A true enterprise class software system must be built upon an architecture that meets minimum requirements, which are often referred to as the “abilities.” Specifically, these “abilities” are:

• Availability. Availability refers to the ability to provide end user access to a service a high percentage of scheduled time while attempting to reduce unscheduled outages. A solution is highly available if it meets the organization's scheduled uptime goals. Availability goals are achieved by reducing unplanned downtime and then working to improve total hours of service operation.
  
  
• Reliability. Reliability refers to the ability to reduce the frequency of system failure, while attempting to provide fault tolerance in case of failure. A solution is highly reliable if it minimizes the number of single points of failure and reduces the risk that failure of a single component/system will result in the outage of the entire service offering. Reliability goals are achieved using redundant, fault tolerant hardware components, and software systems that can be deployed in a manner that fully utilizes a hardware and network fail-over topology.
  
  
• Scalability. Scalability refers to the ability to add resources and computers while attempting to improve performance. A solution is highly scalable if it can be scaled up and out. Individual systems in a service offering can be scaled up by adding more resources (for example, CPUs, memory, disks). The service can be scaled out by adding additional computers.

  The growing utilization of the Internet has placed enormous performance demands on information systems. Poorly architected systems may demonstrate acceptable performance levels when only 50 or 100 people are using the system, but performance often drops precipitously when the number of concurrent users exceeds 150 to 200 users. To the Customer Service Representative, this means very slow system response and an irritated customer. To the web user (often the consumer or field representative), this means frustratingly slow screens and web browser timeout error messages.

  A well architected system will allow a business to inexpensively match hardware resources (servers and client computers) to performance demands. The result is a system that will scale with utilization demands, allowing you to keep the system’s performance quick and pleasant, no matter how many people are accessing it.

• Flexibility. Flexibility is the ability of the architecture to accommodate changes with minimal disruption to the system’s intrinsic stability; the ability to easily extend the system’s functionality with third party software applications or modules; and the ability to expose the system’s core functionality to outside software applications.