AWS DVA-C02 Drill: DynamoDB Query Consistency - Choosing the Right Index and Read Model

Jeff’s Note

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Unlike generic exam dumps, ADH analyzes this scenario through the lens of a Real-World Lead Developer.

For DVA-C02 candidates, the confusion often lies in which index type supports strongly consistent reads and how to guarantee the most up-to-date data. In production, this is about knowing exactly when to use Local Secondary Indexes (LSIs) versus Global Secondary Indexes (GSIs) and how DynamoDB’s read consistency models affect your API calls. Let’s drill down.

The Certification Drill (Simulated Question)

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Scenario

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BrightBox Tech is building a real-time analytics application that stores user activity events in an Amazon DynamoDB table. The development team needs to efficiently query the table by the primary partition key and filter or sort results with a different sort key. Critically, the application requires the latest data including all recent write operations to those keys.

The Requirement:

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How should the development team implement the DynamoDB query to retrieve the most recent and strong-consistency-guaranteed data when querying by a different sort key?

The Options

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• A) Add a Local Secondary Index (LSI) during table creation. Query the LSI using eventually consistent reads.
• B) Add a Local Secondary Index (LSI) during table creation. Query the LSI using strongly consistent reads.
• C) Add a Global Secondary Index (GSI) during table creation. Query the GSI using eventually consistent reads.
• D) Add a Global Secondary Index (GSI) during table creation. Query the GSI using strongly consistent reads.

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Correct Answer

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B.

Quick Insight: The Developer Imperative

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Local Secondary Indexes (LSIs) allow strongly consistent reads, unlike GSIs which only support eventually consistent reads. For workloads demanding the absolutely latest data on alternate sort keys, LSIs queried with strong consistency is the correct pattern.

Content Locked: The Expert Analysis

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You’ve identified the answer. But do you know the implementation details that separate a Junior from a Senior?

The Expert’s Analysis

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Correct Answer

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Option B

The Winning Logic

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• LSIs must be defined at table creation and share the same partition key as the base table, but can have an alternate sort key.
• LSIs support strongly consistent reads because they are within the same partition and use the same transactional write operations as the base table.
• If a developer needs the latest data reflecting all writes, eventually consistent reads risk stale data and should be avoided here.
• GSIs have different partition keys and physically separate storage, so they only offer eventually consistent reads. Strong consistency is not supported on GSIs.

The Trap (Distractor Analysis):

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• Why not A? Using eventually consistent reads risks reading stale data, which violates the requirement for most recent writes.
• Why not C? GSIs only allow eventually consistent reads, so they cannot guarantee the latest updates.
• Why not D? GSIs do NOT support strongly consistent reads; this option is invalid technically and would lead to API errors.

The Technical Blueprint

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# Python example using Boto3 SDK to perform a strongly consistent query on an LSI
import boto3

dynamodb = boto3.resource('dynamodb')
table = dynamodb.Table('BrightBoxEvents')

response = table.query(
    KeyConditionExpression=Key('UserId').eq('user-123') & Key('ActivityTime').between('2024-05-01', '2024-05-10'),
    IndexName='ActivityTypeLSI',       # The LSI with alternate sort key on ActivityType
    ConsistentRead=True                # Strongly consistent read is possible on LSI
)

items = response['Items']
print(items)

The Comparative Analysis

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Real-World Application (Practitioner Insight)

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Exam Rule

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“For the exam, always remember: only LSIs support strongly consistent reads; GSIs do not.”

Real World

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“In production, developers often pick GSIs for flexible partition/sort key patterns and scalability, tolerating eventual consistency. But when fresh data is required immediately, LSIs are the go-to solution. The catch is LSIs must be defined upfront and share the same partition key.”

(CTA) Stop Guessing, Start Mastering

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Disclaimer

This is a study note based on simulated scenarios for the AWS DVA-C02 exam.