DBS-C01 Monitoring and Troubleshooting Practice Test 11 — 15 Questions

Question 1

A database specialist is troubleshooting a performance issue on a self-managed PostgreSQL database that they plan to migrate to Amazon RDS. The database has a high number of 'idle in transaction' connections. What is the impact of these connections on the database?

Accepted Answer

They hold locks and prevent cleanup of dead tuples, leading to bloat.. Option D is correct because idle-in-transaction connections hold locks and can cause bloat, reducing performance. Option A is wrong because they do not prevent new connections from being established. Option B is wrong because CPU usage is low for idle connections. Option C is wrong because they do not affect disk I/O directly.

Answer

They increase CPU usage due to constant polling.

Answer

They prevent new connections from being established.

Answer

They cause increased disk I/O from write-ahead logging.

Question 2

A company is using Amazon DynamoDB with provisioned capacity. They notice an increase in throttled write requests. The workload consists of writes to a single partition key. Which TWO actions would help reduce throttling?

Accepted Answer

Increase the provisioned write capacity units.. Options A and C are correct. Using DynamoDB Accelerator (DAX) reduces read load but does not help with writes; option B is irrelevant. Option D is for read scaling. Option E (using write sharding) distributes writes across partitions. Option C (increasing write capacity) directly addresses throttling. Option A (adaptive capacity) helps but may not fully resolve hot partition; however, it is a valid action. The question expects A and C as the most direct.

Answer

Add a global secondary index with a different partition key.

Answer

Use DynamoDB Accelerator (DAX) for write caching.

Answer

Implement write sharding by adding a suffix to the partition key.

Question 3

A company is monitoring an Amazon RDS for Oracle instance. CloudWatch alarms show that FreeableMemory is consistently below 256 MB. The database has high read and write I/O. Which THREE steps should the database specialist take to diagnose the issue?

Accepted Answer

Check the MemoryPressure and LogFileSyncDuration metrics in CloudWatch.. Options A, C, and D are correct. Checking memory pressure (A), looking at swap usage (C), and reviewing memory advisor (D) are all appropriate. Option B is wrong because storage auto scaling does not address memory. Option E is wrong because RDS uses a fixed SGA/PGA, and increasing instance size might be a solution but not a diagnostic step.

Answer

Enable storage auto scaling to increase allocated storage.

Answer

Increase the DB instance class to allocate more memory.

Question 4

A company uses Amazon ElastiCache for Redis. They want to monitor cache hit ratio. Which TWO metrics should be used to calculate the cache hit ratio?

Accepted Answer

CacheHits. Options A and C are correct. Cache hit ratio = CacheHits / (CacheHits + CacheMisses). CurItems is for memory usage, Evictions is for memory pressure, and GetTypeCmds is total get commands.

Answer

GetTypeCmds

Answer

Evictions

Answer

CurItems

Question 5

Refer to the exhibit. An IAM policy attached to an IAM role used by an AWS Lambda function. The Lambda function is invoked by Amazon API Gateway and reads from a DynamoDB table. The function is experiencing permission errors when calling DynamoDB GetItem. What is the most likely cause?

Accepted Answer

The policy does not grant access to DynamoDB indexes if the function uses a query on a Global Secondary Index.. Option B is correct. The policy grants GetItem and Query on the table ARN, but it does not include the 'dynamodb:GetItem' action? Wait, it does include it. Actually, the correct answer is that the policy is missing the 'dynamodb:GetItem' action? No, it's there. The issue is that the function is using a conditional write? Hmm. Let's analyze: The policy allows GetItem and Query on the table. That should work. But perhaps the function is trying to access a specific index? The policy does not include access to indexes. Option B says missing index ARN. That is plausible. Option A is wrong because the Lambda resource is fine. Option C is wrong because GetItem is included. Option D is wrong because the role trust policy is not shown.

Answer

The IAM role trust policy does not allow Lambda to assume the role.

Answer

The dynamodb:GetItem action is not listed in the policy.

Answer

The Lambda function ARN in the resource field is incorrect.

Question 6

A social media company runs a multi-region application on Amazon DynamoDB with global tables. The application is deployed in us-east-1 and eu-west-1. Recently, they enabled DynamoDB Streams on the table to trigger an AWS Lambda function for real-time analytics. The Lambda function runs in us-east-1. After enabling streams, they notice that the Lambda function is triggered multiple times for the same update, and the application's write latency in eu-west-1 has increased. The company has strict latency requirements. A database specialist is asked to resolve the issue. What should the database specialist recommend?

