Calculating WCU and RCU

WCU

1 WCU → 1 write per second up to 1kb

Examples:

12 writes per second of 2kb = 12 * (2/1) = 24 WCU needed

24 items per minute of 3kb = (24/60) * (3/1) = 1.2 WCU needed

RCU

Two modes: Eventually consistent or strongly consistent

1 RCU = 1 strongly consistent or 2 eventual consistent of items up to 4kb (You have to round up to nearest 4kb if anything in between)

Examples:

20 reads per second of 3.5kb (strong consistent) = 20 * (4/4) = 20 RCU

20 reads per second of 3.5kb (eventual consistent) = (20/2) * (4/4) =10 RCU

DynamoDB –Throttling

• If we exceed provisioned RCUs or WCUs, we get “ProvisionedThroughputExceededException”
• Reasons:
  • Hot Keys – one partition key is being read too many times (e.g., popular item)
  • Hot Partitions
  • Very large items, remember RCU and WCU depends on size of items
• Solutions:
  • Exponential backoff when exception is encountered (already in SDK)
  • Distribute partition keys as much as possible
  • If RCU issue, we can use DynamoDB Accelerator (DAX)

DynamoDB – Conditional Writes

• For PutItem, UpdateItem, DeleteItem, and BatchWriteItem
• You can specify a Condition expression to determine which items should be modified:
  • attribute_exists
  • attribute_not_exists
  • attribute_type
  • contains (for string)
  • begins_with (for string)
  • ProductCategory IN (:cat1, :cat2) and Price between :low and :high
• Note: Filter Expression filters the results of read queries, while Condition Expressions are for write operations