Multi-Region Architectures: Active-Active vs Active-Passive
Architect for catastrophic disaster recovery: RTO and RPO modeling, Active-Passive automated failover and split-brain fencing, Multi-Region Active-Active with CRDTs and Spanner TrueTime, and automated regional evacuation.
01.1. Disaster Recovery Fundamentals: RTO & RPO
When designing multi-region architectures to withstand catastrophic regional cloud outages (e.g., major power grid blackout, hurricane, fiber cut), two non-negotiable metrics define the system's Disaster Recovery (DR) contract:
1. RTO (Recovery Time Objective)
- Definition: The maximum acceptable duration of system downtime between the moment an outage occurs and the moment full operational capability is restored.
- Target Examples:
- Tier-3 internal tools:
RTO = 4-24 hours. - Standard eCommerce:
RTO = 5-15 minutes. - Mission-critical banking/payments:
RTO β 0 seconds.
- Tier-3 internal tools:
2. RPO (Recovery Point Objective)
- Definition: The maximum acceptable data loss measured in time (i.e., the age of data that may be permanently lost during failover).
- Target Examples:
- Nightly backup restore:
RPO = 24 hoursof lost transactions. - Asynchronous cross-region DB replication:
RPO = 1-5 secondsof in-flight writes. - Synchronous multi-region consensus (Spanner/Raft):
RPO = 0(zero committed data loss).
- Nightly backup restore:
Multi-Region Active-Passive vs Active-Active Topologies π
Multi-Region Active-Passive vs Active-Active Topologies π
Single write primary with standby failover vs concurrent multi-master writes utilizing distributed CRDTs and TrueTime consensus.
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