Table capacity modes (on-demand vs provisioned) in DynamoDB - Performance Comparison

Time Complexity: Table capacity modes (on-demand vs provisioned)

O(1)

Understanding Time Complexity

When using DynamoDB, how fast your requests run depends on the table's capacity mode.

We want to understand how the choice between on-demand and provisioned modes affects request speed as data grows.

Scenario Under Consideration

Analyze the time complexity of reading an item from a DynamoDB table in different capacity modes.

// Read item from DynamoDB table
const params = {
  TableName: "MyTable",
  Key: { "id": "123" }
};
const result = await dynamodb.getItem(params).promise();
return result.Item;
    

This code fetches one item by its key from a DynamoDB table.

Identify Repeating Operations

Look for repeated steps that affect speed.

• Primary operation: Single key lookup in the table.
• How many times: Once per request; no loops or recursion here.

How Execution Grows With Input

As the table grows, how does the time to get one item change?

Input Size (n)	Approx. Operations
10 items	1 lookup operation
1,000 items	1 lookup operation
1,000,000 items	1 lookup operation

Pattern observation: The time to get one item stays about the same no matter how big the table is.

Final Time Complexity

Time Complexity: O(1)

This means fetching one item takes about the same time regardless of table size.

Common Mistake

[X] Wrong: "On-demand mode is slower because it handles more requests automatically."

[OK] Correct: Both on-demand and provisioned modes use the same fast key lookup; the difference is in how capacity is managed, not speed per request.

Interview Connect

Knowing how DynamoDB handles capacity helps you explain performance choices clearly and shows you understand cloud database behavior.

Self-Check

"What if we changed the query to scan the whole table instead of a key lookup? How would the time complexity change?"

Practice

1. Which DynamoDB table capacity mode automatically adjusts to your application's traffic without requiring manual capacity settings?

easy

A. On-demand mode

B. Provisioned mode

C. Reserved mode

D. Manual mode

Solution

1. Step 1: Understand capacity modes in DynamoDB

   DynamoDB offers two main capacity modes: on-demand and provisioned. On-demand mode automatically adjusts capacity based on traffic.

2. Step 2: Identify the mode that adjusts automatically

   On-demand mode charges per request and scales automatically without manual settings, unlike provisioned mode which requires fixed capacity.

3. Final Answer:

   On-demand mode -> Option A

4. Quick Check:

   Automatic scaling = On-demand mode [OK]

Hint: Automatic scaling means on-demand mode [OK]

Common Mistakes:

• Confusing provisioned mode as automatic
• Thinking reserved or manual modes exist
• Assuming manual capacity adjustment is automatic

2. Which of the following is the correct way to specify provisioned capacity when creating a DynamoDB table using AWS CLI?

easy

A. --billing-mode PAY_PER_REQUEST

B. --provisioned-throughput ReadCapacityUnits=5,WriteCapacityUnits=5

C. --capacity-mode ON_DEMAND

D. --read-write-capacity 10

Solution

1. Step 1: Recall AWS CLI syntax for provisioned capacity

   Provisioned mode requires specifying read and write capacity units using --provisioned-throughput with ReadCapacityUnits and WriteCapacityUnits.

2. Step 2: Match correct syntax option

   --provisioned-throughput ReadCapacityUnits=5,WriteCapacityUnits=5 uses the correct syntax. The --billing-mode PAY_PER_REQUEST and --capacity-mode ON_DEMAND options specify on-demand mode, while --read-write-capacity is invalid.

3. Final Answer:

   --provisioned-throughput ReadCapacityUnits=5,WriteCapacityUnits=5 -> Option B

4. Quick Check:

   Provisioned throughput syntax = --provisioned-throughput ReadCapacityUnits=5,WriteCapacityUnits=5 [OK]

Hint: Provisioned mode uses --provisioned-throughput flag [OK]

Common Mistakes:

• Using PAY_PER_REQUEST flag for provisioned mode
• Confusing capacity mode flags
• Using invalid flags like --read-write-capacity

3. Consider a DynamoDB table in provisioned mode with ReadCapacityUnits=5 and WriteCapacityUnits=5. If your application suddenly sends 20 read requests per second, what will happen?

medium

A. All 20 read requests succeed without delay

B. The table switches to on-demand mode automatically

C. Some read requests may be throttled due to exceeding capacity

D. DynamoDB automatically increases capacity to handle 20 requests

Solution

1. Step 1: Understand provisioned capacity limits

   Provisioned mode sets fixed read/write capacity units. Here, 5 read units allow about 5 strongly consistent reads per second.

2. Step 2: Analyze traffic exceeding capacity

   Sending 20 read requests exceeds the provisioned 5 units. DynamoDB throttles excess requests instead of automatically scaling or switching modes.

3. Final Answer:

   Some read requests may be throttled due to exceeding capacity -> Option C

4. Quick Check:

   Exceeding provisioned units causes throttling [OK]

Hint: Provisioned mode throttles if requests exceed capacity [OK]

Common Mistakes:

• Assuming automatic scaling in provisioned mode
• Thinking all requests always succeed
• Believing table switches modes automatically

4. You created a DynamoDB table with the following command:

aws dynamodb create-table --table-name MyTable --billing-mode PROVISIONED

But you forgot to specify provisioned throughput. What will happen?

medium

A. Table is created with default capacity of 5 read and 5 write units

B. Table is created but capacity is unlimited

C. Table is created in on-demand mode automatically

D. Table creation fails with an error about missing provisioned throughput

Solution

1. Step 1: Check requirements for provisioned mode

   Provisioned mode requires specifying ReadCapacityUnits and WriteCapacityUnits explicitly during table creation.

2. Step 2: Understand AWS CLI behavior on missing parameters

   Omitting provisioned throughput causes AWS CLI to return an error because required parameters are missing.

3. Final Answer:

   Table creation fails with an error about missing provisioned throughput -> Option D

4. Quick Check:

   Missing provisioned throughput causes creation error [OK]

Hint: Provisioned mode needs capacity units specified [OK]

Common Mistakes:

• Assuming default capacity is assigned
• Thinking table switches to on-demand mode
• Believing capacity is unlimited without settings

5. A startup expects unpredictable traffic spikes but wants to control costs during low usage. Which DynamoDB capacity mode strategy is best to balance cost and performance?

hard

A. Use on-demand mode to automatically handle spikes and pay per request

B. Use provisioned mode with low capacity and manually increase during spikes

C. Use provisioned mode with high capacity to handle spikes

D. Switch between on-demand and provisioned modes daily

Solution

1. Step 1: Analyze startup traffic pattern

   Unpredictable spikes mean traffic varies a lot, so automatic scaling is important to avoid throttling or overpaying.

2. Step 2: Evaluate capacity mode options for cost and performance

   On-demand mode charges per request and scales automatically, ideal for unpredictable traffic and cost control. Provisioned mode requires manual adjustments and risks throttling or wasted capacity.

3. Final Answer:

   Use on-demand mode to automatically handle spikes and pay per request -> Option A

4. Quick Check:

   Unpredictable traffic = On-demand mode best [OK]

Hint: Unpredictable traffic? Choose on-demand mode [OK]

Common Mistakes:

• Choosing high provisioned capacity wastes money
• Manually adjusting capacity is slow and error-prone
• Switching modes daily is not supported