Introduction to high availability¶

High availability systems seek to minimize the following issues:

1. System downtime: Occurs when a user-facing service is unavailable beyond a specified maximum amount of time.
2. Data loss: Accidental deletion or destruction of data.

Most high availability systems guarantee protection against system downtime and data loss only in the event of a single failure. However, they are also expected to protect against cascading failures, where a single failure deteriorates into a series of consequential failures. Many service providers guarantee a Service Level Agreement (SLA) including uptime percentage of computing service, which is calculated based on the available time and system downtime excluding planned outage time.