Read and write capacity units in DynamoDB - Step-by-Step Execution

Concept Flow - Read and write capacity units

Start: Request to DynamoDB

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Check Request Type

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Calculate Read

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Consume Read CUs

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Return Success or Throttle

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End

This flow shows how DynamoDB handles read and write requests by calculating and consuming capacity units, then returning success or throttling.

Execution Sample

DynamoDB

Request: Read 3 items (6 KB each)
Request: Write 2 items (2 KB each)
Calculate Read Capacity Units
Calculate Write Capacity Units
Consume capacity units
Return response

This example calculates how many read and write capacity units are consumed by requests and shows the result.

Execution Table

Step	Request Type	Item Size (KB)	Items Count	Capacity Units Needed	Capacity Units Consumed	Result
1	Read	6	3	6	6	Success
2	Write	2	2	2	2	Success
3	Read	6	3	6	6	Success
4	Write	2	2	2	2	Success
5	Read	6	3	6	6	Success
6	Write	2	2	2	2	Success
7	Read	6	3	6	6	Success
8	Write	2	2	2	2	Success
9	Read	6	3	6	6	Success
10	Write	2	2	2	2	Success
11	Read	6	3	6	6	Throttle
12	Write	2	2	2	2	Throttle

💡 Capacity units exceeded provisioned limits at step 11 and 12, causing throttling.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 10	After Step 12
Read Capacity Units Consumed	0	6	6	30	36
Write Capacity Units Consumed	0	0	2	10	12
Provisioned Read Capacity Units	30	30	30	30	30
Provisioned Write Capacity Units	20	20	20	20	20

Key Moments - 2 Insights

Why does the read request for 3 items of 6 KB each consume 6 read capacity units?

Because DynamoDB calculates strongly consistent read capacity units as ceil(item size in KB / 4) per item. For a 6 KB item: ceil(6/4) = 2 RCUs per item. Therefore, 3 items × 2 RCUs = 6 RCUs.

Why does throttling happen at step 11 and 12 even though each request seems small?

Because the total consumed capacity units have reached the provisioned limits (30 read and 20 write units). After multiple requests, the sum exceeds these limits, causing throttling as shown in execution_table rows 11 and 12.

Visual Quiz - 3 Questions

Look at the execution table, how many read capacity units are consumed at step 5?

A 3

B 6

C 9

D 12

💡 Hint

Check the 'Capacity Units Consumed' column for step 5 in the execution_table.

At which step does throttling first occur according to the execution table?

A Step 9

B Step 10

C Step 11

D Step 12

💡 Hint

Look for the first 'Throttle' result in the 'Result' column of the execution_table.

If each write item size doubled, how would the write capacity units consumed change at step 2?

A It would double

B It would stay the same

C It would halve

D It would quadruple

💡 Hint

Capacity units scale with item size; doubling item size doubles units consumed as seen in variable_tracker.

Concept Snapshot

Read and write capacity units measure how much throughput your DynamoDB requests use.
One read capacity unit = 1 strongly consistent read of 4 KB per second.
One write capacity unit = 1 write of 1 KB per second.
Requests consuming more data use more capacity units.
Exceeding provisioned units causes throttling.
Plan capacity units based on item size and request rate.

Full Transcript

This visual execution shows how DynamoDB calculates and consumes read and write capacity units for requests. Each read or write request consumes capacity units based on item size and consistency. The execution table traces multiple requests consuming capacity units until the provisioned limits are reached, causing throttling. The variable tracker shows how consumed units accumulate over steps. Key moments clarify why capacity units are calculated as they are and why throttling happens. The quiz tests understanding of capacity unit consumption and throttling points. The snapshot summarizes the core rules for read and write capacity units in DynamoDB.