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DynamoDB Capacity

Links: 102 AWS DVA Index

RCU and WCU are specific to a table.

WCU Calculation

  • One Write Capacity Unit (WCU) represents one write per second for an item up to 1 KB in size.
  • If the items are larger than KB, more WCUs are consumed
  • Example calculations:
    • We write 10 items per second, with item size 2 KB - 2 * 10 = 20 WCUs
    • We write 6 items per second, with item size 4.5 KB - 6 * 5 = 30 WCUs (4.5 gets rounded to the upper KB)
    • We write 120 items per minute, with item size 2 KB - 4 WCUs

Strongly Consistent vs Eventually Consistent Reads

  • When we write to DynamoDB we write to one of the servers and our data is replicated to other servers for redundancy.
  • There is chance that we wrote to server 1 and now we are trying to read from server 2.
    • attachments/Pasted image 20220525120751.jpg
  • Eventually Consistent Read (default)
    • If we read just after a write, it's possible we'll get some stale data because of replication
  • Strongly Consistent Read
    • If we read just after a write, we will get the correct data
    • Set ConsistentRead parameter to True in API calls (GetItem, BatchGetItem, Query, Scan)
    • Consumes twice the RCU.
    • slightly higher latency.
If you want low latency reads then go for eventually consistent reads instead for strongly consistent reads.

RCU Calculation

  • 1 Read Capacity Unit (RCU) represents
    • 1 Strongly Consistent Read per second, or
    • 2 Eventually Consistent Reads per second, for an item up to 4 KB in size
  • If the items are larger than 4 KB, more RCUs are consumed rounded up to the next multiple of 4 KB.
  • Example:
    • 10 Strongly Consistent Reads per second, with item size 4 KB: 10 RCUs
    • 16 Eventually Consistent Reads per second, with item size 12 KB: 24 RCUs
    • 10 Strongly Consistent Reads per second, with item size 6 KB: 20 RCUs (6KB is rounded to 8KB)

DynamoDB Partitions Internal

  • Data is stored in partitions
  • Partition Keys go through a hashing algorithm to know to which partition they go to
    • attachments/Pasted image 20220525121609.jpg
WCUs and RCUs are spread evenly across partitions.

If you have 10 partitions and you provision 10 WCUs and 10 RCUs then they are going to be spread evenly across partitions. This means each partition is going to get 1 WCU and 1 RCU.

An application begins to receive a lot of ProvisionedThroughputExceededException exceptions from a DynamoDB table that you manage. After checking the CloudWatch metrics, you found that you haven't exceeded the total provisioned RCU. What is a possible cause for this?

You have a Hot Partition/Hot Key. Remember RCUs and WCUs are spread across all the table's partitions.

Throttling

  • If we exceed provisioned RCUs or WCUs at the partition level, 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)

Last updated: 2023-02-14