Multi Region Bandwidth, Replication Costs, and Sensitivity Analysis

Multi region architectures introduce bandwidth and latency costs that dramatically affect both user experience and operational expenses. Inter region round trip times typically range from 50 to 100ms depending on geographic distance (US East to US West approximately 60ms, US to Europe approximately 80 to 100ms, US to Asia 150 to 200ms). For synchronous write replication, you must add at least one WAN RTT to every write operation. If your single region write latency is 20ms and you add synchronous replication to two additional regions, write latency becomes 20ms plus 100ms equals 120ms assuming sequential replication, or 20ms plus 100ms in parallel if all regions write concurrently. This often forces architects to choose async replication with RPO greater than zero to maintain write latency SLOs below 50ms. Cross region bandwidth costs for replication add up quickly at scale. If your application generates 10 GB per second of write traffic and you replicate to 2 additional regions, inter region bandwidth is 10 GB per second times 2 equals 20 GB per second. At cloud provider egress pricing of $0.02 to $0.05 per GB (region dependent), costs are $0.40 to $1.00 per second or roughly $35,000 to $86,000 per day just for replication bandwidth. For streaming services, a single live event with 100,000 concurrent viewers watching 1080p at 5 Mbps generates 600 Gbps (75 GB per second). If served from CDN edge for 3 hours at $0.02 to $0.05 per GB, egress costs are approximately $6,000 to $15,000 per event. These calculations directly inform decisions about private CDN builds, direct peering agreements, or dedicated interconnects when traffic volumes cross economic thresholds. Sensitivity analysis validates assumptions by varying key parameters by 2x and 10x to identify design breakpoints. If your calculation assumes 70 percent cache hit rate, rerun with 35 percent and 7 percent. If database load becomes unmanageable at 35 percent hit rate, your design is fragile. Similarly, if average object size assumption is 1 KB, test with 2 KB and 10 KB. Storage and bandwidth costs scale linearly, but if 10 KB objects push you over instance memory limits or network interface capacity, the design requires rearchitecture. Production teams at Google, Amazon, and Meta routinely stress test assumptions with 2x to 10x multipliers across traffic, data size, and fan out factors to ensure designs survive unexpected growth or shifts in user behavior without emergency redesigns.

Multi region write latency: Application in US East with synchronous replication to US West (60ms RTT) and Europe (100ms RTT). Parallel replication adds 100ms to writes (max of regional RTTs). Single region write of 20ms becomes 120ms end to end, exceeding 100ms SLO and forcing switch to async replication with 30 second RPO.

Netflix CDN economics: Streaming 10 petabytes per month at $0.03 per GB costs $300,000 monthly. Netflix Open Connect CDN program places edge caches in ISP datacenters to avoid this egress, reducing per gigabyte costs to near zero via direct peering agreements at sustained traffic levels exceeding 100 Gbps.

Sensitivity analysis revealing bottleneck: Social feed design assumes 500 average followers. At 2x (1,000 followers), write amplification doubles storage from 200 GB per day to 400 GB per day (manageable). At 10x (5,000 followers), write amplification reaches 2 TB per day and per shard write QPS exceeds capacity of 15,000 writes per second, requiring rearchitecture to hybrid fan out model before launch.