DBS-C01 Monitoring and Troubleshooting Practice Test 10 — 15 Questions

Question 1

A developer is receiving timeout errors when connecting to an Amazon ElastiCache for Redis cluster from an Amazon EC2 instance. The security group for the EC2 instance allows outbound traffic to the Redis cluster's security group on port 6379. The Redis cluster's security group does not allow inbound traffic from the EC2 instance. What is the most likely cause of the timeout?

Accepted Answer

The Redis cluster security group does not have an inbound rule allowing traffic from the EC2 security group on port 6379. Option A is correct because security groups are stateful; for inbound traffic, the target security group must allow the incoming connection. Option B is incorrect because the subnet routing does not block specific ports. Option C is incorrect because NACLs are stateless but the issue is with security groups. Option D is incorrect because the Redis cluster is in a VPC, not on-premises.

Answer

The network ACL for the Redis subnet is blocking inbound traffic on port 6379

Answer

The subnet route table does not have a route to the Redis cluster

Answer

The Redis cluster is not accessible from within the same VPC

Question 2

A company uses Amazon RDS for PostgreSQL with Multi-AZ deployment. The primary instance fails, and a failover occurs. After the failover, the application is still unable to connect to the database endpoint. The database administrator checks the RDS console and sees that the new primary is in 'available' state. What should the administrator do next to diagnose the connectivity issue?

Accepted Answer

Check the DNS resolution of the RDS endpoint from the application server. Option C is correct because the DNS CNAME of the RDS endpoint should have updated to point to the new primary. If the application is using the old IP or a cached DNS entry, it may not connect. Option A is incorrect because security group rules are usually unchanged. Option B is incorrect because the subnet group is not the issue. Option D is incorrect because the primary is already in available state.

Answer

Verify that the security group for the RDS instance allows inbound traffic from the application

Answer

Check if the subnet group for the RDS instance is correctly configured

Answer

Restart the RDS instance to force a new connection

Question 3

A team is using Amazon DynamoDB Accelerator (DAX) to improve read performance for a table. They notice that DAX is returning stale data even though the TTL is set to 5 minutes. The table is updated frequently by multiple writers. What is the most likely cause of the stale reads?

Accepted Answer

The TTL is too long, causing cached items to remain after updates. Option B is correct because DAX uses a write-through cache; if the TTL is too long, cached data may become stale before invalidation. Option A is incorrect because DAX supports eventual consistency and can return stale data if TTL is long. Option C is incorrect because DAX cache is not sharded across nodes in the same cluster. Option D is incorrect because DAX does not have a separate cache for each AZ.

Answer

The DAX cluster is not large enough to cache all items, causing cache misses

Answer

DAX is configured with eventual consistency, which returns stale data by design

Answer

The DAX cluster is deployed in a different Availability Zone than the application

Question 4

A company is using Amazon RDS for SQL Server with native backup and restore. The backup process is failing with an error indicating insufficient disk space for the backup file. The DB instance has 200 GB of allocated storage, and the backup file is 50 GB. What should the database administrator do to resolve this issue?

Accepted Answer

Increase the allocated storage for the RDS instance. Option A is correct because native backups are stored on the instance's attached storage; if the storage is full, backups fail. Increasing allocated storage provides more space. Option B is incorrect because automated backups are stored in S3 and do not use instance storage. Option C is incorrect because the error is about disk space, not permissions. Option D is incorrect because changing storage type does not add space.

Answer

Change the storage type to Provisioned IOPS for better performance

Answer

Grant the rds_backup user additional permissions to write to S3

Answer

Switch to automated backups instead of native backups

Question 5

A database administrator is troubleshooting a performance issue on an Amazon Aurora MySQL cluster. The application is experiencing high latency on write operations. Which TWO CloudWatch metrics should the administrator analyze to identify the root cause?

Accepted Answer

DMLThroughput. Option A (DMLThroughput) measures the rate of write operations. Option D (CommitLatency) measures the time to commit transactions, which directly affects write latency. Option B is incorrect because it is a read metric. Option C is incorrect because it measures storage. Option E is incorrect because it is a read latency metric.

Answer

ReadLatency

Answer

SelectLatency

Answer

FreeableMemory

Question 6

A company is using Amazon DynamoDB with auto scaling for read and write capacity. During a traffic spike, write requests are being throttled even though the table's write capacity is below the maximum limit. Which TWO actions should the team take to resolve the throttling?

