DBS-C01 Monitoring and Troubleshooting Practice Test 19 — 15 Questions

Question 1

A company is running a MongoDB-compatible Amazon DocumentDB cluster. The application team reports that write operations are failing intermittently with a `WriteConcernError` indicating that the write concern could not be satisfied. The cluster has one primary and two replicas. What is the MOST likely cause of this issue?

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

Accepted Answer

One of the replicas is down or experiencing high replication lag. The WriteConcernError occurs because the write concern requires acknowledgment from a certain number of replicas. With a cluster of one primary and two replicas, if any replica is down or has high replication lag, the write concern requirement cannot be satisfied. Option A is correct because that is the most likely cause. Option B is incorrect because the cluster has enough replicas for majority write concern if all are healthy. Option C is incorrect because read preference does not affect write operations. Option D is incorrect because the primary being overloaded with reads would not cause a WriteConcernError; it would affect performance but not the ability to satisfy write concern.

Answer

The cluster does not have enough replica instances to satisfy the write concern

Answer

The application is using an incorrect read preference

Answer

The primary instance is overloaded with read requests

Question 2

A company is migrating an on-premises Oracle database to Amazon Aurora PostgreSQL. The migration uses AWS Database Migration Service (DMS) with ongoing replication. The team notices that the target Aurora database is falling behind the source during peak hours. Which of the following actions would MOST effectively improve the replication performance?

Accepted Answer

Configure the DMS task to use 'Limited LOB mode' and increase the max LOB size. The most effective action. Using 'Limited LOB mode' in AWS DMS reduces the overhead of replicating large objects by only transferring metadata until the LOB is accessed, and increasing the max LOB size ensures that LOBs fit within a single transaction, preventing fragmentation. Option A is incorrect because disabling Multi-AZ does not directly affect DMS replication performance; Multi-AZ provides high availability but does not impact replication throughput. Option B, increasing the Aurora instance size, may help if the instance is CPU or memory constrained, but the primary bottleneck in replication is often the handling of large objects, making DMS task configuration more targeted. Option C is incorrect because using a smaller instance would worsen performance, not improve it.

Answer

Disable Multi-AZ on the target Aurora cluster

Answer

Increase the Amazon Aurora instance size

Answer

Use a smaller Aurora instance to reduce write latency

Question 3

A company is using Amazon ElastiCache for Redis as a caching layer. The application is experiencing higher latency than expected. The team suspects that cache evictions are occurring due to memory pressure. Which ElastiCache metric should be monitored to confirm this?

Accepted Answer

Evictions. The `Evictions` metric in ElastiCache for Redis tracks the number of keys evicted due to memory pressure, which directly indicates cache evictions. Option A is incorrect because `CurrConnections` shows the number of current client connections, not evictions. Option C is incorrect because `ReplicationLag` measures the delay between the primary and replica nodes, not evictions. Option D is incorrect because `CacheHits` shows the number of successful key lookups, not evictions.

Answer

CurrConnections

Answer

ReplicationLag

Answer

CacheHits

Question 4

A company is using Amazon DynamoDB and has enabled DynamoDB Streams. The application needs to process stream records in real-time. Which AWS service can be used to invoke a Lambda function automatically for each stream record?

Accepted Answer

AWS Lambda. DynamoDB Streams can be directly integrated with AWS Lambda, enabling automatic invocation of a Lambda function for each stream record in near real-time. This is the natural choice for processing stream records without additional services. Option A (Amazon Kinesis Data Firehose) is used to load streaming data into data stores, not to invoke Lambda. Option B (Amazon SQS) is a message queue service; DynamoDB Streams does not directly integrate with SQS, and Lambda would still be needed to poll or process. Option C (AWS Step Functions) orchestrates workflows but does not directly trigger from DynamoDB Streams without Lambda.

Answer

Amazon Kinesis Data Firehose

Answer

Amazon Simple Queue Service (SQS)

Answer

AWS Step Functions

Question 5

A company is running an Amazon RDS for SQL Server instance with Multi-AZ deployment. The database is used by a critical application. During a recent failover test, the application experienced a 2-minute downtime. The application's connection string uses the DB instance endpoint, not the cluster endpoint. Which change would minimize downtime during future failovers?

Accepted Answer

Modify the application to use the cluster endpoint instead of the instance endpoint. Using the cluster endpoint (the DNS name that automatically points to the current primary after failover) allows the application to reconnect to the new primary without waiting for DNS propagation, thus minimizing downtime. Option B is incorrect because increasing the DB instance class size does not reduce failover time. Option C is incorrect because read replicas are used for read scaling, not for reducing failover downtime. Option D is incorrect because Multi-AZ is already enabled, so enabling it again would have no effect.

