Mechanisms and Policies

• One important principle is the separation of policy from mechanism.
  • Mechanisms determine how to do something; policies determine what will be done.
  • For example, the timer construct is a mechanism for ensuring CPU protection, but deciding how long the timer is to be set for a particular user is a policy decision.
• The separation of policy and mechanism is important for flexibility. Policies are likely to change across places or over time.
• For instance, consider a mechanism for giving priority to certain types of programs over others. If the mechanism is properly separated from policy, it can be used to support a policy decision that I/O-intensive programs should have priority over CPU-intensive ones or to support the opposite policy.
• Policy decisions are important for all resource allocation. Whenever it is necessary to decide whether or not to allocate a resource, a policy decision must be made. Whenever the question is how rather than what, it is a mechanism that must be determined.