states that mechanisms (those parts of a system implementation that control the authorization of operations and the allocation of resources) should not dictate (or overly restrict) the policies according to which decisions are made about which operations to authorize, and which resources to allocate. While most commonly discussed in the context of security mechanisms (authentication and authorization), separation of mechanism and policy is applicable to a range of resource allocation problems (e.g. CPU scheduling, memory allocation, quality of service) as well as the design of software abstractions. Per Brinch Hansen introduced the concept of separation of policy and mechanism in operating systems in the RC 4000 multiprogramming system. Artsy and Livny, in a 1987 paper, discussed an approach for an operating system design having an "extreme separation of mechanism and policy". In a 2000 article, Chervenak et al. described the principles of mechanism neutrality and policy neutrality. The separation of mechanism and policy is the fundamental approach of a microkernel that distinguishes it from a monolithic one. In a microkernel, the majority of operating system services are provided by user-level server processes.Raphael Finkel, Michael L. Scott, Artsy Y. and Chang, H. [www.cs.rochester.edu/u/scott/papers/1989_IEEETSE_Charlotte.pdf Experience with Charlotte: simplicity and function in a distributed operating system]. IEEE Trans. Software Engng 15:676-685; 1989. Extended abstract presented at the IEEE Workshop on Design Principles for Experimental Distributed Systems, Purdue University; 1986. It is important for an operating system to have the flexibility of providing adequate mechanisms to support the broadest possible spectrum of real-world security policies. It is almost impossible to envision all of the different ways in which a system might be used by different types of users over the life of the product.