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 a certain SLA based on service type. 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. This means it should be available for consumption by users for 361.35 days. If it becomes less than this, this constitutes a breach of the SLA. Most mission-critical applications define their high-availability SLA with five nines for a year. This means the application should be up, running...