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

Practice - 5 Tasks

1fill in blank

easy

Complete the code to specify the billing mode as on-demand in the table creation.

DynamoDB

aws dynamodb create-table --table-name Music --attribute-definitions AttributeName=Artist,AttributeType=S --key-schema AttributeName=Artist,KeyType=HASH --billing-mode [1]

Drag options to blanks, or click blank then click option'

AON_DEMAND

BPAY_PER_REQUEST

CPROVISIONED

DSTANDARD

Attempts:

3 left

💡 Hint

Common Mistakes

Using PROVISIONED instead of PAY_PER_REQUEST for on-demand billing mode.

Using ON_DEMAND which is not the correct billing mode string.

The billing mode for on-demand capacity in DynamoDB is specified as PAY_PER_REQUEST.

2fill in blank

medium

Complete the code to set the read capacity units for provisioned mode.

DynamoDB

aws dynamodb update-table --table-name Music --provisioned-throughput ReadCapacityUnits=[1],WriteCapacityUnits=5

Drag options to blanks, or click blank then click option'

A5

B20

C0

D10

Attempts:

3 left

💡 Hint

Common Mistakes

Setting read capacity units to 0 which disables reads.

Setting read capacity units too high without need.

Setting ReadCapacityUnits to 5 matches the write capacity units and is a common balanced setting.

3fill in blank

hard

Fix the error in the cost calculation formula for provisioned mode.

DynamoDB

cost = (read_units * 0.00013 + write_units * 0.00065) * [1]

Drag options to blanks, or click blank then click option'

Ahours_per_month

Bdays_per_month

Cminutes_per_month

Dseconds_per_month

Attempts:

3 left

💡 Hint

Common Mistakes

Using days or minutes instead of hours in the formula.

Forgetting to multiply by time period.

The cost is calculated per hour, so multiply by the number of hours in a month.

4fill in blank

hard

Fill both blanks to complete the on-demand cost calculation formula.

DynamoDB

cost = (read_requests * [1] + write_requests * [2]) / 1000000

Drag options to blanks, or click blank then click option'

A0.25

B1.25

C0.00025

D0.00125

Attempts:

3 left

💡 Hint

Common Mistakes

Using per-request costs like 0.00025 instead of per-million costs.

Mixing up read and write request costs.

On-demand read request units cost $0.25 per million and write request units cost $1.25 per million.

5fill in blank

hard

Fill all three blanks to complete the provisioned throughput cost formula with monthly hours.

DynamoDB

cost = (read_units * [1] + write_units * [2]) * [3]

Drag options to blanks, or click blank then click option'

A0.00013

B0.00065

C720

D730

Attempts:

3 left

💡 Hint

Common Mistakes

Using 720 hours instead of 730 hours for monthly calculation.

Swapping read and write unit costs.

Read units cost $0.00013/hr, write units $0.00065/hr, multiplied by 730 hours in a month.

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