Read capacity unit (RCU) — provisioned capacity mode
• First you need to understand what is read request. Each API call to read data from your table is a read request. • Read requests can be strongly consistent, eventually consistent, or transactional. • For provisioned capacity mode, one RCU supports one strongly consistent read per second for an item up to 4 KB. • Larger items consume additional read capacity in 4 KB increments. • For provisioned capacity mode, one RCU supports two eventually consistent reads per second for an item up to 4 KB. • Transactional reads consume two RCUs for one read per second of an item up to 4 KB. • For example, a strongly consistent read of an 8 KB item would require two RCUs, an eventually consistent read of an 8 KB item would require one RCU, and a transactional read of an 8 KB item would require four RCUs.
EXAMPLE: To determine the number of RCUs required to handle 3 strongly consistent reads per second with an average item size of 7 KB, perform the following steps:
- Determine the average item size by rounding up the next multiple of 4KB (7KB rounds up to 8KB).
- Determine the RCU per item by dividing the item size by 4KB (8KB/4KB = 2).
- Multiply the value from step 2 with the number of reads required per second (2x3 = 6).
Write capacity unit (WCU) — provisioned capacity mode
• Each API call to write data to your table is a write request. • For provisioned capacity mode, one WCU supports one standard write per second for an item up to 1 KB. • Larger items consume additional write capacity in 1 KB increments. • Transactional writes consume two WCUs for one write per second of an item up to 1 KB. • For example, a standard write request of a 1 KB item would require one WCU, a standard write request of a 3 KB item would require three WCUs, and a transactional write request of a 3 KB item would require six WCUs.
EXAMPLE: For one standard write per second of an average 7 KB item, the item size is charged in 1 KB increments, so the requirement is 7 WCUs.
