DynamoDB capacity modes (on-demand, provisioned) in AWS - Time & Space Complexity

Time Complexity: DynamoDB capacity modes (on-demand, provisioned)

O(n)

Understanding Time Complexity

When using DynamoDB, how fast your requests get handled depends on the capacity mode you choose.

We want to understand how the number of requests affects the work DynamoDB does in each mode.

Scenario Under Consideration

Analyze the time complexity of handling multiple read requests in different capacity modes.

// Provisioned mode example
aws dynamodb put-item --table-name MyTable --item '{"ID": {"S": "1"}}'
aws dynamodb update-table --table-name MyTable --provisioned-throughput ReadCapacityUnits=5,WriteCapacityUnits=5

// On-demand mode example
aws dynamodb put-item --table-name MyTable --item '{"ID": {"S": "2"}}'
// No capacity units set, DynamoDB scales automatically
    

This shows putting items into a table with provisioned and on-demand capacity modes.

Identify Repeating Operations

Look at what happens when many requests come in.

• Primary operation: Handling each read or write request.
• How many times: Once per request, repeated for every request.
• Dominant factor: The number of requests determines how many operations DynamoDB must process.

How Execution Grows With Input

As you send more requests, DynamoDB processes more operations.

Input Size (n)	Approx. Api Calls/Operations
10	10 requests handled
100	100 requests handled
1000	1000 requests handled

Each request adds one more operation, so the work grows steadily with the number of requests.

Final Time Complexity

Time Complexity: O(n)

This means the time to handle requests grows directly with how many requests you send.

Common Mistake

[X] Wrong: "On-demand mode handles any number of requests instantly without delay."

[OK] Correct: Even on-demand mode processes each request one by one, so more requests still mean more work and time.

Interview Connect

Understanding how request volume affects DynamoDB helps you design systems that stay fast and reliable as they grow.

Self-Check

"What if we batch multiple requests together? How would the time complexity change?"

Practice

1. What is the main advantage of using DynamoDB's on-demand capacity mode?

easy

A. It offers a lower cost for steady, predictable workloads.

B. It automatically scales to handle any workload without manual capacity planning.

C. It requires you to set fixed read and write capacity units.

D. It limits the number of requests per second to a fixed amount.

Solution

1. Step 1: Understand on-demand mode behavior

   On-demand mode automatically adjusts capacity to meet traffic demands without manual input.

2. Step 2: Compare with provisioned mode

   Provisioned mode requires fixed capacity settings, unlike on-demand.

3. Final Answer:

   It automatically scales to handle any workload without manual capacity planning. -> Option B

4. Quick Check:

   On-demand = auto scaling without setup [OK]

Hint: On-demand means auto scale, no manual setup needed [OK]

Common Mistakes:

• Confusing on-demand with provisioned fixed capacity
• Thinking on-demand limits requests
• Assuming on-demand is cheaper for steady workloads

2. Which of the following is the correct way to specify provisioned capacity in a DynamoDB table creation JSON?

easy

A. { "Throughput": { "Reads": 5, "Writes": 10 } }

B. { "CapacityMode": "ON_DEMAND" }

C. { "ProvisionedThroughput": { "ReadCapacityUnits": 5, "WriteCapacityUnits": 10 } }

D. { "ReadUnits": 5, "WriteUnits": 10 }

Solution

1. Step 1: Recall provisioned capacity syntax

   Provisioned mode requires specifying ReadCapacityUnits and WriteCapacityUnits inside ProvisionedThroughput.

2. Step 2: Check options for correct JSON structure

   { "ProvisionedThroughput": { "ReadCapacityUnits": 5, "WriteCapacityUnits": 10 } } matches the correct AWS DynamoDB API format for provisioned throughput.

3. Final Answer:

   { "ProvisionedThroughput": { "ReadCapacityUnits": 5, "WriteCapacityUnits": 10 } } -> Option C

4. Quick Check:

   ProvisionedThroughput with Read/Write units = correct syntax [OK]

Hint: Provisioned capacity uses ProvisionedThroughput with Read/Write units [OK]

Common Mistakes:

• Using CapacityMode for provisioned capacity
• Missing ProvisionedThroughput wrapper
• Incorrect property names like ReadUnits

3. Given a DynamoDB table in provisioned mode with 5 read capacity units and 5 write capacity units, what happens if your application suddenly sends 20 write requests per second?

medium

A. Some write requests are throttled because they exceed the provisioned write capacity.

B. All 20 write requests succeed immediately without throttling.

C. DynamoDB automatically increases write capacity to 20 units instantly.

D. The table switches automatically to on-demand mode to handle the load.

Solution

1. Step 1: Understand provisioned capacity limits

   Provisioned mode limits throughput to the set capacity units (5 writes per second here).

2. Step 2: Analyze effect of exceeding capacity

   Sending 20 writes exceeds 5 units, causing throttling of excess requests.

3. Final Answer:

   Some write requests are throttled because they exceed the provisioned write capacity. -> Option A

4. Quick Check:

   Provisioned mode throttles excess requests [OK]

Hint: Provisioned mode throttles requests above set capacity [OK]

Common Mistakes:

• Assuming automatic capacity increase in provisioned mode
• Thinking on-demand mode applies automatically
• Believing all requests always succeed

4. You configured a DynamoDB table with provisioned capacity: 10 read units and 10 write units. Your application experiences throttling errors during peak hours. What is the best immediate fix to reduce throttling without downtime?

medium

A. Switch the table to on-demand capacity mode.

B. Reduce the size of items stored in the table.

C. Delete and recreate the table with higher capacity.

D. Increase the provisioned read and write capacity units.

Solution

1. Step 1: Identify cause of throttling

   Throttling occurs because provisioned capacity is too low for peak traffic.

2. Step 2: Choose immediate fix without downtime

   Increasing provisioned capacity units can be done live without deleting the table.

3. Final Answer:

   Increase the provisioned read and write capacity units. -> Option D

4. Quick Check:

   Increase capacity units to reduce throttling [OK]

Hint: Increase provisioned units live to fix throttling fast [OK]

Common Mistakes:

• Deleting and recreating table causes downtime
• Assuming switching to on-demand is instant and free
• Reducing item size does not fix capacity limits

5. A startup expects unpredictable traffic spikes and wants to minimize cost during low usage while avoiding throttling during spikes. Which DynamoDB capacity mode should they choose and why?

hard

A. On-demand mode because it charges per request and scales instantly without manual setup.

B. Provisioned mode with auto scaling enabled to adjust capacity automatically.

C. Provisioned mode with fixed capacity to control costs strictly.

D. On-demand mode but with a fixed maximum request limit to save costs.

Solution

1. Step 1: Understand startup's needs

   They want to avoid throttling during spikes and save cost during low usage.

2. Step 2: Compare capacity modes for unpredictable traffic

   On-demand mode charges per request and auto scales instantly, ideal for unpredictable spikes.

3. Step 3: Evaluate other options

   Provisioned with auto scaling helps but requires setup and may lag; fixed capacity risks throttling or cost waste.

4. Final Answer:

   On-demand mode because it charges per request and scales instantly without manual setup. -> Option A

5. Quick Check:

   Unpredictable traffic = on-demand mode best [OK]

Hint: Unpredictable spikes? Choose on-demand for auto scaling [OK]

Common Mistakes:

• Choosing fixed provisioned capacity for unpredictable traffic
• Assuming auto scaling is instant and always perfect
• Thinking on-demand has fixed request limits