Multi-Region vs Multi-AZ: Real Cost and Benefit Analysis

Introduction

Designing for resilience often begins with a choice between multi-AZ and multi-region architectures. Multi-AZ architectures protect against localized failures, while multi-region designs protect against regional outages but add significant complexity and cost. This analysis helps you quantify the trade-offs.

Failure Domains and Risk Profiles

Multi-AZ

• Protects against datacenter failures.
• Lower latency because traffic stays within a region.
• Easier to manage consistent data replication.

Multi-Region

• Protects against regional outages.
• Higher complexity for data consistency.
• Increased latency due to cross-region routing.

Cost Drivers

• Network egress: Cross-region traffic is significantly more expensive.
• Data replication: Multi-region storage often doubles or triples storage costs.
• Operational overhead: Additional environments mean more monitoring, testing, and deployments.

When Multi-Region is Justified

• Regulatory requirements for geographic redundancy.
• Mission-critical workloads with aggressive availability targets.
• Business risk of regional outage outweighs operational cost.

Quantifying the Cost

This JavaScript example compares annual network costs for multi-AZ vs multi-region traffic replication.

const monthlyTrafficGb = 12000;
const intraRegionCostPerGb = 0.01;
const interRegionCostPerGb = 0.08;

const multiAzAnnual = monthlyTrafficGb * intraRegionCostPerGb * 12;
const multiRegionAnnual = monthlyTrafficGb * interRegionCostPerGb * 12;

console.log(`Multi-AZ annual cost: $${multiAzAnnual.toFixed(2)}`);
console.log(`Multi-region annual cost: $${multiRegionAnnual.toFixed(2)}`);
console.log(`Cost delta: $${(multiRegionAnnual - multiAzAnnual).toFixed(2)}`);

Operational Considerations

• Active-active multi-region requires data conflict resolution.
• Active-passive reduces complexity but increases recovery time.
• Consistent observability across regions is mandatory for incident response.

Decision Matrix

• Choose multi-AZ for most production workloads with high availability needs.
• Choose multi-region when regulatory, financial, or reputational risks justify the complexity.

Conclusion

Multi-region architecture is powerful but expensive. Multi-AZ provides strong resilience for most applications and should be the default unless business requirements demand regional redundancy.