Cost estimation for access patterns in DynamoDB - Step-by-Step Execution

Concept Flow - Cost estimation for access patterns

Start: Define Access Pattern

↓

Identify Read/Write Type

↓

Calculate Units Needed

↓

Multiply by Operation Count

↓

Sum Total Units

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Estimate Cost Based on Units

↓

End

This flow shows how to estimate DynamoDB costs by defining access patterns, calculating read/write units, and summing them to find total cost.

Execution Sample

DynamoDB

ReadCapacityUnits = 5
WriteCapacityUnits = 2
ReadsPerSecond = 10
WritesPerSecond = 4
TotalReadUnits = ReadsPerSecond * ReadCapacityUnits
TotalWriteUnits = WritesPerSecond * WriteCapacityUnits
TotalUnits = TotalReadUnits + TotalWriteUnits

Calculates total read and write capacity units consumed per second based on access pattern.

Execution Table

Step	Variable	Value	Calculation/Action	Explanation
1	ReadCapacityUnits	5	Defined	Set read units per operation
2	WriteCapacityUnits	2	Defined	Set write units per operation
3	ReadsPerSecond	10	Defined	Number of read operations per second
4	WritesPerSecond	4	Defined	Number of write operations per second
5	TotalReadUnits	50	10 * 5	Total read units consumed per second
6	TotalWriteUnits	8	4 * 2	Total write units consumed per second
7	TotalUnits	58	50 + 8	Sum of read and write units
8	CostEstimation	$X	TotalUnits * UnitPrice	Multiply units by price per unit

💡 Calculation ends after total units and cost estimation are computed.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	Final
ReadCapacityUnits	undefined	5	5	5	5	5	5	5
WriteCapacityUnits	undefined	undefined	2	2	2	2	2	2
ReadsPerSecond	undefined	undefined	undefined	10	10	10	10	10
WritesPerSecond	undefined	undefined	undefined	undefined	4	4	4	4
TotalReadUnits	undefined	undefined	undefined	undefined	undefined	50	50	50
TotalWriteUnits	undefined	undefined	undefined	undefined	undefined	undefined	8	8
TotalUnits	undefined	undefined	undefined	undefined	undefined	undefined	undefined	58

Key Moments - 3 Insights

Why do we multiply ReadsPerSecond by ReadCapacityUnits?

Because each read operation consumes a certain number of capacity units, multiplying gives total units consumed per second (see execution_table row 5).

What happens if WritesPerSecond increases?

TotalWriteUnits will increase proportionally, raising TotalUnits and cost (see execution_table rows 6 and 7).

Why do we sum TotalReadUnits and TotalWriteUnits?

Because total consumed capacity is the sum of read and write units, which determines overall cost (see execution_table row 7).

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of TotalReadUnits at step 5?

A 10

B 5

C 50

D 58

💡 Hint

Check the 'Value' column at step 5 in the execution_table.

At which step is TotalUnits calculated?

A Step 6

B Step 7

C Step 4

D Step 8

💡 Hint

Look for the step where TotalUnits is assigned in the execution_table.

If WritesPerSecond doubles, what happens to TotalWriteUnits?

A It doubles

B It stays the same

C It halves

D It becomes zero

💡 Hint

Refer to how TotalWriteUnits is calculated in execution_table row 6.

Concept Snapshot

Cost estimation in DynamoDB:
- Define read/write capacity units per operation
- Multiply by number of operations per second
- Sum read and write units for total
- Multiply total units by unit price for cost
- Adjust for access pattern changes

Full Transcript

This visual execution trace shows how to estimate DynamoDB costs based on access patterns. We start by defining the read and write capacity units per operation, then specify how many read and write operations happen each second. We multiply these to find total read and write units consumed per second. Adding these gives total capacity units used. Finally, multiplying total units by the price per unit estimates the cost. This step-by-step calculation helps understand how access patterns affect DynamoDB costs.

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