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.

Practice

1. What do read and write capacity units control in a DynamoDB table?

easy

A. The number of reads and writes the table can handle per second

B. The size of the data stored in the table

C. The number of tables in the database

D. The security settings of the table

Solution

1. Step 1: Understand capacity units meaning

   Read and write capacity units define how many read and write operations a DynamoDB table can perform each second.

2. Step 2: Identify what capacity units control

   They control throughput, not storage size, number of tables, or security settings.

3. Final Answer:

   The number of reads and writes the table can handle per second -> Option A

4. Quick Check:

   Capacity units = throughput control [OK]

Hint: Capacity units = max reads/writes per second [OK]

Common Mistakes:

• Confusing capacity units with storage size
• Thinking capacity units control security
• Assuming capacity units limit number of tables

2. Which of the following is the correct way to specify write capacity units when creating a DynamoDB table using AWS CLI?

easy

A. --capacity ReadUnits=5,WriteUnits=10

B. --provisioned-throughput ReadCapacityUnits=5,WriteCapacityUnits=10

C. --throughput ReadCapacity=5 WriteCapacity=10

D. --provisioned-throughput ReadUnits=5 WriteUnits=10

Solution

1. Step 1: Recall AWS CLI syntax for capacity units

   The correct syntax uses --provisioned-throughput with ReadCapacityUnits and WriteCapacityUnits keys.

2. Step 2: Match options with correct syntax

   --provisioned-throughput ReadCapacityUnits=5,WriteCapacityUnits=10 matches the exact syntax; others use incorrect keys or formats.

3. Final Answer:

   --provisioned-throughput ReadCapacityUnits=5,WriteCapacityUnits=10 -> Option B

4. Quick Check:

   Correct CLI syntax = --provisioned-throughput ReadCapacityUnits=5,WriteCapacityUnits=10 [OK]

Hint: Look for 'ReadCapacityUnits' and 'WriteCapacityUnits' keys [OK]

Common Mistakes:

• Using incorrect keys like ReadUnits or WriteUnits
• Missing commas or using wrong separators
• Confusing throughput with capacity keywords

3. A DynamoDB table has 10 write capacity units. If each write item is 2 KB, how many writes per second can the table handle without throttling?

medium

A. 5 writes per second

B. 10 writes per second

C. 20 writes per second

D. 1 write per second

Solution

1. Step 1: Understand write capacity unit size

   One write capacity unit allows one write per second for an item up to 1 KB.

2. Step 2: Calculate writes for 2 KB items

   Each 2 KB item requires 2 write capacity units. With 10 units, 10 / 2 = 5 writes per second.

3. Final Answer:

   5 writes per second -> Option A

4. Quick Check:

   Write capacity units / item size factor = writes/sec [OK]

Hint: Divide capacity units by item size in KB [OK]

Common Mistakes:

• Assuming 1 unit = 2 KB write
• Not dividing capacity units by item size
• Confusing read and write capacity units

4. You set a DynamoDB table's read capacity units to 5 but your application experiences throttling at 3 reads per second. What is the most likely cause?

medium

A. The table is in the wrong AWS region

B. The table has too many write capacity units

C. Each read item is larger than 4 KB, requiring more capacity units

D. The table's storage size is too large

Solution

1. Step 1: Recall read capacity unit size

   One read capacity unit supports one strongly consistent read per second for an item up to 4 KB.

2. Step 2: Analyze throttling cause

   If items are larger than 4 KB, each read consumes multiple units, so 5 units may not support 3 reads per second.

3. Final Answer:

   Each read item is larger than 4 KB, requiring more capacity units -> Option C

4. Quick Check:

   Item size affects read capacity usage [OK]

Hint: Check item size vs 4 KB read unit size [OK]

Common Mistakes:

• Blaming write capacity units for read throttling
• Ignoring item size impact on capacity
• Thinking region or storage size causes throttling

5. You want to optimize costs for a DynamoDB table with variable traffic. Which approach best uses read and write capacity units efficiently?

hard

A. Set capacity units to zero when not using the table

B. Set very high fixed capacity units to avoid throttling

C. Manually increase capacity units once a month

D. Use on-demand capacity mode to automatically adjust units based on traffic

Solution

1. Step 1: Understand capacity modes

   DynamoDB offers on-demand mode that adjusts capacity automatically to traffic, optimizing cost and performance.

2. Step 2: Compare options for cost efficiency

   Setting high fixed units wastes money; manual changes are slow; zero units disables table. On-demand is best for variable traffic.

3. Final Answer:

   Use on-demand capacity mode to automatically adjust units based on traffic -> Option D

4. Quick Check:

   On-demand mode = auto scaling capacity [OK]

Hint: Choose on-demand for variable traffic [OK]

Common Mistakes:

• Setting fixed high capacity wastes money
• Manually adjusting capacity is inefficient
• Setting zero units disables table access