Accepted Answer

Modify the Lambda function to check the 'awsRegion' attribute in the stream record and ignore records from other regions.. Option C is correct because global tables replicate writes across regions, and each write generates a stream record. The Lambda function should filter on the 'awsRegion' attribute in the stream record to avoid processing the same update from different regions. Option A is wrong because disabling streams in eu-west-1 would break the replication. Option B is wrong because using a dedicated stream for each region is not supported. Option D is wrong because increasing Lambda concurrency doesn't prevent duplicate processing.

Answer

Increase the Lambda function's reserved concurrency to handle duplicate events faster.

Answer

Create a separate Lambda function in eu-west-1 and use a local stream.

Answer

Disable DynamoDB Streams on the eu-west-1 replica table.

Question 7

A retail company uses Amazon RDS for PostgreSQL as the backend for its e-commerce platform. During a flash sale, the database experienced high CPU utilization and increased the number of active connections. The application team reported that some queries timed out. The database specialist reviewed the slow query log and found that several queries were performing sequential scans on large tables due to missing indexes. The specialist created the necessary indexes, but the issue persists for some queries. Upon further investigation, the specialist notices that the query planner is still choosing sequential scans for some queries. What should the database specialist do to ensure the query planner uses the indexes?

Accepted Answer

Run the ANALYZE command to update table statistics.. Option A is correct because running ANALYZE updates table statistics, which helps the query planner choose the best plan. Option B is wrong because increasing maintenance_work_mem helps with autovacuum but not query planning. Option C is wrong because disabling sequential scans is a brute-force approach that may not be optimal. Option D is wrong because setting cpu_tuple_cost changes the cost model but might not be necessary.

Answer

Increase maintenance_work_mem to speed up index creation.

Answer

Decrease the random_page_cost to make indexes more attractive.

Answer

Set enable_seqscan to off to force index usage.

Question 8

A startup is using Amazon ElastiCache for Redis to cache session data. They deployed a single Redis node (cache.t3.micro) in us-west-2. The application reports high latency when reading session data. CloudWatch metrics show CPUUtilization at 90% and Evictions at 100 per minute. The cache hit ratio is 80%. The database specialist suspects the node is overloaded. What should the specialist do to improve performance?

Accepted Answer

Scale up to a larger node type, such as cache.m5.large.. Option D is correct because increasing the node size provides more CPU and memory, reducing evictions and latency. Option A is wrong because a read replica helps with read scaling but the node is a primary. Option B is wrong because Redis Cluster mode requires multiple shards and may add complexity. Option C is wrong because reducing TTL might increase cache misses.

Answer

Add a read replica to offload read traffic.

Answer

Enable cluster mode and add more shards.

Answer

Decrease the TTL for session keys to reduce memory usage.

Question 9

A company runs a document management system using Amazon S3 and Amazon DynamoDB. The application writes document metadata to DynamoDB and stores the document in S3. Recently, users report that occasionally documents are saved in S3 but the corresponding metadata is missing in DynamoDB. The application writes to DynamoDB first, then to S3. If the S3 upload fails, the application retries. The database specialist suspects a transaction consistency issue. The application is running on multiple EC2 instances behind an Application Load Balancer. What should the specialist recommend to ensure that both the metadata and document are stored consistently?

Accepted Answer

Use DynamoDB transactions to write metadata and initiate S3 upload within the same transaction.. Option B is correct because using DynamoDB transactions ensures atomicity; if the S3 upload fails, the transaction can be rolled back. Option A is wrong because DynamoDB Streams with Lambda introduces eventual consistency and possible duplicates. Option C is wrong because writing to S3 first still leaves inconsistency if DynamoDB fails. Option D is wrong because idempotency tokens help with duplicates but not atomicity.

Answer

Use DynamoDB Streams to trigger an AWS Lambda function that performs the S3 upload.

Answer

Reverse the order: upload to S3 first, then write to DynamoDB.

Answer

Implement an idempotency key in the application to retry the entire operation.

Question 10

A financial services company is using Amazon Aurora MySQL as its primary database. The database has a table 'transactions' that receives high inserts during business hours. The table is partitioned by date. Recently, the application team noticed an increase in lock wait timeouts. The database specialist reviewed the InnoDB status and found that there are frequent gap locks on the 'transaction_date' column. The isolation level is REPEATABLE READ. What should the specialist do to reduce lock waits while maintaining data consistency?

Accepted Answer

Change the transaction isolation level to READ COMMITTED.. Option A is correct because changing the isolation level to READ COMMITTED eliminates gap locks for non-unique indexes, reducing lock contention. Option B is wrong because increasing innodb_lock_wait_timeout only increases timeout, not reduces locks. Option C is wrong because adding an index on transaction_date may increase lock granularity? Actually, if the index is not unique, gap locks still occur. Option D is wrong because partitioning does not affect gap locks.

Answer

Add a secondary index on transaction_date.

Answer

Increase the innodb_lock_wait_timeout parameter.

