AWS Certified Solutions Architect (SAA-C03) - Resilience & High Availability

High Availability refers to the ability of a system to operate continuously without failure for a long time. In AWS, it often involves distributing resources across multiple Availability Zones (AZs) to withstand single points of failure.

Resilience is the ability of a system to recover from failures and continue to function, even if in a degraded state. It encompasses fault tolerance, disaster recovery, and the ability to adapt to changing conditions.

<ul><li>RTO: The maximum acceptable delay between the interruption of service and restoration of service. It's about time to recover.</li><li>RPO: The maximum acceptable amount of data loss measured in time. It's about data loss tolerance.</li></ul>

This strategy involves backing up data to another region or S3 and restoring it when a disaster occurs. It has the highest RTO and RPO among common DR strategies, as it requires full restoration and data might be lost since the last backup.

In Pilot Light, a minimal version of the application is always running in the DR region (e.g., databases replicated, but no application servers). When a disaster strikes, you "light up" the remaining components (e.g., launch EC2 instances, scale up services) to fully restore functionality. It offers lower RTO/RPO than Backup and Restore.

Warm Standby involves having a scaled-down but fully functional version of your application running in the DR region. It's ready to take on traffic with minimal scaling up. This provides even lower RTO/RPO than Pilot Light, as more resources are already provisioned and running.

This strategy involves running a full-scale, active application in multiple regions simultaneously, serving traffic in both. It offers the lowest RTO and RPO, often near zero, but is the most complex and expensive to implement.

Choose Pilot Light when cost is a significant concern, and you can tolerate a slightly higher RTO/RPO. Warm Standby costs more because more resources are running, but provides faster recovery. Pilot Light is a good balance between cost and recovery time for non-critical applications.