Cost estimation for access patterns in DynamoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Calculate read capacity units for strongly consistent reads

A DynamoDB table has items of size 3 KB each. You want to perform 100 strongly consistent read operations per second. How many read capacity units (RCUs) are required?

A 100 RCUs

B 300 RCUs

C 200 RCUs

D 400 RCUs

💡 Hint

Each strongly consistent read of up to 4 KB consumes 1 RCU. Reads larger than 4 KB consume additional RCUs proportionally.

Each strongly consistent read can read up to 4 KB per RCU. Since each item is 3 KB, each read consumes 1 RCU. For 100 reads per second, total RCUs = 100 * 1 = 100. But since the item size is 3 KB, which is less than 4 KB, each read consumes 1 RCU. So total is 100 RCUs.

Correction: Actually, the item size is 3 KB, so each read consumes 1 RCU. For 100 reads, 100 RCUs are needed. The correct answer is 100 RCUs.

❓ query_result

intermediate

Write capacity units for batch write operations

You want to write 50 items per second to a DynamoDB table. Each item is 2 KB in size. How many write capacity units (WCUs) do you need to provision?

A 200 WCUs

B 100 WCUs

C 25 WCUs

D 50 WCUs

💡 Hint

Each write capacity unit represents one write per second for items up to 1 KB. Larger items consume more WCUs.

Each write of up to 1 KB consumes 1 WCU. Since each item is 2 KB, each write consumes 2 WCUs. For 50 writes per second, total WCUs = 50 * 2 = 100 WCUs.

❓ optimization

advanced

Optimizing cost for eventually consistent reads

You have a table with items of size 8 KB. You need to perform 120 read operations per second. You can choose between strongly consistent or eventually consistent reads. Which option minimizes your read capacity units (RCUs) cost?

A Use eventually consistent reads, requiring 60 RCUs

B Use strongly consistent reads, requiring 240 RCUs

C Use eventually consistent reads, requiring 120 RCUs

D Use strongly consistent reads, requiring 120 RCUs

💡 Hint

Eventually consistent reads consume half the RCUs of strongly consistent reads for the same item size.

Each read of 8 KB consumes 2 RCUs for strongly consistent reads (8 KB / 4 KB = 2). For 120 reads, total RCUs = 120 * 2 = 240.

Eventually consistent reads consume half the RCUs, so 240 / 2 = 120 RCUs.

However, the option C says 60 RCUs, which is half of 120. This is incorrect. The correct calculation is 120 RCUs for eventually consistent reads.

Correction: Actually, each eventually consistent read consumes half the RCUs of strongly consistent. So for 8 KB item, strongly consistent read consumes 2 RCUs, eventually consistent consumes 1 RCU. For 120 reads, eventually consistent reads require 120 RCUs.

Therefore, option C is correct.

❓ schema

advanced

Choosing partition key to optimize cost

You have a DynamoDB table with a hot partition causing throttling. Which partition key design helps reduce cost by distributing traffic evenly?

A Use a single partition key with high traffic

B Use a partition key based on timestamp only

C Use a partition key with low cardinality

D Use a composite key with a random suffix to spread traffic

💡 Hint

Distributing traffic evenly across partitions reduces throttling and cost.

A single partition key with high traffic causes hot partitions and throttling.

Using a composite key with a random suffix spreads traffic across multiple partitions, reducing throttling and cost.

Low cardinality keys concentrate traffic, increasing throttling.

Timestamp-only keys can cause traffic spikes on recent partitions.

🔧 Debug

expert

Identify the cause of unexpectedly high write costs

A DynamoDB table shows unexpectedly high write capacity usage. The application writes 10 items per second, each 1 KB in size. The table has a global secondary index (GSI) with a write capacity of 50 WCUs. What is the likely cause of the high write costs?

A Writes to the GSI double the write capacity units consumed

B The GSI write capacity is unrelated to the main table writes

C Each write consumes only 1 WCU regardless of GSI presence

D The application is writing items larger than 1 KB

💡 Hint

Writes to a table with GSIs consume write capacity on both the table and the GSIs.

When writing to a table with GSIs, each write consumes WCUs on the main table and on each GSI.

Since the GSI has a write capacity of 50 WCUs, writes to the GSI add to the total write capacity usage.

This causes higher overall write costs than expected from the main table writes alone.