A service level objective (SLO) subsystem that includes an SLO engine in a network storage system that allows new degrees of freedom to be added to the SLO subsystem, without requiring any modification of an associated SLO engine or the system's existing SLOs, and without requiring the SLO engine to have any a priori knowledge of the new degrees of freedom. In certain embodiments, the system has a modular design which includes the SLO engine and one or more SLO modules that interface with the SLO engine. Each SLO module corresponds to a different degree of freedom. Each SLO module evaluates SLO compliance and determines what specific action(s) within its corresponding degree of freedom should be taken to comply with an SLO.