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.