Peering Costs More Than the Database
A company runs a web tier in one cloud region and a reporting service in a second region. The reporting service queries the primary database in region A continuously, transferring about 40 TB per month across regions. A network engineer proposes VPC peering between the two regions to "keep traffic private and cut costs." The monthly bill shows data transfer out from region A at a significant line item, plus per-GB processing on a NAT gateway that the reporting service uses for outbound calls. The security team separately requires that reporting traffic never traverse the public internet. The finance owner wants the transfer line item reduced by at least half within one quarter, and the application owner says the reporting queries cannot be rewritten this quarter.
1. Evaluate whether cross-region peering will reduce the transfer cost, and identify what actually drives the bill.
Read the worked answer
Peering changes the path and privacy of traffic, not the fundamental cross-region data-transfer charge: inter-region transfer is billed regardless of whether it flows over peering or the public internet, so peering alone will not halve the line item. It does satisfy the security requirement that reporting traffic avoid the public internet, which is valuable but separate from cost. The NAT gateway processing charge is a different cost driver: if the reporting service's outbound calls route through NAT, those per-GB charges are avoidable by using private endpoints or gateway endpoints for supported services. The real lever for the 40 TB is reducing cross-region volume-caching, replicating a read-only copy into region B, or aggregating queries-but the application owner says queries cannot change this quarter. So the honest answer is that peering is justified for privacy and may remove NAT charges, but it will not by itself meet the 50% transfer reduction target.
2. Given the constraints, propose a plan that meets the security requirement and makes measurable progress on cost.
Read the worked answer
First, implement private connectivity so reporting traffic stays off the public internet-peering or a private interconnect-and document that this satisfies the security requirement. Second, route supported outbound calls through private or gateway endpoints to eliminate NAT processing charges, which is a real and quick cost win. Third, attack cross-region volume without rewriting queries: stand up a read replica or replicated dataset in region B so reporting reads locally, and cache repeated report results. Even partial local reads cut inter-region transfer proportionally. Track transfer volume and NAT charges separately so progress is measurable against the quarterly target. If the 50% goal still looks unreachable without query changes, present the data to finance and negotiate a phased target rather than promising a number the architecture cannot deliver this quarter.