Separation of mechanism from policy

The Separation of mechanism¹ and policy is a fundamental design principle in the field of 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 (authentication and authorization) mechanisms, 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.

Artsy, in a 1987 paper, discussed an approach for an operating system design having an "extreme separation of mechanism and policy".⁴⁵

In a 2000 article, Chervenak et al described the principles of mechanism neutrality and policy neutrality.⁶

Rationale and Implications

The separation of mechanism and policy is the fundamental approach of a micro kernel that distinguishes it from a monolithic one. In a microkernel the majority of operating system services are provided by user-level server processes.⁷ 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.⁸

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) 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.

There is always a cost, and a risk associated with changes to any system implementation. If it is possible to enable new policies without changing the implementing mechanisms, the costs and risks of such policy changes can be greatly reduced. This could be accomplished merely by segregating mechanisms and their policies into distinct modules, but in cases where a wide or variable range of policies are anticipated, it makes sense to create some non-code means for specifying policies. We might, for instance, parametrize our policies (e.g. UNIX user/group/other read/write/execute file protections). Alternatively we could design our implementing mechanisms to include an interpreter for a new policy specification language. These are usually accompanied by a deferred binding mechanism (e.g. configuration files, or APIs) that permits policy specifications to be added to the system after it has been delivered to the customer.

A good every-day example of Mechanism/Policy Separation is the use of card-keys to gain access to locked doors. The mechanisms (magnetic card readers, remote controlled locks, connections to a security server) do not impose any limitations on entrance policy (which people should be allowed to enter which doors, at which times). These decisions are made by a centralized security server, which (in turn) probably makes its decisions by consulting a database of room access rules. Specific authorization decisions can be changed by updating a room access database. If the rule schema of that database proved too limiting, the entire security server could be replaced while leaving the fundamental mechanisms (readers, locks, and connections) unchanged. This architecture achieves a high degree of mechanism/policy separation

Notes

^ Butler W. Lampson and Howard E. Sturgis. Reflections on an Operating System Design [1] Communications of the ACM 19(5):251-265 (May 1976)
^ Wulf 74 pp.337-345
^ Brinch Hansen 70 pp.238-241
^ Miller, M. S., & Drexler, K. E. (1988). Markets and computation: Agoric open systems. In Huberman (1988), pp. 133{176. (Huberman, B. A. (Ed.). (1988). The Ecology of Computation. North-Holland.)
^ Artsy, Yeshayahu et al, 1987
^ Chervenak 2000 p.2
^ 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.
^ R. Spencer, S. Smalley, P. Loscocco, M. Hibler, D. Andersen, and J. Lepreau The Flask Security Architecture: System Support for Diverse Security Policies In Proceedings of the Eighth USENIX Security Symposium, pages 123–139, Aug. 1999.

Contents

Rationale and Implications

Notes

References

See also

External links