On-demand vs provisioned cost comparison in DynamoDB - When to Use Which

The Big Idea

What if your database could pay only for what it actually uses, no more, no less?

The Scenario

Imagine running a small online store and trying to guess how many customers will visit each day. You decide to prepare a fixed number of checkout counters (servers) to handle the rush. Sometimes, many customers wait in long lines; other times, counters sit idle.

The Problem

Manually guessing traffic means you either waste money paying for unused capacity or frustrate customers with slow service during busy times. Adjusting capacity constantly is tiring and error-prone.

The Solution

With on-demand and provisioned capacity modes, DynamoDB lets you choose how to pay for database usage. On-demand automatically adjusts to traffic, charging only for what you use. Provisioned lets you set capacity ahead, saving money if traffic is steady and predictable.

Before vs After

✗ Before

Set fixed capacity: 100 reads/sec, 50 writes/sec
Pay for all even if unused

✓ After

Use on-demand mode
Pay only for actual reads and writes
Or provision capacity based on expected load

What It Enables

You can optimize costs and performance by matching your database capacity to real user demand without guesswork.

Real Life Example

A mobile app with unpredictable user spikes uses on-demand mode to handle sudden traffic without downtime or overspending.

Key Takeaways

Manual capacity planning wastes money or causes slowdowns.

On-demand mode charges per actual use, perfect for variable traffic.

Provisioned mode saves cost when traffic is steady and predictable.

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