Cost estimation for access patterns in DynamoDB - Mini Project: Build & Apply

Cost Estimation for Access Patterns in DynamoDB

📖 Scenario: You are managing a DynamoDB table for an online bookstore. You want to estimate the cost of different access patterns based on read and write operations.

🎯 Goal: Build a simple DynamoDB cost estimation model that calculates the total read and write capacity units (RCUs and WCUs) needed for given access patterns.

📋 What You'll Learn

Create a dictionary with exact read and write counts for different operations

Add a configuration variable for the cost per capacity unit

Calculate total RCUs and WCUs using the given counts

Calculate the total estimated cost based on capacity units and cost per unit

💡 Why This Matters

🌍 Real World

Estimating costs helps businesses plan budgets and optimize their DynamoDB usage to avoid surprises in billing.

💼 Career

Understanding cost estimation is important for cloud engineers, database administrators, and developers working with AWS DynamoDB.

1

Setup Access Pattern Counts

Create a dictionary called access_counts with these exact entries: 'reads': 1500, 'writes': 300.

DynamoDB

# Create the access_counts dictionary with reads and writes
# Your code here

Hint

Use curly braces to create a dictionary with keys 'reads' and 'writes' and their respective values.

2

Add Cost Per Capacity Unit Configuration

Create two variables: cost_per_rcu set to 0.00013 and cost_per_wcu set to 0.00065.

DynamoDB

access_counts = {'reads': 1500, 'writes': 300}
# Define cost_per_rcu and cost_per_wcu variables
# Your code here

Hint

Assign the exact float values to the variables as shown.

3

Calculate Total Capacity Units

Create two variables: total_rcu assigned from access_counts['reads'] and total_wcu assigned from access_counts['writes'].

DynamoDB

access_counts = {'reads': 1500, 'writes': 300}
cost_per_rcu = 0.00013
cost_per_wcu = 0.00065
# Calculate total_rcu and total_wcu from access_counts
# Your code here

Hint

Use dictionary key access to assign values to total_rcu and total_wcu.

4

Calculate Total Estimated Cost

Create a variable total_cost that calculates the sum of total_rcu * cost_per_rcu and total_wcu * cost_per_wcu.

DynamoDB

access_counts = {'reads': 1500, 'writes': 300}
cost_per_rcu = 0.00013
cost_per_wcu = 0.00065
total_rcu = access_counts['reads']
total_wcu = access_counts['writes']
# Calculate total_cost based on capacity units and cost per unit
# Your code here

Hint

Multiply the counts by their costs and add them to get total_cost.

Practice

1. In DynamoDB, what primarily determines the cost of your database usage?

easy

A. The number of read and write operations per second

B. The number of tables you create

C. The size of your database backup files

D. The number of indexes you define

Solution

1. Step 1: Understand DynamoDB cost factors

   DynamoDB charges mainly based on how many read and write requests you perform per second.

2. Step 2: Identify the main cost driver

   While backups and indexes can affect cost, the primary cost is from read/write capacity usage.

3. Final Answer:

   The number of read and write operations per second -> Option A

4. Quick Check:

   Cost depends on read/write operations [OK]

Hint: Focus on read/write operations for cost estimation [OK]

Common Mistakes:

• Thinking table count directly affects cost
• Confusing backup size with operational cost
• Assuming indexes alone drive cost

2. Which of the following is the correct way to calculate the read capacity units (RCUs) for a strongly consistent read of a 3 KB item in DynamoDB?

easy

A. 4 RCUs per 3 KB read

B. 2 RCUs per 3 KB read

C. 0.5 RCU per 3 KB read

D. 1 RCU per 3 KB read

Solution

1. Step 1: Recall RCU calculation for strongly consistent reads

   One RCU allows reading up to 4 KB per second for strongly consistent reads.

2. Step 2: Calculate RCUs for 3 KB item

   Since 3 KB is less than 4 KB, it requires 1 RCU.

