3.1. Building Resilient Cloud Solutions

This section focuses on the architectural patterns and AWS services required to build systems that are highly available, scalable, and fault-tolerant. We will cover multi-AZ and multi-Region designs, disaster recovery strategies, scaling patterns, and deployment approaches that minimize risk.

What happens when your single-AZ application loses its availability zone? Everything goes down — and you discover your "highly available" architecture was actually a single point of failure wearing a Multi-AZ label. Consider the difference: a system running in one AZ with "plans to add a second" is not highly available. Availability is a property of running systems, not of architecture diagrams.

Think of resilience like a building's structural engineering. A skyscraper doesn't become earthquake-resistant after the earthquake — the resistance is designed in from the foundation. Similarly, you can't bolt on high availability after a production outage exposes your single points of failure. The patterns in this section — N+1 AZ sizing, stateless design, external state management — must be architectural decisions, not afterthoughts.

The key trade-off throughout this section is cost versus recovery capability. A Backup-and-Restore strategy costs almost nothing but accepts hours of downtime. Active-Active costs significantly more but delivers near-zero downtime. Neither is universally "right" — the right choice depends on your RPO, RTO, and budget constraints. How do you decide? By understanding each pattern's mechanics deeply enough to match them to business requirements.