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.

This is most commonly discussed in the context of security mechanisms (authentication and authorization), but is actually applicable to a much wider range of resource allocation problems (e.g. CPU scheduling, memory allocation, quality of service), and the general question of good object abstraction.

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. It is considered 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. This means that any hard-coded policies are likely to be inadequate or inappropriate for some (or perhaps even most) potential users. Decoupling the mechanism implementations from the policy specifications makes it possible for different applications to use the same mechanism implementations with different policies. This means that those mechanisms are likely to better meet the needs of a wider range of users, for a longer period of time.

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