DVA-C02 Troubleshooting and Optimization Practice Test 14 — 15 Questions

Question 1

A developer optimized an Amazon S3 bucket for high request rates. The bucket receives over 5,000 PUT requests per second. Recently, some requests are failing with a 503 Slow Down error. What is the most likely cause and how should the developer fix it?

Accepted Answer

Add a random prefix to the object keys to distribute across partitions.. Option D is correct because S3 returns 503 when request rates exceed partition limits. Prefix randomization spreads requests across partitions. Option A is wrong because 503 is not due to permissions. Option B is wrong because 503 is not a quota limit exceeded error (that would be 400). Option C is wrong because multipart upload is for large objects, not rate limits.

Answer

Use multipart upload for all objects to improve throughput.

Answer

The request rate exceeds the account-level PUT quota; request a quota increase.

Answer

The bucket policy is too permissive; restrict access to prevent abuse.

Question 2

A developer is troubleshooting an AWS Lambda function that is failing with a timeout error. The function has a 15-minute timeout and processes messages from an Amazon SQS queue. Which TWO actions should the developer take to resolve the issue?

Accepted Answer

Increase the SQS batch size in the Lambda event source mapping.. Options A and D are correct. A: Increasing batch size reduces the number of invocations, but each invocation processes more messages quickly. D: Using SQS batch operations can improve throughput. Option B is wrong because dead-letter queue is for failed messages after retries. Option C is wrong because reducing visibility timeout would cause more retries. Option E is wrong because Lambda can have up to 15 minutes; the issue is processing speed.

Answer

Reduce the SQS visibility timeout.

Answer

Increase the Lambda function timeout to 15 minutes.

Answer

Configure a dead-letter queue for the SQS queue.

Question 3

A company's application runs on Amazon EC2 instances in an Auto Scaling group. The application experiences intermittent failures, and the developer suspects the application is not properly handling termination notifications. Which TWO steps should the developer take to diagnose the issue?

Accepted Answer

Configure a CloudWatch Events rule to capture Auto Scaling termination events.. Options B and D are correct. B: Lifecycle hooks allow the instance to perform actions before termination. D: CloudWatch Events can capture lifecycle transitions. Option A is wrong because detailed monitoring does not capture termination signals. Option C is wrong because CloudWatch Logs agent is for logs, not for termination notifications. Option E is wrong because replacing instances does not diagnose the issue.

Answer

Enable detailed monitoring on the Auto Scaling group.

Answer

Install the CloudWatch Logs agent on the instances to capture application logs.

Answer

Use an Elastic Load Balancer to replace instances automatically.

Question 4

A developer is optimizing an Amazon S3 bucket that stores millions of small objects. The application frequently lists objects with prefix-based queries. Which THREE strategies should the developer implement to improve performance?

Accepted Answer

Use Amazon S3 Inventory to generate a daily list of objects and query that list.. Option A is correct because Amazon S3 Inventory provides a daily or weekly CSV/Parquet report listing all objects and their metadata. By querying this inventory file (e.g., with Athena or SQL), you avoid making thousands of individual ListObjects API calls, which is far more efficient for prefix-based queries on millions of small objects.

Answer

Move infrequently accessed objects to Amazon S3 Glacier Deep Archive.

Answer

Increase the TPS limit for ListObjects requests by requesting a quota increase.

Question 5

A developer invoked a Lambda function and saw the above output. What is the root cause of the error?

Accepted Answer

The function code expects a property that is missing from the event payload.. Option C is correct because the error message 'Cannot read property 'length' of undefined' suggests that the code expects a property that does not exist in the event object. Option A is wrong because there is no timeout error. Option B is wrong because the log shows the function started and ran. Option D is wrong because there is no mention of missing permissions.

Answer

The Lambda function lacks permission to access the event payload.

Answer

The Lambda function's handler name is incorrect.

Answer

The Lambda function timed out.

Question 6

A developer attached the above IAM policy to an AWS Lambda function that writes encrypted objects to an S3 bucket. The function fails with an 'AccessDenied' error when putting objects. What is the most likely reason?

Accepted Answer

The policy does not include s3:PutObjectAcl permission.. Option C is correct because the policy allows s3:PutObject but does not include s3:PutObjectAcl or other permissions that may be needed by the Lambda function or the encryption context. Option A is wrong because the policy is not missing; it is there. Option B is wrong because KMS permissions are granted. Option D is wrong because s3:GetObject is not needed for writing; but missing s3:PutObjectAcl is common when bucket policies enforce specific ACLs.

Answer

The policy is missing a statement for the S3 bucket itself (not the objects).

Answer

The policy allows s3:GetObject, which is unnecessary and conflicts with PutObject.

Answer

The KMS key policy does not allow the Lambda function to decrypt.

Question 7

A developer deployed the above AWS SAM template. Messages are not being processed; they end up in the DeadLetterQueue after 3 receives. The Lambda function timeout is 30 seconds. What is the most likely cause?

Accepted Answer

The SQS visibility timeout is equal to the Lambda function timeout.. Option A is correct because the VisibilityTimeout (30 seconds) equals the Lambda timeout (30 seconds). If the function takes near 30 seconds, the message becomes visible again before processing completes, causing another receive and eventually moving to DLQ. Option B is wrong because the function has the SQSPollerPolicy. Option C is wrong because BatchSize 10 is fine. Option D is wrong because a higher maxReceiveCount would delay DLQ, not prevent it.

Answer

The maxReceiveCount is too low; increase it to 5.

Answer

The batch size is too high, causing the function to timeout.

Answer

The Lambda function does not have permission to poll the SQS queue.

