Read and write capacity units in DynamoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Read Capacity Unit (RCU) in DynamoDB?

A Read Capacity Unit represents one strongly consistent read per second, or two eventually consistent reads per second, for items up to 4 KB in size.

beginner

What does a Write Capacity Unit (WCU) represent in DynamoDB?

A Write Capacity Unit represents one write per second for items up to 1 KB in size.

intermediate

How does item size affect the number of capacity units consumed?

If an item is larger than the base size (4 KB for reads, 1 KB for writes), DynamoDB rounds up and consumes additional capacity units accordingly.

intermediate

What is the difference between strongly consistent and eventually consistent reads in terms of capacity units?

Strongly consistent reads consume double the capacity units compared to eventually consistent reads for the same item size.

beginner

Why is it important to correctly estimate read and write capacity units?

Correct estimation helps avoid throttling, ensures good performance, and controls costs by not over-provisioning or under-provisioning capacity.

How many strongly consistent reads per second does 5 Read Capacity Units allow for items up to 4 KB?

A 20

B 5

C 2.5

D 10

Each Read Capacity Unit allows one strongly consistent read per second for items up to 4 KB.

If you write an item of 3 KB, how many Write Capacity Units are consumed?

A 3

B 2

C 1

D 4

Each WCU covers 1 KB, so 3 KB requires 3 WCUs.

Which type of read consumes fewer capacity units?

A Strongly consistent read

B Both consume the same

C Depends on item size only

D Eventually consistent read

Eventually consistent reads consume half the capacity units of strongly consistent reads.

What happens if your application exceeds provisioned capacity units?

A Requests are queued indefinitely

B Requests succeed without delay

C DynamoDB throttles requests

D DynamoDB increases capacity automatically

Exceeding provisioned capacity causes throttling, which limits request throughput.

Why might you choose on-demand capacity mode instead of provisioned capacity?

A To avoid managing capacity units manually

B To reduce storage costs

C To increase item size limits

D To enable strongly consistent reads only

On-demand mode automatically adjusts capacity, removing the need to manage capacity units.

Explain what Read Capacity Units and Write Capacity Units represent in DynamoDB and how item size affects their consumption.

Think about how many reads or writes per second one unit supports and how bigger items use more units.

Describe why it is important to estimate read and write capacity units correctly when designing a DynamoDB table.

Consider what happens if you set capacity too low or too high.