DVA-C02 Troubleshooting and Optimization Practice Test 6 — 15 Questions

Question 1

A developer is using Amazon S3 to host a static website. The developer updates the files, but users still see the old version. What is the most likely cause?

Accepted Answer

The browser is caching the old files.. When users see old content after updating S3 static website files, the most likely cause is browser caching. Browsers often cache static files like HTML, CSS, and JavaScript to improve performance. Users may need to clear their cache or perform a hard refresh to see the latest version. Option A is incorrect because S3 does not enable server-side caching at the bucket level; it stores objects but does not cache responses. Option C is incorrect as CloudFront is not mentioned in the scenario and would be an additional service. Option D is incorrect because if files were stored in a different bucket, the website would not serve the updated files at all, causing a mismatch.

Answer

The S3 bucket has server-side caching enabled.

Answer

The files are being served from a CloudFront distribution with TTL.

Answer

The files are stored in a different S3 bucket.

Question 2

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 3

Why is the Lambda function not being invoked?

Accepted Answer

The S3 bucket does not have a notification configuration for the Lambda function.. The S3 bucket has no notification configuration to trigger the Lambda function. The Lambda permission allows S3 to invoke, but the bucket must have a NotificationConfiguration event. Option D is correct. Option A is wrong because the execution role is irrelevant to S3 invocation; invocation is controlled by resource-based policy. Option B is wrong because the permission is correctly configured (or the issue is not about source account). Option C is wrong because the runtime is supported.

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 4

A developer deploys a Lambda function that transforms incoming JSON payloads and writes results to DynamoDB. After a recent code update, the function frequently times out with 5-second durations. The function has a 15-second timeout and 512 MB memory. CloudWatch Logs show no errors. The DynamoDB table has autoscaling enabled. What is the MOST likely cause of the increased duration?

Accepted Answer

A new dependency in the deployment package increased the function's initialization time beyond the duration of the function's reserved concurrency warm start window.. Introducing a new dependency, especially a large library, can increase the function's initialization time (cold start). Since the function has a 15-second timeout but is timing out at 5 seconds, this suggests the cold start duration is extending beyond the expected window, causing timeouts despite sufficient timeout setting. Option A is incorrect because DynamoDB autoscaling handles capacity, and CloudWatch Logs would show throttling errors if that were the issue. Option B is incorrect because the question doesn't mention S3, and reading from S3 would not cause a 5-second timeout if properly configured. Option D is incorrect because unhandled exceptions would produce error logs in CloudWatch.

Answer

DynamoDB write capacity is insufficient, causing throttling and retries.

Answer

The function is reading the input payload from S3 instead of API Gateway, causing network latency.

Answer

The function code throws an unhandled exception that is not logged.

Question 5

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 6

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.. The most likely cause is a delay in S3 Event Notifications. S3 is strongly consistent for new object writes, so option B is incorrect. Notifications can have a latency of a few seconds to several minutes, and the application may be polling for the processed image before the notification triggers Lambda, causing the image to appear unavailable. Option A (Lambda cold start) typically adds only a few seconds, not a significant delay. Option D (DynamoDB throttling) is unrelated to image availability.

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 7

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 8

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 9

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 10

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.

Question 11

A developer is using Amazon DynamoDB as the database for a web application. The application experiences occasional spikes in traffic, and some write requests fail with a ProvisionedThroughputExceededException. What is the MOST cost-effective way to handle these spikes without manual intervention?

Accepted Answer

Enable DynamoDB auto scaling for the table.. DynamoDB auto scaling automatically adjusts the provisioned read and write capacity based on actual traffic patterns, handling spikes without manual intervention and only paying for the capacity needed at peak times. Option A (on-demand mode) avoids capacity management but can be more expensive for predictable workloads or sustained traffic. Option C (increasing to peak) leads to over-provisioning and higher cost during low traffic. Option D (DAX) is a caching layer for reads, not writes, and does not address write throughput limitations.

Answer

Switch to on-demand mode for the table.

Answer

Increase the provisioned write capacity to the peak expected value.

Answer

Use DynamoDB Accelerator (DAX) to cache writes.

Question 12

The exhibit shows the output of invoking a Lambda function from the AWS CLI. The function returned a status code of 200 but included a FunctionError field set to 'Unhandled'. What does this indicate?

