Many developers are shocked when receiving their first DynamoDB bill. The amount often runs 10× or 100× higher than expected for three reasons — RCU/WCU calculation mistakes, wrong choice between on-demand and provisioning modes, and wasted capacity from incomplete work. In this day, we understand RCU/WCU calculation precisely through mathematics, how DAX manages caching, and how DynamoDB Streams connects with Lambda.
RCU (Read Capacity Unit) calculation formula:
Strongly Consistent RCU = ceil(item size / 4KB) × 1
Eventually Consistent RCU = ceil(item size / 4KB) × 0.5
Transactional Read RCU = ceil(item size / 4KB) × 2