On-demand vs provisioned cost comparison in DynamoDB - Practice Questions

Challenge - 5 Problems

❓ query_result

intermediate

Calculate monthly cost for On-demand mode

You have a DynamoDB table in On-demand mode. It receives 1 billion read requests and 500 million write requests per month. Reads cost $1.25 per million and writes cost $1.25 per million. What is the total monthly cost?

A $1,875

B $2,500

C $1,250

D $3,125

💡 Hint

Calculate cost by multiplying requests by cost per million, then add reads and writes.

Reads: (1,000,000,000 / 1,000,000) * $1.25 = 1,000 * $1.25 = $1,250; Writes: (500,000,000 / 1,000,000) * $1.25 = 500 * $1.25 = $625; Total = $1,250 + $625 = $1,875.

❓ query_result

intermediate

Calculate monthly cost for Provisioned mode with auto scaling

A DynamoDB table is in provisioned mode with 1,500 read capacity units (RCUs) and 300 write capacity units (WCUs). Reads cost $0.00013 per RCU-hour and writes cost $0.00065 per WCU-hour. Calculate the monthly cost assuming 30 days.

A $140.40

B $280.80

C $560.00

D $468.00

💡 Hint

Calculate hourly cost for reads and writes, then multiply by hours in 30 days (720 hours).

Reads: 1,500 * $0.00013 = $0.195/hr; Writes: 300 * $0.00065 = $0.195/hr; Total/hr = $0.39; Monthly = $0.39 * 720 = $280.80.

🧠 Conceptual

advanced

Identify the main cost difference between On-demand and Provisioned modes

Which statement best describes the main cost difference between DynamoDB On-demand and Provisioned capacity modes?

A On-demand requires capacity planning, Provisioned automatically scales with usage

B On-demand charges a fixed monthly fee, Provisioned charges per request

C On-demand charges per request, Provisioned charges per capacity unit regardless of usage

D On-demand is cheaper for high traffic, Provisioned is cheaper for low traffic

💡 Hint

Think about how billing works for each mode.

On-demand mode bills you for each read/write request you make. Provisioned mode bills you for the capacity units you reserve, whether you use them or not.

📝 Syntax

advanced

Identify the error in a cost calculation query

Given this SQL-like pseudocode to calculate monthly cost for provisioned mode:
SELECT (RCU * 0.00013 + WCU * 0.00065) * 720 AS monthly_cost FROM table_capacity;
What is the main issue with this query?

DynamoDB

SELECT (RCU * 0.00013 + WCU * 0.00065) * 720 AS monthly_cost FROM table_capacity;

A It lacks a GROUP BY clause to aggregate multiple rows

B It assumes 720 hours per month which may not be accurate

C It does not multiply by the number of days in the month

D It incorrectly adds RCU and WCU costs before multiplying by hours

💡 Hint

Consider if the query aggregates multiple rows or not.

If table_capacity has multiple rows, the query will return multiple monthly_cost values instead of a total. A GROUP BY or aggregation is needed to sum costs.

❓ optimization

expert

Optimize cost for a variable traffic DynamoDB table

You manage a DynamoDB table with highly variable traffic: low usage at night and high during the day. Which strategy best optimizes cost while ensuring performance?

A Use On-demand mode to pay only for requests made

B Use Provisioned mode with fixed capacity to avoid scaling delays

C Use On-demand mode with reserved capacity units for peak hours

D Use Provisioned mode with auto scaling to adjust capacity dynamically

💡 Hint

Think about balancing cost and performance with changing traffic.

Provisioned mode with auto scaling adjusts capacity based on traffic, saving cost during low usage and providing capacity during peaks.

Practice

1. Which DynamoDB billing mode charges you only for the read and write requests you actually use, without any upfront capacity planning?

easy

A. Provisioned mode

B. Reserved mode

C. On-demand mode

D. Manual mode

Solution

1. Step 1: Understand billing modes in DynamoDB

   DynamoDB offers two main billing modes: on-demand and provisioned. On-demand charges per request without pre-allocating capacity.

2. Step 2: Identify the mode with no upfront capacity planning

   On-demand mode does not require capacity planning and bills you based on actual usage.

