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

Question 1

An application running on Amazon EC2 generates logs that need to be streamed to Amazon CloudWatch Logs. The developer installs and configures the CloudWatch agent. However, logs are not appearing in the log group. What is the most likely cause?

Choose an answer to begin — your selection is scored in the full session.

Accepted Answer

The EC2 instance does not have an IAM role with CloudWatch Logs write permissions.. The CloudWatch agent requires IAM permissions to call CloudWatch Logs APIs (CreateLogGroup, CreateLogStream, PutLogEvents). On EC2, these permissions are provided via an instance profile attached to the instance. If the instance lacks an IAM role with CloudWatch Logs write permissions, the agent cannot publish logs, and they will not appear in the log group.

Answer

The CloudWatch agent cannot be installed on Amazon Linux 2.

Answer

The CloudWatch agent must be configured from the AWS Management Console.

Answer

The log group must be created manually before the agent can send logs.

Question 2

A developer is troubleshooting an AWS Elastic Beanstalk environment that is failing health checks. The environment runs a web application on Tomcat. The developer checks the logs and finds no errors. What is the most likely cause of the health check failure?

Accepted Answer

The application's health check URL is returning a non-200 status code.. The most likely cause is that the application's health check URL is returning a non-200 status code. Elastic Beanstalk uses the load balancer to perform health checks against a configurable path (default: /). If the application responds with any status other than 200 OK, the load balancer marks the instance as unhealthy, even if the application logs show no errors. This is a common misconfiguration where the health check endpoint is not implemented or returns an unexpected status.

Answer

The security group for the instances does not allow traffic from the load balancer.

Answer

The application is throwing exceptions that are not logged.

Answer

The application is listening on a port other than 80.

Question 3

A developer is using Amazon DynamoDB for a new application. The developer wants to reduce read latency. Which design pattern should the developer use?

Accepted Answer

Use DynamoDB Accelerator (DAX) as a cache for frequently read items.. DynamoDB Accelerator (DAX) is an in-memory cache designed specifically for DynamoDB, providing microsecond read latency for frequently accessed items. By caching read-heavy workloads, DAX offloads requests from the DynamoDB table, reducing read latency without requiring application-level caching logic. This directly addresses the developer's goal of reducing read latency.

Answer

Create a global secondary index (GSI) for the table.

Answer

Increase the provisioned read capacity units (RCUs) for the table.

Answer

Use DynamoDB Global Tables to replicate data to multiple regions.

Question 4

A developer is deploying a serverless application using AWS CloudFormation. The stack creation fails with the error 'CREATE_FAILED: The following resource(s) failed to create: [MyLambdaFunction]'. The developer checks the CloudFormation events and sees 'Resource creation cancelled'. What is the most likely cause?

Accepted Answer

Another resource in the stack failed, triggering a rollback and cancelling the Lambda creation.. When CloudFormation creates a stack, resources are provisioned in dependency order. If any resource fails, CloudFormation initiates a rollback and cancels in-progress creations of other resources — producing the 'Resource creation cancelled' message for MyLambdaFunction. The Lambda itself did not fail; it was cancelled because a sibling resource failed first.

Answer

The Lambda function code is too large and exceeds the deployment limit.

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The Lambda function creation timed out due to a network issue.

Answer

The Lambda function's execution role is missing permissions.

Question 5

A developer is troubleshooting a slow-performing Amazon RDS for MySQL database. Which TWO actions should the developer take to improve query performance?

Accepted Answer

Increase the instance size to provide more CPU and memory.. Option C is correct because a slow-performing RDS for MySQL instance is often constrained by CPU, memory, or IOPS, and vertically scaling to a larger instance class provides more vCPU, RAM, and baseline EBS throughput, which directly improves query execution and buffer pool caching. Option D is correct because enabling the MySQL slow query log (via the slow_query_log and long_query_time parameters in a custom parameter group) captures queries exceeding the threshold, letting the developer identify and then optimize the specific poorly performing SQL statements. Option A is not appropriate because deleting indexes generally hurts read performance and only marginally reduces write overhead, and unused indexes are not the typical cause of slow queries. Option B is wrong because Multi-AZ is a high-availability/failover feature that maintains a synchronous standby, not a read-scaling mechanism, so it does not improve read performance. Option E is wrong because purging binary logs only frees storage and does not address query performance, and it can break point-in-time recovery and replication.

Answer

Delete unused indexes to reduce write overhead.

Answer

Enable Multi-AZ deployment for better read performance.

Answer

Delete the binary log files to free up storage.

Question 6

A developer is using AWS CodePipeline to deploy a web application. The pipeline has a source stage from GitHub and a deploy stage to Elastic Beanstalk. The deploy stage fails with the error 'The S3 bucket does not allow access to the artifact'. Which THREE actions could resolve this issue?