Question 8

A developer is troubleshooting a Lambda function that times out sporadically. The function reads from an S3 bucket and processes CSV files. Which of the following is the MOST likely cause?

Accepted Answer

S3 GET request throttling during high traffic.. Sporadic timeouts often occur due to throttling or limits. S3 throttling (especially for GET requests) can cause delays. Cold starts would cause initial delays but not sporadic ones. Memory exhaustion would cause failures, not timeouts. VPC configuration would cause consistent timeouts.

Answer

VPC configuration missing a NAT gateway.

Answer

Cold starts due to low invocation frequency.

Answer

Insufficient memory allocated to the Lambda function.

Question 9

An application running on EC2 instances behind an Application Load Balancer (ALB) occasionally returns HTTP 503 errors. The instances are in an Auto Scaling group. Which action should be taken to resolve this issue?

Accepted Answer

Review the ALB access logs to identify the target response codes.. HTTP 503 errors from an ALB indicate that the targets (EC2 instances) are not responding successfully. Reviewing ALB access logs reveals the specific target response codes (e.g., 503 from the target itself or connection timeouts), which helps pinpoint whether the issue is due to overloaded instances, application errors, or health check failures. This diagnostic step is essential before making any configuration changes.

Answer

Enable cross-zone load balancing on the ALB.

Answer

Increase the ALB idle timeout setting.

Answer

Increase the size of the EC2 instances.

Question 10

A developer is deploying a new version of a Lambda function using an alias for blue/green deployment. Traffic is gradually shifted to the new version. During the shift, a high error rate is observed. What should the developer do to minimize impact?

Accepted Answer

Configure the alias with a canary deployment and an error rate alarm for automatic rollback.. Option C is correct because it automates the rollback process using AWS CodeDeploy's canary deployment with an Amazon CloudWatch alarm on the error rate. When the alarm triggers, CodeDeploy automatically shifts traffic back to the previous version, minimizing impact without manual intervention. This is the recommended approach for safe blue/green deployments with Lambda aliases.

Answer

Use the Lambda function's provisioned concurrency to pre-warm the new version.

Answer

Manually revert the alias to point back to the old version.

Answer

Delete the new version and redeploy after fixing the issue.

Question 11

A company uses AWS CodePipeline with CodeBuild to test and deploy a web application. The pipeline has been failing at the deploy stage with an error: 'Access Denied'. CloudTrail shows the CodePipeline service role is making the call. What is the MOST likely cause?

Accepted Answer

The CodePipeline service role lacks permissions for the deploy action.. The error 'Access Denied' in the deploy stage, with CloudTrail showing the CodePipeline service role making the call, indicates that the IAM role assumed by CodePipeline does not have the necessary permissions to perform the deploy action against the target provider (e.g., ECS, S3, Elastic Beanstalk). CodePipeline uses its service role to invoke the deploy action, and if that role lacks the required `codedeploy:*`, `s3:PutObject`, or `ecs:UpdateService` permissions, the API call will be denied.

Answer

The CodeBuild project does not have internet access.

Answer

The deploy provider (e.g., ECS, S3) is not in the same AWS region.

Answer

The source code repository does not have the correct branch.

Question 12

A developer notices that an application is generating duplicate entries in a DynamoDB table. The application uses a Lambda function triggered by an SQS queue. Messages are processed with at-least-once delivery. Which design change will reduce duplicates?

Accepted Answer

Implement idempotent processing by checking a condition expression in DynamoDB PutItem.. SQS provides at-least-once delivery, so duplicates are possible. Making the Lambda function idempotent using a unique message deduplication ID (e.g., using the message ID as a condition in DynamoDB) prevents duplicates.

Answer

Increase the visibility timeout of the SQS queue.

Answer

Use a FIFO SQS queue with content-based deduplication.

Answer

Use DynamoDB Transactions to ensure atomic writes.

Question 13

A developer is troubleshooting a slow-running query in Amazon RDS for MySQL. The query is used by a reporting dashboard. Which AWS service should the developer use to identify the bottleneck?

Accepted Answer

Amazon RDS Performance Insights. Amazon RDS Performance Insights provides a detailed analysis of database performance, including wait events and SQL query performance, helping identify bottlenecks.

Answer

AWS X-Ray

Answer

AWS CloudTrail

Answer

Amazon CloudWatch Logs

Question 14

A developer has deployed a serverless application using AWS SAM. After a recent update, the API Gateway endpoints return 500 errors. The Lambda function logs show no errors. What should the developer investigate first?

Accepted Answer

Verify the API Gateway integration response and mapping templates.. Option D is correct because when API Gateway returns 500 errors but Lambda logs show no errors, the issue is typically in the API Gateway integration response or mapping templates. API Gateway may fail to transform the Lambda response into the expected format, causing an internal server error without the Lambda function ever throwing an exception.

Answer

Increase the Lambda function timeout.

Answer

Check the Lambda function's reserved concurrency.

Answer

Review the CloudFormation stack events for any failures.

Question 15

A company uses an S3 bucket to store sensitive documents. The bucket policy allows access only from a specific VPC endpoint. However, a developer in the same VPC is unable to access the bucket from an EC2 instance. What is the MOST likely cause?

Accepted Answer

The EC2 instance is routing traffic to S3 through the internet instead of the VPC endpoint.. S3 bucket policies that restrict access to a specific VPC endpoint require that requests originate from that endpoint. The EC2 instance must route S3 traffic through the VPC endpoint. If the instance has a public IP and routes directly to S3, the request won't go through the endpoint.

Answer

The S3 bucket policy requires encryption in transit, which is not configured.

Answer

The VPC endpoint policy does not grant access to the developer's IAM role.

Answer

The EC2 instance does not have an IAM role assigned.