Answer

Increase the DB instance class size

Answer

Create a read replica in a different Availability Zone

Answer

Enable Multi-AZ on the DB instance

Question 6

A company is using Amazon RDS for MySQL and needs to monitor for slow queries. Which TWO AWS services can be used to capture and analyze slow query logs? (Choose TWO.)

Accepted Answer

Amazon RDS Performance Insights. Amazon RDS Performance Insights (Option B) provides database performance analysis and can help identify slow queries by visualizing database load. Amazon CloudWatch Logs (Option E) can ingest and analyze RDS slow query logs by streaming them from RDS. Amazon S3 (Option A) is an object storage service, not a monitoring or analysis service. AWS Config (Option C) is for recording configuration changes, not database logs. AWS CloudTrail (Option D) records API calls for governance, not database-level query logs.

Answer

Amazon S3

Answer

AWS Config

Answer

AWS CloudTrail

Question 7

A company is using Amazon DynamoDB with auto scaling enabled. Despite auto scaling, the application is still experiencing throttling during traffic spikes. Which THREE actions should the company take to resolve this issue? (Choose THREE.)

Accepted Answer

Implement exponential backoff in the application code. Exponential backoff (A) is a best practice to retry throttled requests gracefully, reducing retry storms. DAX (B) caches read-heavy workloads, reducing read capacity unit consumption and mitigating hot key issues. Global tables (C) distribute write traffic across multiple regions, alleviating write throttling. Option D (on-demand capacity) could help but may be cost-prohibitive and does not address hot keys. Option E (disable auto scaling) would worsen throttling by fixing capacity. Therefore, A, B, and C are correct.

Answer

Switch to on-demand capacity mode

Answer

Disable auto scaling and set fixed capacity

Question 8

A company is using Amazon Redshift and notices that queries are running slowly. Which TWO system views should be used to identify the cause of the slow queries? (Choose TWO.)

Accepted Answer

STL_QUERY. The correct answers are C and E. STL_QUERY contains detailed logs of all queries, including execution times, enabling identification of slow queries after execution. SVV_QUERY_STATE shows currently running queries, helping identify those that are running slowly. Option A (STV_TBL_PERM) is for table permissions, not performance. Option B (PG_TABLE_DEF) shows table definitions. Option D (STL_LOAD_COMMITS) shows load commit information.

Answer

STV_TBL_PERM

Answer

PG_TABLE_DEF

Answer

STL_LOAD_COMMITS

Question 9

A company runs an e-commerce platform on AWS using a multi-tier architecture. The application tier consists of Auto Scaling groups of EC2 instances behind an Application Load Balancer. The database tier uses Amazon RDS for MySQL with Multi-AZ deployment. Recently, the operations team noticed that during flash sales, the application becomes unresponsive and users receive 503 errors. The team checks CloudWatch metrics and sees that the RDS instance's CPU utilization spikes to 100%, and the `DatabaseConnections` metric also spikes to the maximum allowed value of 500. The application uses connection pooling with a maximum of 200 connections, but the metric shows 500 connections. The team suspects that the connection pooling configuration is not being honored. The application code is written in Python and uses SQLAlchemy with a connection pool size of 10 per application instance. There are 20 application instances in the Auto Scaling group during peak times. The team wants to resolve the issue without increasing the database instance size. What should the team do?

Accepted Answer

Set the `max_connections` parameter in the RDS parameter group to 200 and configure the application to handle connection errors with retry logic. Setting the `max_connections` parameter in the RDS parameter group enforces a hard limit at the database level, preventing the database from accepting more than 200 connections. With 20 application instances each using a connection pool of 10, the total intended connections are 200. By setting `max_connections` to 200, any excess connection attempts will be rejected, and the application should handle connection errors with retry logic, preventing the database from being overwhelmed. Option A is wrong: reducing the Auto Scaling group's desired capacity to 10 instances would reduce the total connection pool to 100, which might temporarily reduce load but is not a scalable solution and does not address the root cause that the application's connection pooling is not being honored. Option C is wrong: migrating to Amazon Aurora MySQL with Auto Scaling does not directly solve the connection pooling issue. Aurora may allow more connections, but the root cause is that the application is opening more connections than configured, and a migration is a costly change that doesn't fix the configuration problem. Option D is wrong: increasing the SQLAlchemy pool size per instance to 25 would increase the total potential connections to 500 (20 instances × 25), which would worsen the connection spike and exceed the database's maximum, increasing contention and load.