Answer

Modify the partitioning key to use a hash-based partition.

Question 11

A company uses Amazon DynamoDB as a session store for a web application. The application uses a TTL attribute to expire old sessions. The company noticed that expired sessions are not being deleted promptly, causing the table size to grow and increasing costs. The TTL attribute is defined as 'expire_time' with a Unix epoch timestamp. The database specialist verified that TTL is enabled. What should the specialist do to ensure expired sessions are deleted in a timely manner?

Accepted Answer

Increase the provisioned write capacity on the table to allow TTL to delete items faster.. Option C is correct because TTL deletion can be delayed if the table has a high write rate; increasing provisioned write capacity can speed up the deletion process. Option A is wrong because DynamoDB does not have a 'ttl_deletion_lag' parameter. Option B is wrong because TTL handles deletion automatically; a Lambda function is unnecessary. Option D is wrong because the attribute type is correct (Number).

Answer

Change the TTL attribute type to String format.

Answer

Configure the 'ttl_deletion_lag' parameter to a lower value.

Answer

Create an AWS Lambda function that scans the table and deletes expired items.

Question 12

A database administrator notices that an Amazon RDS for MySQL DB instance is experiencing high CPU utilization and increased latency during peak hours. The administrator wants to identify the queries causing the issue. Which TWO actions should be taken to diagnose the problem? (Select TWO.)

Accepted Answer

Enable Performance Insights on the DB instance.. Option A: Enabling Performance Insights provides a performance schema and wait events to identify problematic queries. Option C: Enabling the slow query log captures queries that take longer than a specified time, helping to pinpoint slow queries. Option B: RDS Enhanced Monitoring is for OS-level metrics, not query-level. Option D: Amazon DevOps Guru is for broader anomaly detection, not specific query identification. Option E: RDS Proxy is for connection pooling, not query analysis.

Answer

Enable Amazon DevOps Guru for RDS.

Answer

Use RDS Proxy to manage connections.

Answer

Enable RDS Enhanced Monitoring.

Question 13

A company is running a production Amazon Aurora MySQL-Compatible Edition database. The database has recently experienced several failovers due to replica lag. The DBA needs to implement monitoring to detect replica lag early. Which THREE metrics should be monitored to assess replication health? (Select THREE.)

Accepted Answer

DatabaseConnections. Option A: AuroraReplicaLag is the direct metric for replica lag. Option B: ReplicaLag is the standard metric for MySQL replication lag. Option D: DatabaseConnections can indirectly indicate issues if application retries increase. Option C: BufferCacheHitRatio is about cache efficiency, not replication. Option E: ActiveTransactions is about transactions, not replication.

Answer

ActiveTransactions

Answer

BufferCacheHitRatio

Question 14

A company uses Amazon DynamoDB for a highly transactional application. The application is experiencing increased latency and throttled requests. The operations team notices that the DynamoDB table's read and write capacity utilization is consistently near 100%. The table uses on-demand capacity mode. What is the MOST likely cause of the throttling?

Accepted Answer

The table has reached the per-table throughput limit for on-demand mode.. Option D: On-demand mode has a per-table throughput limit (e.g., 40,000 read/write units per second). When sustained traffic exceeds that, requests are throttled. Option A: Global tables replicate writes to other regions but do not cause throttling. Option B: DynamoDB Accelerator (DAX) reduces latency but can still see throttling if the table itself is throttled. Option C: Adaptive capacity automatically scales partitions but cannot exceed the on-demand limit.

Answer

The table is configured as a global table and cross-region replication is causing write conflicts.

Answer

The application is not using DynamoDB Accelerator (DAX) to cache reads.

Answer

The table's partition key design is causing hot partitions, and adaptive capacity is not enabled.

Question 15

A company is running a self-managed MongoDB cluster on Amazon EC2. The cluster consists of three replica set members in different Availability Zones. The primary node recently experienced a crash, and the cluster failed over to a secondary. However, the new primary is showing significantly higher latency. The operations team wants to ensure that failover is fast and consistent. What should be done to improve the failover reliability?

Accepted Answer

Use Amazon EBS io2 Block Express volumes with provisioned IOPS.. Option C: Using Amazon EBS io2 Block Express volumes with higher IOPS and lower latency can reduce the time for the secondary to catch up. Option A: EBS Multi-Attach is not supported for io2? Actually io2 supports multi-attach but for shared volumes, not typically used for MongoDB. Option B: Using instance store volumes may have higher performance but are ephemeral and data loss is possible. Option D: Snapshots are for backups, not for improving failover performance.

Answer

Use Amazon EBS Multi-Attach to allow all replicas to share the same volume.

Answer

Switch to instance store volumes for better I/O performance.

Answer

Configure EBS snapshots to be taken every 5 minutes.