Service availability is an extension of high availability that implies a service is always available regardless of hardware, software or user fault.

Key principles of service availability:

• Redundancy - "backup" capability in case of need to failover due to a fault
• Stateful and seamless recovery from failures
• Minimization of Mean time to repair (MTTR) - time to restore service after an outage
• Fault prediction & avoidance - take action before something fails

The traditional definitions of high availability

High availability

High availability is a system design approach and associated service implementation that ensures a prearranged level of operational performance will be met during a contractual measurement period....

have their roots in hardware systems where redundancy of equipment was the primary mechanism for achieving uptime over a specific period. As software has come to dominate the landscape, the probability of failure is often much higher for applications than it is for hardware and so these concepts have been extended encompass an overall view of Service Availability where downtime, irrespective of its cause, is an exceptionally rare event. Services and applications should always be available, whether it is during abnormal system operation, scheduled maintenance, or software upgrade, for example.

External Links

• Service Availability Reporting - A Guide To Service Availability Reporting