Answer

Reduce the Auto Scaling group's desired capacity to 10 instances during flash sales

Answer

Migrate the database to Amazon Aurora MySQL with Auto Scaling enabled

Answer

Increase the SQLAlchemy pool size to 25 per instance to reduce connection contention

Question 10

A database administrator notices that the CPU utilization on an Amazon RDS for PostgreSQL instance is consistently above 90% during peak hours. Which CloudWatch metric should be checked first to identify the cause of the high CPU usage?

Accepted Answer

DatabaseConnections. A high number of database connections can lead to increased CPU usage as each connection requires processing. Option B is wrong because NetworkThroughput measures network traffic, which is not a primary cause of high CPU. Option C is wrong because SwapUsage indicates memory pressure, not CPU. Option D is wrong because ReadIOPS measures disk I/O and may not directly cause high CPU.

Answer

NetworkThroughput

Answer

SwapUsage

Answer

ReadIOPS

Question 11

A company is running a production Amazon Aurora MySQL database. The database performance has degraded over the past week. The DBA suspects an increase in lock waits. Which tool should be used to identify queries experiencing lock waits?

Accepted Answer

Amazon RDS Performance Insights. Amazon RDS Performance Insights provides a database load visualization and detailed wait event analysis, allowing you to identify queries experiencing lock waits. Option A is incorrect because CloudWatch Logs captures log streams but does not directly show lock wait events. Option B is incorrect because Enhanced Monitoring provides OS-level metrics (CPU, memory, disk I/O) but not database-specific lock waits. Option D is incorrect because AWS Trusted Advisor offers best-practice recommendations but does not monitor real-time database performance or lock waits.

Answer

Amazon CloudWatch Logs

Answer

Amazon RDS Enhanced Monitoring

Answer

AWS Trusted Advisor

Question 12

A company is using Amazon DynamoDB with auto scaling. The application is experiencing increased read latency. The monitoring shows that the ReadThrottleEvents metric is high. Which action should the company take to reduce read throttling?

Accepted Answer

Implement DynamoDB Accelerator (DAX) for caching.. DynamoDB Accelerator (DAX) provides an in-memory cache that reduces read latency and alleviates read throttling by serving frequently accessed items from cache, reducing the load on the underlying table. Option B is incorrect because switching to on-demand capacity mode may help with unpredictable traffic patterns but does not specifically address read throttling caused by high read demand; it can also increase costs. Option C is incorrect because enabling DynamoDB Streams does not directly reduce read throttling and may add overhead. Option D is incorrect because increasing write capacity units does not affect read throttling since read and write capacities are separate.

Answer

Switch the table to on-demand capacity mode.

Answer

Enable DynamoDB Streams on the table.

Answer

Increase the write capacity units.

Question 13

A developer reports that an application is unable to connect to an Amazon RDS for Oracle database. The security group for the database allows inbound traffic on port 1521 from the application's security group. The database is publicly accessible. What should be checked next?

Accepted Answer

Verify that the DB subnet group includes a public subnet with an internet gateway.. The database is publicly accessible, meaning it must be in a public subnet that has an internet gateway attached. Even if the security group allows traffic, the subnet must be configured correctly for public access. Option A is incorrect because the 'remote_listener' parameter affects Oracle listener registration, not basic network connectivity. Option B is incorrect because backup retention is unrelated to connection issues. Option D is incorrect because CloudWatch Logs may not capture failed connection attempts at the network level.

Answer

Check the DB parameter group for the 'remote_listener' parameter.

Answer

Check the automated backup retention period.

Answer

Review the CloudWatch Logs for error logs.

Question 14

A company is running a MongoDB database on Amazon EC2. The database is experiencing high disk I/O latency. Which AWS service can be used to monitor the disk I/O metrics at the instance level?

Accepted Answer

Amazon CloudWatch. Amazon CloudWatch provides disk I/O metrics for EC2 instances. Option A is wrong because Amazon RDS is a managed database service. Option C is wrong because Amazon DynamoDB is a NoSQL database service. Option D is wrong because Amazon S3 is an object storage service.

Answer

Amazon RDS

Answer

Amazon DynamoDB

Answer

Amazon S3

Question 15

A company is experiencing slow query performance on an Amazon RDS for MySQL database. The DBA wants to identify the most time-consuming queries. Which TWO actions should the DBA take? (Choose two.)

Accepted Answer

Use Amazon RDS Performance Insights.. Amazon RDS Performance Insights (Option D) provides a database performance tuning and monitoring feature that visualizes database load and identifies the most time-consuming queries by breaking down wait events, SQL statements, and hosts. It directly helps the DBA pinpoint the specific queries causing performance degradation without additional configuration or overhead.

Answer

Enable the audit log.

Answer

Enable the general log and review it.

Answer

Use Amazon RDS Enhanced Monitoring.