Accepted Answer

Review the table's partition key design to ensure even distribution of write traffic. Option A is correct because auto scaling may have a lag in provisioning capacity; pre-warming can help. Option C is correct because a hot partition can cause throttling even if overall capacity is not exhausted. Option B is incorrect because write sharding is not a built-in DynamoDB feature. Option D is incorrect because global tables add complexity and are not a direct fix for throttling. Option E is incorrect because decreasing read capacity does not help writes.

Answer

Enable DynamoDB Streams on the table to offload writes to a Lambda function

Answer

Create a DynamoDB global table to distribute writes across regions

Answer

Decrease the read capacity to free up resources for writes

Question 7

A company is using Amazon Redshift for analytics. The database administrator notices that some queries are slow and the system is running out of memory. Which THREE steps should the administrator take to improve performance?

Accepted Answer

Optimize the table design by choosing appropriate distkeys and sortkeys. Option A is correct because adding nodes increases memory. Option B is correct because distkey and sortkey optimization reduces data movement. Option C is correct because WLM queues can limit concurrency to reduce memory contention. Option D is incorrect because vacuum is for disk space, not memory. Option E is incorrect because increasing node count (scale out) is better than scale up for memory.

Answer

Increase the node size (scale up) to get more memory per node

Answer

Run the VACUUM command to reclaim space from deleted rows

Question 8

A company runs a critical application on Amazon RDS for PostgreSQL with Multi-AZ deployment. The database is a db.r5.xlarge instance with 500 GB of Provisioned IOPS (io1) storage. The application writes a large volume of time-series data every 5 minutes. Recently, the team noticed that the replica lag between the primary and standby instance has increased to over 10 seconds during peak write periods. The application requires that the standby be within 5 seconds of the primary for disaster recovery purposes. The team has already verified that network latency between AZs is minimal, and the standby instance is the same size as the primary. The CloudWatch metric 'TransactionLogsDiskUsage' shows that the primary is generating logs at a high rate. Which combination of actions should the database administrator take to reduce the replica lag? (Choose the best option.)

Accepted Answer

Increase the Provisioned IOPS on both the primary and standby instances to improve log write throughput. Option B is correct because increasing the storage IOPS on both instances reduces the time to write transaction logs to disk, which is the bottleneck. The primary generates logs, and the standby applies them; both need fast I/O. Option A is insufficient because disabling synchronous commit might cause data loss. Option C is incorrect because the standby is already the same size; increasing its size alone may not help if the bottleneck is I/O. Option D is incorrect because reducing the batch size reduces write volume, affecting application performance.

Answer

Disable synchronous commit on the primary to reduce log generation

Answer

Change the standby instance to a larger instance type (e.g., db.r5.2xlarge) to improve apply throughput

Answer

Reduce the batch write frequency to every 10 minutes to lower the log generation rate

Question 9

A company is using Amazon DynamoDB for a gaming leaderboard. The table has a partition key of 'game_id' and a sort key of 'score'. The table is configured with on-demand capacity. During a major tournament, the application experiences high latency and some requests return 'ProvisionedThroughputExceededException' errors. The CloudWatch metric 'ThrottledRequests' spikes. The application uses a single partition key for all writes during the tournament (game_id = 'tournament_final'). What is the most likely cause of the throttling, and what is the best solution?

Accepted Answer

The application is using a single partition key, causing all writes to go to one partition. The team should redesign the partition key to distribute writes across multiple partitions. Option C is correct because using a single partition key creates a hot partition, leading to throttling even with on-demand capacity (which has per-partition limits). The best solution is to redesign the partition key to distribute writes. Option A is incorrect because auto scaling is not needed for on-demand. Option B is incorrect because DAX caches reads, not writes. Option D is incorrect because creating a GSI does not affect write throttling on the base table.

Answer

The application is using a single partition key, causing all writes to go to one partition. The team should implement DAX to cache writes

Answer

The table has a global secondary index that is throttling writes; the team should remove the GSI

Answer

The table is using on-demand capacity, which has a maximum throughput limit per partition; the team should switch to provisioned capacity with auto scaling

Question 10

A company runs an Amazon Redshift cluster with 8 dc2.large nodes for its data warehouse. The data engineering team loads data daily using COPY commands from S3. Recently, the load times have increased significantly. The cluster's CloudWatch metric 'CPUUtilization' is high during the load. The administrator runs the STL_LOAD_ERRORS table and finds no errors. The SVL_S3LOG shows that the COPY command is scanning many small files. The data in S3 is stored as 10,000 small CSV files (each ~100 KB). Which action will MOST improve the COPY performance?