3. Final Answer:

   1 RCU per 3 KB read -> Option D

4. Quick Check:

   Strongly consistent read 4 KB = 1 RCU [OK]

Hint: 1 RCU per 4 KB strongly consistent read [OK]

Common Mistakes:

• Confusing strongly consistent with eventually consistent reads
• Miscalculating item size rounding
• Assuming 1 RCU per KB

3. You have a DynamoDB table with items of size 6 KB. You perform 100 eventually consistent reads per second. How many read capacity units (RCUs) do you need?

medium

A. 150 RCUs

B. 100 RCUs

C. 300 RCUs

D. 200 RCUs

Solution

1. Step 1: Calculate RCUs per read for eventually consistent reads

   One RCU allows reading 8 KB per second for eventually consistent reads, which is half the cost of strongly consistent reads.

2. Step 2: Calculate RCUs needed for 6 KB item

   6 KB item requires 2 RCUs for strongly consistent reads (since 6 KB > 4 KB, round up to 2). For eventually consistent reads, it costs half, so 1 RCU per read.

3. Step 3: Calculate total RCUs for 100 reads

   100 reads x 1 RCU = 100 RCUs.

4. Step 4: Correct calculation

   Since 6 KB is more than 4 KB, for eventually consistent reads, each read consumes ceil(6/4) = 2 RCUs * 0.5 = 1 RCU per read. So total 100 RCUs.

5. Final Answer:

   100 RCUs -> Option B

6. Quick Check:

   6 KB item eventually consistent read = 1 RCU per read [OK]

Hint: Eventually consistent reads cost half RCUs [OK]

Common Mistakes:

• Using strongly consistent read calculation for eventually consistent
• Not rounding up item size to nearest 4 KB
• Multiplying incorrectly by number of reads

4. A developer estimates write capacity units (WCUs) for 1 KB items but forgets to account for item size correctly. The code calculates 1 WCU per write regardless of size. What is the likely issue?

medium

A. Incorrectly calculating read capacity units instead

B. Overestimating WCUs for items smaller than 1 KB

C. Underestimating WCUs for items larger than 1 KB

D. No issue, 1 WCU per write is always correct

Solution

1. Step 1: Understand WCU calculation for item size

   One WCU allows writing 1 KB per second. Larger items require more WCUs, rounded up.

2. Step 2: Identify problem with fixed 1 WCU per write

   If item size > 1 KB, fixed 1 WCU underestimates needed capacity, causing throttling or errors.

3. Final Answer:

   Underestimating WCUs for items larger than 1 KB -> Option C

4. Quick Check:

   WCU depends on item size, fixed 1 WCU is wrong [OK]

Hint: Always multiply WCU by item size in KB rounded up [OK]

Common Mistakes:

• Assuming 1 WCU fits all item sizes
• Confusing read and write capacity units
• Ignoring item size when estimating capacity

5. You want to estimate the monthly cost of a DynamoDB table with 50 writes per second of 2 KB items and 100 strongly consistent reads per second of 3 KB items. The price per WCU is $0.00065 and per RCU is $0.00013. What is the approximate monthly cost (30 days)?

hard

A. $56.16

B. $18.72

C. $28.08

D. $36.00

Solution

1. Step 1: Calculate WCUs needed

   Each 2 KB write requires 2 WCUs (ceil(2/1 KB)). 50 writes/sec x 2 = 100 WCUs.

2. Step 2: Calculate RCUs needed

   Each 3 KB strongly consistent read requires 1 RCU (ceil(3/4 KB)). 100 reads/sec x 1 = 100 RCUs.

3. Step 3: Hourly cost

   WCUs: 100 x $0.00065 = $0.065. RCUs: 100 x $0.00013 = $0.013. Total: $0.078/hr.

4. Step 4: Monthly cost

   $0.078 x 24 hrs/day x 30 days = $56.16.

5. Final Answer:

   $56.16 -> Option A

6. Quick Check:

   Calculate WCUs and RCUs x price x time [OK]

Hint: Multiply capacity units by price and hours in month [OK]

Common Mistakes:

• Not rounding item size to nearest KB
• Mixing hourly and monthly rates
• Ignoring strongly consistent read cost