Accepted Answer

Add a bucket policy that grants the pipeline's service role access to the artifact bucket.. The deploy stage fails because the CodePipeline service role cannot access the artifact S3 bucket. To resolve this, you can: (1) Attach a bucket policy that grants the pipeline's service role access to the bucket (Option B). (2) Ensure the pipeline's IAM role has the necessary S3 permissions: s3:GetObject and s3:PutObject on the artifact bucket (Option C). (3) If the artifact bucket uses AWS KMS encryption, the pipeline role also needs kms:Decrypt permission to read the encrypted artifacts (Option D). Option A is not a direct fix — specifying a different bucket may avoid the issue but does not address the access problem with the current bucket. Option E is irrelevant because bucket versioning does not affect access permissions.

Answer

Specify a different artifact bucket in the pipeline configuration.

Answer

Enable versioning on the artifact bucket.

Question 7

A developer is using AWS X-Ray to trace requests through a microservices application. The developer notices that some traces are incomplete. Which TWO actions can help ensure complete traces?

Accepted Answer

Use the X-Ray SDK to instrument the application code.. Option A is correct because the X-Ray SDK must be used to instrument the application code so that it emits segment data and propagates trace headers across service calls; without instrumentation, downstream services cannot contribute subsegments and traces remain incomplete. Option E is correct because the X-Ray daemon must be running on every EC2 instance (or equivalent compute) to receive UDP segment data from the SDK on port 2000 and forward it to the X-Ray API; a missing daemon on any instance means those segments are dropped, producing incomplete traces. Option B is not correct because opening port 2000 in a security group is not required for the daemon to receive local UDP traffic from the SDK on the same host, and it does not by itself fix incomplete traces. Option C is not correct because the X-Ray daemon is designed to run locally on each instance (or as a sidecar/daemon set), not as a single centralized service, so centralizing it would not ensure all instances' segments are captured. Option D is not correct because the CloudWatch agent collects metrics and logs, not X-Ray trace segments, so it does not contribute to complete X-Ray traces.

Answer

Open port 2000 on the security groups for TCP traffic.

Answer

Deploy the X-Ray daemon as a centralized service in a separate instance.

Answer

Install the CloudWatch agent on all instances.

Question 8

Why is the Lambda function not being invoked?

Accepted Answer

The S3 bucket does not have a notification configuration for the Lambda function.. The most likely reason the Lambda function is not being invoked is that the S3 bucket does not have a notification configuration for the Lambda function. For S3 to invoke a Lambda function, you must configure an event notification on the bucket that specifies the Lambda function as the destination. Without this configuration, S3 will not send events to Lambda, and the function will never be invoked.

Answer

The Lambda execution role does not have permission to be invoked by S3.

Answer

The Lambda permission does not specify the correct source account.

Answer

The Lambda function has a runtime that is not supported.

Question 9

A developer notices that an EC2 instance running a web application is unreachable via its public IP. The instance passes status checks but security group rules appear correct. What should the developer check NEXT?

Accepted Answer

Check the network ACL associated with the subnet for rules that may block traffic.. The instance passes status checks, the route table is confirmed to have a route to an internet gateway, and security group rules appear correct. Since the instance is still unreachable via its public IP, the next logical step is to check the network ACL (NACL) associated with the subnet. NACLs are stateless and can block inbound or outbound traffic even if security groups allow it. NACLs evaluate rules in order by rule number, and a deny rule (or missing allow rule) for the required ephemeral ports (e.g., 1024-65535 for return traffic) could silently drop packets.

Answer

Verify that the instance has an Elastic IP associated.

Answer

Review the route table for a route to an internet gateway.

Answer

Inspect the IAM role attached to the instance for network permissions.

Question 10

A company's application uses Amazon S3 to store user-uploaded images. Users report that recently uploaded images are sometimes not immediately available for viewing. The application uses S3 Event Notifications to trigger a Lambda function that processes images and stores metadata in DynamoDB. What is the MOST likely cause of the delay?

Accepted Answer

S3 Event Notifications may have a slight delay, and the application polls for the processed image before the notification triggers Lambda.. S3 Event Notifications are delivered asynchronously and can take seconds to minutes to reach Lambda, so if the application polls for the processed image or metadata immediately after upload, it may query before Lambda has run. This race condition between the upload and the notification-driven processing is the most likely cause of the intermittent delay.

Answer

Lambda function has a cold start that adds several seconds to processing time.

Answer

S3 is eventually consistent for new object writes, so the object may not be immediately available.

Answer

DynamoDB has insufficient read capacity causing throttling on metadata retrieval.

Question 11

A developer is troubleshooting an AWS Lambda function that processes messages from an Amazon SQS queue. The function is configured with a batch size of 10 and a maximum concurrency of 5. The function frequently reports errors related to message processing timeouts. The function code is idempotent. Which combination of actions will reduce the number of timeouts and improve processing efficiency?

