Read and write capacity units in DynamoDB - Time & Space Complexity

Time Complexity: Read and write capacity units

O(n)

Understanding Time Complexity

When working with DynamoDB, it's important to understand how read and write capacity units affect performance.

We want to know how the number of capacity units used grows as we read or write more data.

Scenario Under Consideration

Analyze the time complexity of the following DynamoDB operations.

// Writing multiple items
const items = [...]; // list of items
for (const item of items) {
  await dynamodb.putItem({ TableName: 'MyTable', Item: item });
}

// Reading multiple items
const keys = [...]; // list of keys
const result = await dynamodb.batchGetItem({
  RequestItems: { 'MyTable': { Keys: keys } }
});
    

This code writes many items one by one and reads many items in a batch from DynamoDB.

Identify Repeating Operations

Look for repeated actions that cost time.

• Primary operation: Writing each item individually in a loop.
• How many times: Once per item in the list.
• Secondary operation: Reading multiple items in one batch request.
• How many times: One batch call for all keys.

How Execution Grows With Input

As you add more items, the number of capacity units used grows.

Input Size (n)	Approx. Operations
10	About 10 write units for writes, 1 batch read
100	About 100 write units for writes, 1 batch read
1000	About 1000 write units for writes, 10 batch reads

Writing scales linearly with the number of items because each item costs capacity units. Reading in batch is one operation but capacity units depend on total data size.

Final Time Complexity

Time Complexity: O(n)

This means the capacity units and time grow directly with the number of items you read or write.

Common Mistake

[X] Wrong: "Batch reading many items costs the same as reading one item."

[OK] Correct: Even though batch reads are one request, the capacity units used depend on the total size of all items read, so cost grows with data size.

Interview Connect

Understanding how capacity units scale helps you design efficient DynamoDB usage and shows you can think about performance in real systems.

Self-Check

"What if we switched from writing items one by one to using batch write operations? How would the time complexity change?"

Practice

1. What do read and write capacity units control in a DynamoDB table?

easy

A. The number of reads and writes the table can handle per second

B. The size of the data stored in the table

C. The number of tables in the database

D. The security settings of the table

Solution

1. Step 1: Understand capacity units meaning

   Read and write capacity units define how many read and write operations a DynamoDB table can perform each second.

2. Step 2: Identify what capacity units control

   They control throughput, not storage size, number of tables, or security settings.

3. Final Answer:

   The number of reads and writes the table can handle per second -> Option A

4. Quick Check:

   Capacity units = throughput control [OK]

Hint: Capacity units = max reads/writes per second [OK]

Common Mistakes:

• Confusing capacity units with storage size
• Thinking capacity units control security
• Assuming capacity units limit number of tables

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

easy

A. --capacity ReadUnits=5,WriteUnits=10

B. --provisioned-throughput ReadCapacityUnits=5,WriteCapacityUnits=10

C. --throughput ReadCapacity=5 WriteCapacity=10

D. --provisioned-throughput ReadUnits=5 WriteUnits=10

Solution

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

   The correct syntax uses --provisioned-throughput with ReadCapacityUnits and WriteCapacityUnits keys.

2. Step 2: Match options with correct syntax

   --provisioned-throughput ReadCapacityUnits=5,WriteCapacityUnits=10 matches the exact syntax; others use incorrect keys or formats.

3. Final Answer:

   --provisioned-throughput ReadCapacityUnits=5,WriteCapacityUnits=10 -> Option B

4. Quick Check:

   Correct CLI syntax = --provisioned-throughput ReadCapacityUnits=5,WriteCapacityUnits=10 [OK]

Hint: Look for 'ReadCapacityUnits' and 'WriteCapacityUnits' keys [OK]

Common Mistakes:

• Using incorrect keys like ReadUnits or WriteUnits
• Missing commas or using wrong separators
• Confusing throughput with capacity keywords

3. A DynamoDB table has 10 write capacity units. If each write item is 2 KB, how many writes per second can the table handle without throttling?

medium

A. 5 writes per second

B. 10 writes per second

C. 20 writes per second

D. 1 write per second

Solution

1. Step 1: Understand write capacity unit size

   One write capacity unit allows one write per second for an item up to 1 KB.

2. Step 2: Calculate writes for 2 KB items

   Each 2 KB item requires 2 write capacity units. With 10 units, 10 / 2 = 5 writes per second.

3. Final Answer:

   5 writes per second -> Option A

4. Quick Check:

   Write capacity units / item size factor = writes/sec [OK]

Hint: Divide capacity units by item size in KB [OK]

Common Mistakes:

• Assuming 1 unit = 2 KB write
• Not dividing capacity units by item size
• Confusing read and write capacity units

4. You set a DynamoDB table's read capacity units to 5 but your application experiences throttling at 3 reads per second. What is the most likely cause?

medium

A. The table is in the wrong AWS region

B. The table has too many write capacity units

C. Each read item is larger than 4 KB, requiring more capacity units

D. The table's storage size is too large

Solution

1. Step 1: Recall read capacity unit size

   One read capacity unit supports one strongly consistent read per second for an item up to 4 KB.

2. Step 2: Analyze throttling cause

   If items are larger than 4 KB, each read consumes multiple units, so 5 units may not support 3 reads per second.

3. Final Answer:

   Each read item is larger than 4 KB, requiring more capacity units -> Option C

4. Quick Check:

   Item size affects read capacity usage [OK]

Hint: Check item size vs 4 KB read unit size [OK]

Common Mistakes:

• Blaming write capacity units for read throttling
• Ignoring item size impact on capacity
• Thinking region or storage size causes throttling

5. You want to optimize costs for a DynamoDB table with variable traffic. Which approach best uses read and write capacity units efficiently?

hard

A. Set capacity units to zero when not using the table

B. Set very high fixed capacity units to avoid throttling

C. Manually increase capacity units once a month

D. Use on-demand capacity mode to automatically adjust units based on traffic

Solution

1. Step 1: Understand capacity modes

   DynamoDB offers on-demand mode that adjusts capacity automatically to traffic, optimizing cost and performance.

2. Step 2: Compare options for cost efficiency

   Setting high fixed units wastes money; manual changes are slow; zero units disables table. On-demand is best for variable traffic.

3. Final Answer:

   Use on-demand capacity mode to automatically adjust units based on traffic -> Option D

4. Quick Check:

   On-demand mode = auto scaling capacity [OK]

Hint: Choose on-demand for variable traffic [OK]

Common Mistakes:

• Setting fixed high capacity wastes money
• Manually adjusting capacity is inefficient
• Setting zero units disables table access