Cost estimation for access patterns in DynamoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a key factor in estimating DynamoDB costs for access patterns?

The number of read and write capacity units consumed by your queries and operations is the key factor in estimating costs.

beginner

How does the size of an item affect DynamoDB cost estimation?

Larger items consume more capacity units because DynamoDB charges based on the size of data read or written, not just the number of items.

intermediate

What is the difference between strongly consistent and eventually consistent reads in cost?

Strongly consistent reads consume twice the read capacity units compared to eventually consistent reads for the same data size.

beginner

Why is it important to design efficient access patterns in DynamoDB?

Efficient access patterns reduce the number of capacity units consumed, lowering costs and improving performance.

intermediate

How do partition keys influence cost estimation in DynamoDB?

Good partition key design distributes workload evenly across partitions, preventing hot partitions that can increase costs and reduce performance.

Which factor directly affects the number of capacity units consumed in DynamoDB?

A Number of indexes defined

B Number of attributes in the table

C Number of tables in the database

D Size of the data item

DynamoDB charges capacity units based on the size of the data read or written, so item size directly affects cost.

How do strongly consistent reads affect read capacity units compared to eventually consistent reads?

A They consume the same read capacity units

B They consume twice the read capacity units

C They consume half the read capacity units

D They consume no read capacity units

Strongly consistent reads consume twice the read capacity units of eventually consistent reads for the same data size.

What happens if your partition key design causes hot partitions?

A Costs increase and performance may degrade

B No effect on cost or performance

C Costs decrease due to less data access

D DynamoDB automatically fixes hot partitions

Hot partitions cause uneven workload distribution, increasing costs and reducing performance.

Which access pattern helps reduce DynamoDB costs?

A Reading large amounts of unnecessary data

B Using inefficient queries that scan the whole table

C Designing queries to fetch only needed data

D Writing duplicate data multiple times

Fetching only needed data reduces capacity units consumed, lowering costs.

DynamoDB charges for write capacity units based on:

A Size of data written per request

B Number of indexes on the table

C Number of tables in the account

D Number of write requests only

Write capacity units depend on the size of data written, not just the number of requests.

Explain how read and write capacity units affect DynamoDB cost estimation for different access patterns.

Think about how much data you read or write and how often.

Describe why good partition key design is important for cost and performance in DynamoDB.

Imagine a busy road with traffic evenly spread versus all cars on one lane.

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