Multi-cloud replication: ongoing egress math
Hot replicas across two clouds means every write triggers an egress charge from the source. For a 100 GB / day write workload (3 TB / month), AWS to Azure costs ~$270 / month in egress alone. Three-cloud active-active triples that and adds cross-region reads.
Two-cloud DR (one-way replication)
| Write volume / mo | AWS to Azure | AWS to GCP | AWS to Cloudflare R2 |
|---|---|---|---|
| 1 TB | $83.70 | $83.70 | $83.70 |
| 10 TB | $911.36 | $911.36 | $911.36 |
| 100 TB | $8,072.94 | $8,072.94 | $8,072.94 |
AWS-side egress dominates all three; the destination ingress is free. R2 advantage emerges only when you also read from the replica.
Active-active read-from-anywhere multiplier
Two-cloud active-active means writes go to both clouds AND reads can land on either. Reads from the secondary cloud trigger egress from that cloud's side. For a workload where 50 percent of reads hit the secondary, you double the egress bill versus single-cloud.
The Cloudflare R2 bridge pattern: keep the source-of-truth in AWS, replicate writes to R2, read all secondary traffic from R2 (zero egress). For a 10 TB / month write + 50 TB / month read workload, R2 cuts the multi-cloud bill from ~$5,800 / month (AWS-to-Azure replicate + 50 TB Azure egress) to ~$910 / month (AWS-to-R2 replicate, R2 reads free).
CDC vs full snapshot replication
For database replication, Change Data Capture (CDC) tools like Debezium, Airbyte, or AWS DMS only transfer the delta. For a typical OLTP workload, deltas are 5 to 20 percent of full snapshots. A 1 TB database with 100 GB / day of writes generates ~3 TB / month CDC versus ~30 TB / month full snapshot replication, a 10x egress cost saving.
See also: AWS to GCP math, Cloudflare R2 zero egress.