Application Service Quality Requirements

The key application service quality requirements for a target application should be specified with clear definitions for unambiguous measurement and quantified minimum expectations. This chapter considers the fundamental application service quality performance requirements for application instances. Service availability is the most fundamental quality requirement because if the application is not available to serve users then little else matters. End users typically experience an application one transaction at a time, and the latency between the user's action and the application service's response is a critical service quality characteristic. Service reliability requirements are most conveniently specified as defective (or failed) operations per million attempts, or DPM. Service accessibility metrics consider the probability than any individual user can successfully acquire service on demand. Service retainability is an application specific metric for session-oriented applications. The chapter also discusses timestamp accuracy, elasticity, release management, and disaster recovery requirements.