Accepted Answer

The function executed but threw an unhandled exception.. The FunctionError field set to 'Unhandled' indicates that the Lambda function executed successfully (as shown by the 200 status code) but an unhandled exception was thrown during execution. Option B is incorrect because LogResult contains the execution logs, not the error itself. Option C is incorrect because a timeout would result in a different FunctionError value (e.g., 'Unhandled' for uncaught exceptions, but timeouts are typically indicated by a specific error). Option D is incorrect because a network error would prevent invocation, not result in a 200 response with an unhandled error.

Answer

The function returned an error in the LogResult field.

Answer

The function timed out during execution.

Answer

The function was invoked but there was a network error.

Question 13

The exhibit shows an IAM policy attached to a Lambda function's execution role. The function writes objects to an S3 bucket that is encrypted with a KMS key (the key specified in the policy). When the function tries to write an object, it receives an access denied error. What is the MOST likely missing permission?

Accepted Answer

kms:GenerateDataKey is missing.. When writing an object to an S3 bucket encrypted with a KMS key, the Lambda function needs kms:GenerateDataKey permission to generate a data key for encryption. Option B is misleading: although the KMS key policy might not allow the role, the most likely missing permission directly on the execution role is kms:GenerateDataKey. Option C is incorrect because s3:GetObject is not needed for writing. Option D is incorrect because kms:ReEncrypt is not used during new object writes. Therefore, option A is the missing permission.

Answer

The KMS key policy does not allow the Lambda function role.

Answer

s3:GetObject is missing for the bucket.

Answer

kms:ReEncrypt is missing.

Question 14

The exhibit shows a CloudFormation template that creates an S3 bucket with versioning enabled. After deploying the stack, a developer uploads an object to the bucket. Later, the developer updates the object by uploading a new version. The developer wants to retrieve the original object. What is the correct way to do this?

Accepted Answer

Use the S3 console or CLI to list object versions and retrieve the version ID of the original object.. With S3 versioning enabled, each object has a unique version ID. When you upload a new version, the original is preserved. To retrieve it, use the S3 console or CLI to list object versions and specify the version ID of the original. Option A is wrong because S3 Object Lambda applies transformations on data retrieval, but it does not restore previous versions. Option B is wrong because S3 Batch Operations perform bulk actions, but there is no built-in 'revert' operation; you would need to copy the original version manually. Option C is wrong because the original object is not overwritten; it remains as a previous version.

Answer

Restore the original object using the S3 Object Lambda.

Answer

Use the S3 Batch Operations to revert to the original version.

Answer

The original object is overwritten and cannot be retrieved.

Question 15

A company has a production application running on Amazon ECS with Fargate. The application consists of a front-end service and a backend service that processes orders. The backend service consumes messages from an Amazon SQS queue and writes order records to an Amazon DynamoDB table. Recently, during a marketing campaign, traffic increased significantly, causing the backend service to fall behind processing messages. The SQS queue depth grew to over 100,000 messages, and some orders were not processed in time, leading to customer complaints. The operations team noticed that the ECS service's CPU utilization never exceeded 60%, and memory utilization was around 50%. The service is configured with a desired count of 2 tasks and a target tracking scaling policy based on average CPU utilization. The DynamoDB table has on-demand capacity mode. After analyzing the logs, the development team found that each message processing takes about 2 seconds, but the backend service has a bottleneck: it makes an HTTP call to a third-party API that sometimes takes up to 10 seconds to respond. The team wants to optimize the architecture to handle traffic spikes better without over-provisioning resources.

Which solution is MOST effective?

Accepted Answer

Refactor the backend service to send the order processing request to a separate SQS queue and have a dedicated set of tasks poll that queue to make the HTTP call. This decouples the main processing from the slow API call.. The correct solution is Option B. The bottleneck is the third-party API call that can take up to 10 seconds. By refactoring the backend service to send the order processing request to a separate SQS queue and having a dedicated set of tasks poll that queue, the main processing is decoupled from the slow API call. This allows the ECS tasks to quickly process messages from the original queue and offload the HTTP call, increasing overall throughput without over-provisioning. Option A (increasing batch size) would not help because the bottleneck is the API latency, not the polling efficiency. Option C (increasing desired count and scaling on queue depth) could help but is over-provisioning and less efficient than decoupling. Option D (scaling on memory) does not address the root cause of the API call latency.

Answer

Increase the batch size of messages polled from SQS to 20 to process more messages per task.

Answer

Increase the desired count of the ECS service to 10 tasks and set a target tracking scaling policy based on SQS queue depth.

Answer

Change the scaling policy to use memory utilization instead of CPU, since CPU is underutilized.