Accepted Answer

Reduce the batch size to 5 and increase the maximum concurrency to 10.. Reducing the batch size to 5 decreases the number of messages processed per invocation, lowering the processing time and reducing the likelihood of timeouts. Increasing maximum concurrency to 10 allows more Lambda functions to run in parallel, improving overall throughput. Option A is wrong: increasing the Lambda timeout to 30 seconds alone does not address the root cause (overloaded invocations), and setting the SQS visibility timeout to 6 minutes may cause delayed retries if messages fail. Option B is wrong: increasing the batch size to 20 would increase the processing time per invocation, exacerbating timeouts. Option D is wrong: increasing concurrency to 10 helps parallelism but does not reduce the per-invocation workload; setting visibility timeout to 30 seconds is too short, risking message duplication if processing exceeds that time.

Answer

Increase the function timeout to 30 seconds and set the SQS visibility timeout to 6 minutes.

Answer

Increase the batch size to 20 and increase the function timeout to 30 seconds.

Answer

Increase the maximum concurrency to 10 and set the SQS visibility timeout to 30 seconds.

Question 12

A developer runs a script that uses the AWS CLI to copy a large number of files from an on-premises server to an S3 bucket. The copy operation fails partway through with a 'RequestTimeout' error. What is the MOST efficient way to resume the copy and ensure all files are transferred?

Accepted Answer

Use the aws s3 sync command to synchronize the source directory with the S3 bucket.. The `aws s3 sync` command is the most efficient way to resume the copy because it automatically compares the source directory with the destination S3 bucket and transfers only the files that are missing or have been modified. This avoids re-uploading already transferred files, directly addressing the partial failure without manual intervention or unnecessary overhead.

Answer

Delete the S3 bucket and restart the copy operation.

Answer

Use the cp command with the --recursive flag to copy the remaining files.

Answer

Increase the --cli-read-timeout value in the AWS CLI configuration and retry the original command.

Question 13

A developer deploys an application on EC2 instances behind an Application Load Balancer (ALB). The application uses sticky sessions (session affinity) based on a cookie. Users report that they are intermittently logged out during their session. What is the MOST likely cause?

Accepted Answer

The stickiness cookie expiration duration is set too low, causing the cookie to expire before the user's session ends.. If the stickiness cookie expiration duration is set too low, the cookie will expire before the user's session ends, causing the load balancer to route the user to a different instance and losing session state. Option A is wrong because deregistration delay affects how long an instance remains in service during scaling events, but does not directly cause intermittent logouts during a session. Option B is wrong because a short health check interval might cause healthy instances to be marked unhealthy, but this would result in dropped connections, not specifically intermittent logouts due to session stickiness. Option C is wrong because cross-zone load balancing affects traffic distribution across zones, not session stickiness.

Answer

The deregistration delay value is too low, causing connections to be dropped during scaling events.

Answer

The ALB health check interval is too short, causing healthy instances to be marked unhealthy frequently.

Answer

Cross-zone load balancing is disabled, causing uneven traffic distribution.

Question 14

A company runs a microservices application on Amazon ECS with Fargate. The application includes a service that processes messages from an SQS queue. The service's CPU utilization is consistently above 80%, and messages are accumulating in the queue. The service is configured with a desired count of 2 tasks and auto scaling based on CPU utilization. What should a developer do to improve message processing throughput?

Accepted Answer

Change the auto scaling metric to use the SQS queue's ApproximateNumberOfMessagesVisible.. The correct answer because using the SQS queue's ApproximateNumberOfMessagesVisible metric for auto scaling is more responsive to the actual workload than CPU utilization. When messages accumulate in the queue, scaling based on queue depth triggers task additions sooner, improving throughput. Option A is wrong because simply increasing the desired count without a dynamic scaling policy may not adapt to varying load and could lead to over-provisioning or under-provisioning. Option B is wrong because increasing task size (CPU/memory) does not directly address the scaling trigger; it might help a single task process more, but the bottleneck is the number of tasks. Option D is wrong because decreasing the batch size reduces the number of messages processed per poll, which would decrease throughput, not improve it.

Answer

Increase the desired count of tasks to 5.

Answer

Increase the task size to use more CPU and memory.

Answer

Decrease the batch size of messages polled from SQS.

Question 15

A developer is troubleshooting an AWS Lambda function that writes to an S3 bucket. The function is configured with a resource-based policy that allows the S3 service to invoke the function. However, the function fails with an access denied error when trying to write to S3. What is the MOST likely cause?

Accepted Answer

The Lambda function's execution role does not have an IAM policy that allows s3:PutObject.. The Lambda function's execution role lacks the necessary IAM permissions (s3:PutObject) to write to the S3 bucket. The resource-based policy only allows S3 to invoke the function, not the function to write. Option A is incorrect because a VPC endpoint would not cause an access denied error for writing if the execution role had permissions. Option C is incorrect because trigger misconfiguration would prevent invocation, not cause access denied during execution. Option D is incorrect because the S3 bucket policy is not required if the execution role grants write access; the bucket policy controls who can access the bucket, but the execution role is the primary mechanism for Lambda permissions.

Answer

The Lambda function is configured in a VPC without an S3 VPC endpoint.

Answer

The Lambda function's trigger (S3 event notification) is misconfigured.

Answer

The S3 bucket policy does not grant the Lambda function write access.