A developer is designing an online bookstore's catalog system where inventory updates and … — AWS Certified Developer - Associate

A developer is designing an online bookstore's catalog system where inventory updates and search queries must be highly optimized. The application writes $40$ book metadata updates per second to an Amazon DynamoDB table, with each write payload size averaging $2.7\text{ KB}$ . To support real-time inventory dashboards, the application runs $15$ strongly consistent `Query` operations per second. Each `Query` returns a list of $12$ book items associated with a specific publisher (the partition key), and each book item has an average size of $1.5\text{ KB}$ . Which of the following represents the minimum provisioned read capacity units (RCUs) and write capacity units (WCUs) required to support this workload?

$75\text{ RCUs}$ and $120\text{ WCUs}$ Answer
B
$180\text{ RCUs}$ and $120\text{ WCUs}$
C
$38\text{ RCUs}$ and $120\text{ WCUs}$
D
$75\text{ RCUs}$ and $108\text{ WCUs}$

Answer

The correct answer is

75\text{ RCUs}

and

120\text{ WCUs}

The correct answer is

75\text{ RCUs}

and

120\text{ WCUs}

. Each write payload of

2.7\text{ KB}

is rounded up to

3\text{ KB}

. Writing

40

items per second requires

40 \times 3 = 120\text{ WCUs}

. For reads, a single Query returns

12

items of

1.5\text{ KB}

each, totaling

18\text{ KB}

of data. This cumulative size is rounded up to the nearest

4\text{ KB}

boundary, which is

20\text{ KB}

. Since strongly consistent reads require

1\text{ RCU}

per

4\text{ KB}

block, each Query consumes

20\text{ KB} / 4\text{ KB} = 5\text{ RCUs}

. Executing

15

queries per second requires

15 \times 5 = 75\text{ RCUs}

Step-by-Step Solution

Calculate the Write Capacity Units (WCUs) needed per second.

Each write payload of

2.7\text{ KB}

is rounded up to the nearest

1\text{ KB}

boundary, which is

3\text{ KB}

. Since the application performs

40

writes per second, the total write capacity required is

40 \times 3\text{ KB} = 120\text{ WCUs}

One WCU supports one write per second for items up to

1\text{ KB}

Calculate the cumulative size of retrieved items per `Query` operation.

Each `Query` returns

12

items with an average size of

1.5\text{ KB}

each. The cumulative size is

12 \times 1.5\text{ KB} = 18\text{ KB}

For Query and Scan operations, DynamoDB calculates read capacity based on the cumulative size of all items returned rather than rounding up each item individually.

Round up the cumulative query size and calculate the RCUs per Query.

The cumulative size of

18\text{ KB}

is rounded up to the nearest

4\text{ KB}

boundary, which is

20\text{ KB}

. For a strongly consistent read, each

4\text{ KB}

requires

1\text{ RCU}

, so each query consumes

20\text{ KB} / 4\text{ KB} = 5\text{ RCUs}

Read capacity for strongly consistent reads is calculated by dividing the rounded cumulative size in KB by

4\text{ KB}

Calculate the total Read Capacity Units (RCUs) needed per second.

The application performs

15

queries per second. The total read capacity required is

15 \times 5\text{ RCUs} = 75\text{ RCUs}

Multiplying the RCUs consumed per query by the number of queries per second gives the total provisioned read throughput.

Key Concept

Calculation of DynamoDB Read and Write Capacity Units (RCUs and WCUs) for Batch/Query Operations

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Topics

More questions from this topic