HADR

High Availability Disaster Recovery (HADR) is a set of strategies and technologies designed to maintain service availability and data integrity in the face of hardware failures, software faults, or site-wide disasters. By maintaining redundant systems and replicating data to one or more secondary locations, HADR aims to minimize downtime and prevent data loss.

Key concepts include replication (often synchronous or asynchronous), failover and switchover procedures, and defined recovery objectives such as recovery time objective (RTO) and recovery point objective (RPO). HADR environments typically use monitoring, automated failover, fencing to prevent split-brain, and sometimes quorum or witness mechanisms.

Common architectures include active-passive configurations, where a secondary site takes over after a failure, and active-active configurations, where multiple sites serve traffic and replicate changes bidirectionally. Technologies span log shipping, streaming replication, and continuous data replication. HADR is implemented across database platforms such as SAP HANA (system replication), Oracle Data Guard, Microsoft SQL Server Always On, and PostgreSQL streaming replication.

Operational considerations include planning, testing, and regular disaster drills, as well as procedures for switchover, failover, and data reconciliation. While HADR reduces downtime and data loss due to site outages, it is not a substitute for regular backups; independent backups remain necessary for data protection and recovery from data corruption.

Latency, bandwidth, and replication lag can affect RPO and RTO. Complex application workloads, stateful services, and external dependencies require careful architectural design, consistent change control, and thorough documentation.