Egress Path Analysis
Tracing where data physically moves in an architecture, because transfer charges follow paths that appear nowhere on the diagram.
Architecture diagrams show logical relationships. Transfer bills follow physical paths: which zone, which region, which provider, and whether traffic left through a NAT gateway, a private endpoint or the public internet.
The exercise is to take each significant data flow and answer four questions. Does it cross an availability zone? Does it cross a region? Does it leave the provider? And does it pass through a NAT gateway, which charges per gigabyte processed on top of everything else?
What this reliably surfaces: a chatty service pair that ended up in different zones, replication factors multiplying a stream's volume by three, a workload calling object storage over NAT instead of a private endpoint, log shipping to a central region, and backups replicated cross-region on a default setting nobody chose.
Do it at design review, when moving a component is a diagram edit. Done after the fact, the same findings are a migration.