Cloud cost
Data egress
The fee a cloud provider charges for data that leaves its network. On most providers, this includes transfer between availability zones.
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Definition
Providers bill data egress per gigabyte. It applies to traffic that leaves the network for the public internet or another cloud. On most providers it also applies to traffic that crosses to a different availability zone in the same region. Egress is one of the least visible lines on a bill because it follows application behavior. A chatty microservice design or an unoptimized data pipeline can build up large egress with no single obvious cause.
Cross-AZ transfer catches teams off guard. Traffic that never leaves the provider's network still pays a per-GB charge when it crosses an availability-zone boundary. This is easy to miss when you design for high availability across zones.
Where it shows up
Data egress appears on an AWS bill under DataTransfer usage types. They split by direction: out to the internet, to another region, or across Availability Zones. Cost Explorer's default Service grouping often folds these into a broader EC2-Other bucket. Switch to the Usage type grouping to see the charge on its own.
What makes it expensive
Splitting a chatty service across Availability Zones buys high availability. It also adds a per-gigabyte bill that nobody priced. That charge bills both legs of a move. One service spread across zones can double the transfer charge for the same request volume. If DataTransfer-Regional-Bytes climbs with request volume and not with any capacity change, the architecture is the cause.
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