AWS DVA-C02 Drill: Lambda-DynamoDB Integration - IAM Permissions Pitfall

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 understanding AWS IAM permission boundaries for Lambda functions when accessing other services. In production, this is about knowing exactly which permissions your Lambda function needs to perform data operations on DynamoDB tables without failing at runtime. Let’s drill down.

The Certification Drill (Simulated Question)

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Scenario

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At NextGen Analytics, a lead developer built a Python AWS Lambda function designed to process incoming files uploaded to an S3 bucket and then write processed results into an Amazon DynamoDB table. The function triggers correctly upon new S3 object creation events. However, during execution, the function fails with errors when attempting to write data into DynamoDB.

The Requirement:

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Determine the most likely cause of the Lambda function’s failure when writing to the DynamoDB table, assuming the function is triggered successfully by S3 events.

The Options

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• A) The Lambda function’s concurrency limit has been exceeded.
• B) DynamoDB table requires a global secondary index (GSI) to support writes.
• C) The Lambda function does not have IAM permissions to write to DynamoDB.
• D) The DynamoDB table is not running in the same Availability Zone as the Lambda function.

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Correct Answer

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C) The Lambda function does not have IAM permissions to write to DynamoDB.

Quick Insight: The Developer Imperative

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Lambda functions require explicit IAM permissions to interact with DynamoDB tables. Without the necessary dynamodb:PutItem (or relevant write) permission attached to the Lambda execution role, any write attempts will fail — even if the event trigger and DynamoDB table exist correctly. This is a common stumbling block for developers new to AWS.

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 C

The Winning Logic

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The most frequent reason a Lambda function cannot write to DynamoDB is insufficient IAM permissions granted to the Lambda’s execution role. AWS Lambda relies entirely on its IAM role trust and attached policy to allow any API calls to AWS services. Even if the Lambda function is triggered successfully by S3 events, the attempt to write to DynamoDB (PutItem, UpdateItem, etc.) will fail with an access denied error if those permissions are missing.

• From a developer perspective, the IAM role must explicitly include policies that allow dynamodb:PutItem or broader dynamodb:* actions scoped to the specific table or ARN.
• IAM permissions are checked at every API request. Unlike concurrency limits or AZ placement, denied permissions lead to immediate failure at the API call.

The Trap (Distractor Analysis):

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• Why not A? Lambda concurrency limits affect invocation availability or throttling, but if the function is invoked successfully by S3, concurrency limits are unlikely to cause failures specifically on DynamoDB writes.
• Why not B? DynamoDB GSIs are used for indexing, not a prerequisite for accepting writes to the base table. Lack of a GSI will not block writes to the primary table.
• Why not D? DynamoDB is a regional service abstracted across AZs. Lambda functions and DynamoDB tables do not need to be in the same AZ because DynamoDB offers high availability and replication across multiple AZs in a region.

The Technical Blueprint

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Dev / SysOps Relevant IAM Policy Snippet Example:

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{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": [
                "dynamodb:PutItem",
                "dynamodb:UpdateItem"
            ],
            "Resource": "arn:aws:dynamodb:us-east-1:123456789012:table/NextGenAnalyticsTable"
        }
    ]
}

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 pick IAM permissions issues when you see AWS Lambda function failures during service API calls.

Real World

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In reality, developers also face permission issues when combining multiple AWS SDK calls or when policies use restrictive conditions. Properly scoped IAM roles and testing with least-privilege helps avoid production issues.

(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.