Situatie
High availability and disaster recovery are frequently discussed together, but they address different infrastructure challenges.
High availability focuses on maintaining service continuity despite the failure of individual components.Disaster recovery focuses on restoring services after disruptive events that exceed the protection provided by normal availability mechanisms.Confusing these objectives can lead to expensive infrastructure designs that still fail during major incidents.
High Availability Architecture
High availability typically relies on redundancy, health monitoring, automated failover, and mechanisms that reduce single points of failure.
Examples include clustered application servers, redundant network devices, load-balanced services, and database availability groups.However, redundant components do not automatically create a resilient system.Two servers connected to the same storage device may remain vulnerable to a shared storage failure.
Similarly, redundant applications depending on one authentication or network service may still experience a common outage.
Understanding Failure Domains
A failure domain represents a collection of components that can be affected by a shared event.Common failure domains include physical hosts, storage systems, network switches, power supplies, data centers, and cloud regions. Infrastructure architects should identify where redundancy actually exists and where dependencies remain shared.Availability design should be based on realistic failure scenarios rather than the number of deployed components.
Disaster recovery architectures may use backup restoration, storage replication, database replication, or standby environments.Synchronous replication can reduce data loss under suitable latency and connectivity conditions, but it may introduce performance and geographic constraints.
Asynchronous replication allows greater flexibility but generally introduces a potential gap between the latest committed production data and the replicated state.Replication must not be confused with backup.Accidental deletions, corrupt data, or malicious changes may also propagate to replicated environments.
Defining RPO and RTO
Recovery Point Objective establishes the maximum acceptable amount of data loss.Recovery Time Objective establishes how quickly business services must be restored.For example, a transaction-processing platform may require tighter recovery objectives than an internal reporting application.These requirements should be defined with business stakeholders and translated into technical architecture.
Failover and Failback Engineering
Failover transfers services to an alternative infrastructure environment.Failback returns operations to the original or newly restored primary environment.Both operations require documented procedures.DNS changes, application configurations, authentication dependencies, database consistency, and network routing must be considered.
Failback can be particularly challenging if production data has changed significantly while services operated in the recovery environment.
Testing Recovery Capability
A disaster recovery plan that has never been tested provides limited assurance.Exercises should verify the complete recovery sequence, including infrastructure startup, application dependencies, data consistency, and user access.
Testing results should be measured against defined recovery objectives.
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