3 Creating Elaborate DynamoDB tables

3.1 Storage consistency

DDB manages capacity

1 WCU (Write Capacity Unit) = 1 write of <=1 kB op per second
- write sizes are rounded up to 1kB: 1 w/sec of 500B counts as 1 WCU
1 RCU (Read Capacity Unit) = 1 or 2 read of <=4 kB ops per second
- read sizes are rounded up to 4kB: 1 r/sec of 500B counts as 1 RCU
- 2 types of reads for 1 RCU:
- eventually consistent: 2 read/sec
- strongly consistent: 1 read/sec

DDB replicates all writes, and uses eventual consistency:

eventual consistency uses the most available replicate, and is good for
- data that may be stale
- apps that do not query soon after writing
strong consistency checks all replicas for the most recent update, and is food for
- data that must be up-to-date
- applications that have a pattern of reading after writing

DDB provides adjustement of WCU/RCU capacity with no downtime, by setting minimum and maximum RCU/WCU expected for a table
To calculate these, we need:
- size of items in kB
- quantity of items read/written per second to the table
- required read consistency
Example: app reads 10 eventually consistent items/sec around 1kB each
- Round up item size to next 4kB -> 4kB = 1 Read Unit (RU)
- RU * rps: 1 * 10 = 10 RCU
- eventual consistency: divide RCU by 2 -> 5 RCU
Example: app reads 10 strongly consistent items/sec around 14kB each
- Round: 14kB -> 16kB = 4 RU
- RU * rps: 4 * 10 = 40 RCU
- strong consistency: don't divide -> 40 RCU
Example: app writes 10 items around (below) 7kB/second
- Round up : 7 -> 7 WU
- WU * rps: 7 * 10 = 70 WCU

Provisioned Autoscaling	On-demande Autoscaling
Predictable traffic patterns	Unpredictable traffic patterns
Pay for provisioned capacity	Pay for amount of request
Cap performand and price	Unlimited performance and price