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.
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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.