Cloud Egress Cost
The asymmetric pricing where data entering a cloud is free and data leaving is charged, which shapes architecture more than most teams expect.
The asymmetry is deliberate and it is the single most architecture-distorting item in cloud pricing. Ingress is free; egress to the internet is charged per gigabyte; cross-region transfer is charged; and cross-availability-zone transfer is charged in both directions, which is the one that appears in bills without anyone having made a decision.
The consequences follow directly. Data gravity is real: once a large dataset is in a provider, moving it out has a price that grows with the dataset, which is the concrete mechanism behind lock-in far more than any API incompatibility. Multi-cloud architectures that move data between providers pay this continuously rather than once. And chatty microservices spread across zones can spend more on network transfer than on compute, invisibly, because the cost appears in a line item nobody owns.
The design responses that work: keep processing next to the data rather than moving data to processing, use zone-aware routing so calls stay local by default, put a CDN in front of anything high-volume and internet-facing since CDN egress is usually much cheaper than origin egress, compress aggressively on paths that cross a billing boundary, and use private endpoints where they avoid NAT charges.
Regulatory pressure has recently reduced or removed exit fees at several providers, which changes the one-off migration calculation and leaves the ongoing cross-zone and cross-region economics unchanged — those remain the ones that shape day-to-day design.