Read and write capacity units in DynamoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Calculating Read Capacity Units for Strongly Consistent Reads

You have a DynamoDB table with items of size 3 KB each. You want to perform 100 strongly consistent read operations per second. How many read capacity units (RCUs) do you need to provision?

A 100 RCUs

B 200 RCUs

C 300 RCUs

D 400 RCUs

💡 Hint

Remember that one strongly consistent read capacity unit can read up to 4 KB per second.

Each strongly consistent read capacity unit (RCU) can read up to 4 KB per second. Since each item is 3 KB, one RCU can handle one read per second. For 100 reads per second, you need 100 RCUs. Because the item size is less than 4 KB, each read consumes 1 RCU. So total RCUs = 100 * (3 KB / 4 KB rounded up) = 100 * 1 = 100. The question expects the calculation for strongly consistent reads, so the correct answer is 100 RCUs.

❓ query_result

intermediate

Write Capacity Units for Large Items

You want to write 50 items per second to a DynamoDB table. Each item is 2 KB in size. How many write capacity units (WCUs) do you need to provision?

A 150 WCUs

B 50 WCUs

C 100 WCUs

D 200 WCUs

💡 Hint

One write capacity unit can write up to 1 KB per second.

Each write capacity unit (WCU) can write 1 KB per second. Since each item is 2 KB, each write consumes ceil(2/1) = 2 WCUs. For 50 writes per second, total WCUs = 50 * 2 = 100.

📝 Syntax

advanced

Identify the Error in Capacity Unit Calculation Expression

Which of the following expressions correctly calculates the number of read capacity units needed for eventually consistent reads of 150 items per second, each 5 KB in size?

A RCUs = 150 * CEIL(5 / 4) / 2

B RCUs = 150 * CEIL(5 / 4) * 2

C RCUs = 150 / 2 * CEIL(5 / 4)

D RCUs = CEIL(150 * 5 / 4) / 2

💡 Hint

Eventually consistent reads consume half the RCUs of strongly consistent reads.

For eventually consistent reads, the formula is: RCUs = (number_of_reads * ceil(item_size/4 KB)) / 2. Option A correctly applies this formula. Option A multiplies by 2 instead of dividing. Option A changes the order incorrectly. Option A divides after ceiling, which is incorrect.

🔧 Debug

advanced

Debugging Provisioned Capacity Calculation for Mixed Reads

A developer wrote this formula to calculate total RCUs for 100 strongly consistent reads and 200 eventually consistent reads per second, each item 3 KB:

total_RCU = (100 + 200) * CEIL(3 / 4) / 2

What is the problem with this formula?

DynamoDB

total_RCU = (100 + 200) * CEIL(3 / 4) / 2

A It should add 2 to the reads before multiplying

B It incorrectly divides the sum of reads by 2, applying eventually consistent factor to all reads

C It should use item size 4 KB instead of 3 KB

D It should multiply by 2 instead of dividing by 2

💡 Hint

Strongly consistent reads consume full RCUs, eventually consistent reads consume half.

The formula divides the total reads by 2, which incorrectly applies the eventually consistent read factor to both strongly and eventually consistent reads. The correct approach is to calculate RCUs separately for each type and then sum them.

🧠 Conceptual

expert

Optimizing Capacity Units for Variable Item Sizes

You have a DynamoDB table with items of varying sizes: 50% are 1 KB, 30% are 3 KB, and 20% are 7 KB. You expect 100 writes per second. How should you provision write capacity units (WCUs) to minimize cost while avoiding throttling?

A Provision WCUs based on median item size (3 KB) for all writes: 100 * 3 = 300 WCUs

B Calculate weighted average item size and provision WCUs accordingly: (0.5*1 + 0.3*3 + 0.2*7) * 100 = 3.2 * 100 = 320 WCUs

C Provision WCUs based on the smallest item size (1 KB) for all writes: 100 * 1 = 100 WCUs

D Provision WCUs based on the largest item size (7 KB) for all writes: 100 * 7 = 700 WCUs

💡 Hint

DynamoDB charges WCUs per 1 KB chunk per write. Underprovisioning causes throttling.

Because item sizes vary, and DynamoDB charges WCUs per 1 KB chunk, you must provision capacity for the largest item size to avoid throttling. Using averages or medians risks underprovisioning and throttling. So provisioning 700 WCUs (100 writes * 7 KB) is safest.

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