Service Qualities

As Internet and e-commerce services have become more critical to business operations, the performance, availability, security, scalability, and serviceability of these services have become key quality-of-service requirements of large-scale, high-performance deployment architectures.

To design a successful software solution, you have to establish relevant quality-of-service requirements and design an architecture that meets those requirements. A number of important service qualities are used to specify quality-of-service requirements. These service qualities are summarized in the following table.

Table 2–2 Service Qualities Impacting Solution Architecture

System Service Qualities	Description
Performance	The measurement of response time and latency with respect to user load conditions.
Availability	A measure of how often a system’s resources and services are accessible to end users (the uptime of a system).
Security	A complex combination of factors that describe the integrity of a system and its users. Security includes physical security of systems, network security, application and data security (authentication and authorization of users), as well as the secure transport of information.
Scalability	The ability to add capacity to a deployed system over time. Scalability typically involves adding resources to the system but should not require changes to the deployment architecture.
Latent capacity	The ability of a system to handle unusual peak load usage without additional resources.
Serviceability	The ease by which a deployed system can be maintained, including monitoring the system, repairing problems that arise, and upgrading hardware and software components.

The quality-of-service dimension strongly impacts a solution’s deployment architecture: how application components and infrastructure components are deployed in a physical environment.

The service qualities that affect deployment architecture are closely interrelated: Requirements for one system quality often affect the design for other service qualities. For example, higher levels of security might affect performance, which in turn might affect availability. Adding additional computers to address availability issues through redundancy often affects maintenance costs (serviceability).

Understanding how service qualities are interrelated and which trade-offs to make is key to designing deployment architectures that satisfy both business requirements and business constraints.