Availability is the time frame in which the system functions normally and without failures. Availability is measured as the percentage of total application downtime over a defined time period. Availability is affected by failures, exceptions, infrastructure issues, malicious attacks, and maintenance and upgrades.

It is the uptime or the amount of time the system is operational and available for use. This is specified because some systems are architected with expected downtime for activities like database upgrades and backups.

Availability also conveys the number of hours or days per week or weeks per year the application will be available to its end customers, as well as how rapidly it can recover from faults. Since the architecture establishes software, hardware, and networking entities, this requirement extends to all of them. Hardware availability, recoverability, and reliability definitions measure system uptime.

For example, it is specified in terms of Mean Time Between Failures (MTBF).

The following attributes are:

• Availability: Application availability considering the weekends, holidays, and maintenance times and failures
• Locations of operation: Geographic location, connection requirements, and if the restrictions of the network prevail
• Offline requirement: Time available for offline operations including batch processing and system maintenance
• Length of time between failures: This is the predicted elapsed time between inherent failures of a system during operation
• Recoverability: Time required by the system to resume operation in the event of failure
• Resilience: The reliability characteristics of the system and sub-components