The reliability of the service is how it performs its functions under specified conditions for a specified period of time. Two main requirements should be agreed between all interested parties: availability and recoverability.
The expected availability of the service is the degree to which it is operational and accessible when required for use. Whilst the ideal goal is a consistent 24/7/365 service provision, this is not always a realistic target due to the potential need for disaster recovery, maintenance or upgrades. As well as agreeing on the acceptable levels of (non)availability, contingency plans need also to be agreed in order that operations can continue seamlessly and safely when any downtime is required.
Recoverability concerns how the service is re-established in the event of an interruption or a failure.
Monitoring and measuring these characteristics provides a means to identify where improvements are required in terms of reliability.