On-demand vs provisioned cost comparison in DynamoDB

Introduction

You want to understand how much it costs to use DynamoDB with different billing methods so you can choose the best one for your needs.

When you have unpredictable or variable traffic to your database.

When you have steady and predictable traffic and want to save money.

When you want to avoid paying for unused capacity.

When you want to control costs by setting limits on read/write capacity.

When you want to quickly start without capacity planning.

Syntax

DynamoDB

No direct query syntax; cost comparison is done by analyzing usage and pricing models.

On-demand charges you per request, no capacity planning needed.

Provisioned requires setting read/write capacity units in advance.

Examples

You pay only for what you use, good for unpredictable workloads.

DynamoDB

On-demand pricing: Pay per read/write request unit used.

You pay for the capacity you reserve, good for steady workloads.

DynamoDB

Provisioned pricing: Set read/write capacity units (RCUs/WCUs) ahead.

Sample Program

This example shows how to calculate monthly costs for both pricing models using sample usage and rates.

DynamoDB

-- Example calculation for cost comparison
-- Assume 1 million read requests and 500,000 write requests in a month
-- On-demand pricing (example rates):
-- Read request unit = $0.00025
-- Write request unit = $0.00125
-- Provisioned pricing:
-- 50 RCUs at $0.00013 each per hour
-- 25 WCUs at $0.00065 each per hour

-- Calculate on-demand cost:
SELECT
  1000000 * 0.00025 AS on_demand_read_cost,
  500000 * 0.00125 AS on_demand_write_cost,
  (1000000 * 0.00025) + (500000 * 0.00125) AS on_demand_total_cost;

-- Calculate provisioned cost for 30 days (720 hours):
SELECT
  50 * 0.00013 * 720 AS provisioned_read_cost,
  25 * 0.00065 * 720 AS provisioned_write_cost,
  (50 * 0.00013 * 720) + (25 * 0.00065 * 720) AS provisioned_total_cost;

Important Notes

On-demand is flexible but can be more expensive if usage is high and steady.

Provisioned is cheaper for steady workloads but you pay even if you don't use all capacity.

You can switch between modes based on your usage patterns.

Summary

On-demand charges per request, no setup needed.

Provisioned requires capacity planning but can save money for steady use.

Choose based on your workload predictability and cost preferences.

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