High Availability

High Availability is one of the major architectural concerns for any architect. It forms one of the core non-functional technical requirements for any serious service and its deployment. High Availability refers to the feature of a service or application that keeps it operational on a continuous basis, meeting or surpassing its promised defined service level agreement (SLA). Users are promised certain SLA on the availability of a service. The service should be available for consumption based on its SLA. For example, an SLA can have 99% availability for an application for the entire year (assuming 365 days). It means it should be available for consumption by users for 361.35 days. If it goes less than this, there is a breach of SLA. Most mission-critical applications define their High Availability SLA with five nines for a year. It means the application should...