Accepted Answer

Consolidate the small files into fewer, larger files (e.g., 100 files of 10 MB each). Option B is correct because Redshift performs better with fewer, larger files. Consolidating small files into larger ones reduces the overhead of opening many files. Option A is incorrect because the issue is not with the COPY command syntax. Option C is incorrect because changing the distribution style does not affect COPY performance directly. Option D is incorrect because the issue is not with the column data type.

Answer

Use the MANIFEST option to specify the files explicitly

Answer

Use the JSON format instead of CSV to reduce parsing overhead

Answer

Change the table's distribution style to ALL to avoid data redistribution

Question 11

A company is using Amazon RDS for MySQL and has enabled Enhanced Monitoring. The database administrator wants to identify the top contributors to disk I/O. Which THREE metrics from Enhanced Monitoring should they examine?

Accepted Answer

DirtyBufferFlushRate. Option A is correct because physical reads cause disk I/O. Option C is correct because write operations generate I/O. Option D is correct because dirty buffer flushes cause writes. Option B is incorrect because logical reads are from memory, not disk. Option E is incorrect because network throughput is not disk I/O.

Answer

NetworkThroughput

Answer

LogicalReads

Question 12

A company is using Amazon RDS for MySQL with Multi-AZ deployment. The application team reports intermittent connection timeouts. CloudWatch shows increased DatabaseConnections and CPU Utilization during peak hours. Which action should the database specialist take to troubleshoot the issue?

Accepted Answer

Enable slow query log and analyze queries.. Option B is correct because enabling slow query log helps identify long-running queries causing high CPU and connections. Option A is wrong because Multi-AZ failover does not resolve performance issues. Option C is wrong because enhancing CloudWatch metrics does not directly fix timeouts. Option D is wrong because read replicas are for read scaling, not for reducing connection timeouts on the primary.

Answer

Add enhanced monitoring to collect additional metrics.

Answer

Create a read replica and redirect read traffic.

Answer

Failover to the standby instance to refresh connections.

Question 13

A financial services company runs a critical application on Amazon DynamoDB. Recently, they observed increased latency and throttled requests. Upon reviewing CloudWatch metrics, they see that WriteCapacityUnits consumed is consistently below the provisioned capacity, but ReadCapacityUnits consumed is frequently at 100%. The application performs equal mix of strongly consistent reads and writes. What is the most likely cause of the throttling?

Accepted Answer

Hot partitions causing uneven read traffic distribution.. Option A is correct because throttling on reads with provisioned capacity still available indicates hot partitions. Option B is wrong because writes are not throttled. Option C is wrong because adaptive capacity helps but does not eliminate hot partitions. Option D is wrong because strongly consistent reads consume twice the read capacity units, but the issue is hot partitions.

Answer

Adaptive capacity is automatically adjusting partition throughput.

Answer

Write capacity is insufficient for the workload.

Answer

Strongly consistent reads are consuming twice the RCU.

Question 14

A developer is troubleshooting an application that writes to an Amazon ElastiCache for Redis cluster. The application occasionally fails with 'OOM command not allowed when used memory > maxmemory'. What is the most likely cause?

Accepted Answer

The cluster's maxmemory-policy is set to noeviction.. Option C is correct because this error indicates the cache has reached its memory limit and eviction policy is not set or set to noeviction. Option A is wrong because that error is about keyspace notifications. Option B is wrong because cluster mode may affect sharding but not this specific error. Option D is wrong because connection limit would show a different error.

Answer

The cluster's maxclients limit has been reached.

Answer

The cluster has too many keyspace notifications enabled.

Answer

The cluster is in cluster mode and cross-slot commands are used.

Question 15

A company uses Amazon Aurora MySQL. They notice that the DB cluster's failover took longer than expected during a recent primary instance failure. CloudWatch shows Failover latency of 120 seconds. Which configuration change would most likely reduce the failover time?

Accepted Answer

Configure the application to use the cluster endpoint with Aurora JDBC driver's fast failover feature.. Option A is correct because Aurora's fast failover requires the JDBC driver to use the cluster endpoint with the Aurora hostlist provider. Option B is wrong because increasing instance size does not reduce failover time. Option C is wrong because Multi-AZ is already inherent in Aurora. Option D is wrong because increasing backup retention does not affect failover.

Answer

Increase the instance class of the primary and replica instances.

Answer

Increase the backup retention period to 35 days.

Answer

Enable Multi-AZ on the DB cluster.