| Category | Single‑AZ | Multi‑AZ (Single Region) | Multi‑Region |
|---|---|---|---|
| Availability Level | Low – No AZ fault tolerance | High – Survives AZ failure | Very High – Survives region failure |
| Fault Tolerance | Instance‑level only | AZ‑level redundancy | Region‑level redundancy |
| Data Replication | Local or single‑node | Synchronous across AZs | Asynchronous or semi‑sync across regions |
| RPO (Recovery Point Objective) | Minutes to hours (backup-based) | Near‑zero (sync replication) | Seconds to minutes (async replication) |
| RTO (Recovery Time Objective) | Hours (manual recovery) | Seconds to minutes (auto failover) | Minutes to hours (regional failover) |
| Latency Between Nodes | Lowest (same AZ) | Low (high‑speed inter‑AZ network) | Highest (cross‑region/geo latency) |
| Service Continuity During Failure | Outage if AZ fails | No major impact – automatic AZ failover | Continues from secondary region after failover |
| Compliance & Data Residency | Basic | Regional compliance only | Full geo‑compliance and DR support |
| Cost | Lowest | Moderate (AZ redundancy) | Highest (duplicate infra across regions) |
| Use Cases | Dev/Test, low‑critical apps | Business‑critical workloads requiring HA | Mission‑critical systems requiring full DR |
| Strengths | Simple & cost‑effective | High availability & zero data loss | Maximum resilience & geography‑level protection |
| Weaknesses | No AZ/Region protection | No region‑level DR | Expensive and more operational complexity |
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