Amazon DynamoDB - Throughput (RCU & WCU)

Provisioned Mode (default)

• You specify the number of reads/writes per second
• You need to plan capacity beforehand
• Pay for provisioned read & write capacity units

On-Demand Mode

• Read/writes automatically scale up/down with your workloads
• No capacity planning needed
• Pay for what you use, more expensive
• You can switch between different modes once every 24 hours

Write Capacity Units (WCU)

• One Write Capacity Unit (WCU) represents one write per second for an item up to I KB in size
• If the items are larger than I KB, more WCUs are consumed

Example I: we write 10 items per second, with item size 2 KB

• We need 10 * (2 kb / 1 kb ) = 20 WCUs

Example 2: we write 6 items per second, with item size 4.5 KB

• We need 6 * ( 5 kb / 1 kb ) = 30 WCUs (4.5 gets rounded to the upper KB)

Example 3: we write 120 items per minute, with item size 2 KB

• We need (120 / 60 ) * ( 2 kb / 1 kb ) = 4 WCUs

Strongly Consistent Read vs. Eventually Consistent Read

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 (Getltem, BatchGetltem, Query, Scan)
• Consumes twice the RCU

DynamoDB - Read Capacity Units (RCU)

• One Read Capacity Unit (RCU) represents one Strongly Consistent Read per second, or two 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

Example I: 10 Strongly Consistent Reads per second, with item size 4 KB

• We need 10 * (4 kb / 4 kb) = 10 RCUs

Example 2: |6 Eventually Consistent Reads per second, with item size 12 KB

• We need (16 / 2) * ( 12 KB / 4 KB ) = 24 RCUs

Example 3: 10 Strongly Consistent Reads per second, with item size 6 KB

• We need 10 * (8 kb/ 4 kb) = 20 RCUs (we must round up 6 KB to 8 KB)

DynamoDB-Throttling

• If we exceed provisioned CUs 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)