3. Final Answer:

   On-demand mode -> Option C

4. Quick Check:

   On-demand charges per request = On-demand mode [OK]

Hint: On-demand means pay per request, no setup needed [OK]

Common Mistakes:

• Confusing provisioned mode as pay-per-request
• Thinking reserved mode exists in DynamoDB billing
• Assuming manual mode is a billing option

2. Which of the following is the correct way to specify provisioned capacity in a DynamoDB table creation command?

easy

A. ProvisionedThroughput: { ReadCapacityUnits: 5, WriteCapacityUnits: 5 }

B. BillingMode: 'ON_DEMAND'

C. CapacityUnits: { Read: 5, Write: 5 }

D. Throughput: { Read: 5, Write: 5 }

Solution

1. Step 1: Recall DynamoDB table creation syntax for provisioned mode

   Provisioned mode requires specifying ProvisionedThroughput with ReadCapacityUnits and WriteCapacityUnits.

2. Step 2: Match the correct syntax

   ProvisionedThroughput: { ReadCapacityUnits: 5, WriteCapacityUnits: 5 } uses the exact correct keys: ProvisionedThroughput with ReadCapacityUnits and WriteCapacityUnits.

3. Final Answer:

   ProvisionedThroughput: { ReadCapacityUnits: 5, WriteCapacityUnits: 5 } -> Option A

4. Quick Check:

   Provisioned capacity uses ProvisionedThroughput [OK]

Hint: Provisioned capacity uses ProvisionedThroughput keys [OK]

Common Mistakes:

• Using BillingMode for provisioned capacity
• Incorrect key names like CapacityUnits or Throughput
• Confusing ON_DEMAND with provisioned syntax

3. Consider a DynamoDB table with provisioned capacity of 10 read units and 5 write units. If your application performs 8 strongly consistent reads and 3 writes per second, which billing mode will likely cost less?

medium

A. Cannot determine without data size

B. On-demand mode

C. Both cost the same

D. Provisioned mode

Solution

1. Step 1: Analyze usage vs provisioned capacity

   The app uses 8 reads and 3 writes per second, which is below the provisioned 10 reads and 5 writes capacity.

2. Step 2: Compare cost implications

   Provisioned mode charges fixed capacity regardless of usage, so if usage is steady and below provisioned, it is cheaper than on-demand which charges per request.

3. Final Answer:

   Provisioned mode -> Option D

4. Quick Check:

   Steady low usage < provisioned = Provisioned cheaper [OK]

Hint: If usage steady and below capacity, provisioned is cheaper [OK]

Common Mistakes:

• Assuming on-demand is always cheaper
• Ignoring provisioned capacity limits
• Thinking cost depends only on data size

4. You set a DynamoDB table to provisioned mode with 5 read and 5 write capacity units, but your app suddenly needs 20 reads per second. What is the likely problem and how to fix it?

medium

A. Provisioned capacity too low; increase capacity or switch to on-demand

B. On-demand mode is enabled; switch to provisioned mode

C. Table is in on-demand mode; reduce read requests

D. Provisioned capacity too high; reduce capacity units

Solution

1. Step 1: Identify the mismatch between capacity and usage

   The app needs 20 reads per second but provisioned capacity is only 5 reads, causing throttling.

2. Step 2: Determine the fix

   To fix, increase provisioned capacity or switch to on-demand mode which scales automatically.

3. Final Answer:

   Provisioned capacity too low; increase capacity or switch to on-demand -> Option A

4. Quick Check:

   Low provisioned capacity causes throttling; increase or switch [OK]

Hint: If usage > provisioned, increase capacity or use on-demand [OK]

Common Mistakes:

• Confusing on-demand and provisioned modes
• Reducing capacity when usage is high
• Ignoring throttling symptoms

5. A startup expects unpredictable traffic spikes but wants to control costs. They plan to use DynamoDB. Which billing strategy best balances cost and flexibility?

hard

A. Use provisioned mode with high capacity to handle spikes

B. Use provisioned mode with low capacity and enable auto scaling

C. Use on-demand mode but limit requests manually

D. Use on-demand mode to pay only for actual requests

Solution

1. Step 1: Understand startup needs

   They want to handle unpredictable spikes but also control costs.

2. Step 2: Evaluate billing options

   Provisioned mode with auto scaling adjusts capacity automatically, balancing cost and flexibility better than fixed provisioned or pure on-demand.

3. Step 3: Compare with other options

   High fixed provisioned is costly; on-demand can be expensive at scale; manual limits reduce flexibility.

4. Final Answer:

   Use provisioned mode with low capacity and enable auto scaling -> Option B

5. Quick Check:

   Auto scaling balances cost and spikes well [OK]

Hint: Auto scaling provisioned mode balances cost and spikes [OK]

Common Mistakes:

• Choosing high fixed provisioned capacity unnecessarily
• Assuming on-demand always cheaper for spikes
• Trying to limit requests manually in on-demand mode