DBS-C01 Practice Test 9 — 10 Questions

Question 1

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?

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

Accepted Answer

The cluster's maxmemory-policy is set to noeviction.. The error 'OOM command not allowed when used memory > maxmemory' occurs when Redis has reached its configured memory limit and the eviction policy is set to 'noeviction', which prevents any further writes. Option B is correct because 'noeviction' causes Redis to return an error instead of evicting keys. Option A is incorrect because the 'maxclients' limit produces a different error ('max number of clients reached'). Option C is incorrect because keyspace notifications do not affect memory limits. Option D is incorrect because cross-slot commands relate to cluster mode, not memory errors.

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 2

A company is designing a new e-commerce platform using Amazon DynamoDB. The workload requires single-digit millisecond latency for user session data, which is accessed by session token. The session data is temporary and should be automatically deleted after 24 hours. Which DynamoDB design should the database specialist recommend?

Accepted Answer

Enable DynamoDB Time to Live (TTL) on the session token attribute. DynamoDB Time to Live (TTL) automatically deletes expired items after a specified timestamp, making it ideal for session data that must be removed after 24 hours. This approach requires no additional infrastructure, meets the single-digit millisecond latency requirement by using the session token as the primary key, and ensures automatic cleanup without manual intervention or added cost.

Answer

Create an AWS Lambda function that runs every hour and deletes expired session data

Answer

Store session data in Amazon S3 with a lifecycle policy to delete objects after 24 hours

Answer

Use DynamoDB Accelerator (DAX) to cache session data and set a 24-hour TTL on the cache

Question 3

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.. Idle-in-transaction connections hold locks and prevent PostgreSQL's autovacuum from cleaning up dead tuples, leading to table bloat and performance degradation. Option A is wrong because idle transactions do not cause constant polling; CPU usage remains low. Option C is wrong because they do not prevent new connections from being established unless the max_connections limit is reached. Option D is wrong because idle transactions do not significantly increase write-ahead logging.

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 4

A company uses Amazon RDS for MySQL to store e-commerce order data. The orders table has millions of rows and is frequently queried by order_id. The company also runs periodic reports that aggregate data by order_date. The reports are slow. The database has a primary key on order_id. The company needs to improve report performance without affecting OLTP queries. Which design change should be made?

Accepted Answer

Create a secondary index on order_date.. Creating a secondary index on order_date allows MySQL to quickly locate rows matching the report's date range without scanning the entire table, significantly improving aggregation performance. This index is separate from the primary key on order_id, so OLTP queries that filter by order_id remain unaffected. The index provides a balanced approach: it accelerates read-heavy reporting while adding minimal overhead to write operations.

Answer

Upgrade to a larger instance type.

Answer

Create a read replica and run reports on the replica.

Answer

Partition the table by order_date.

Question 5

A developer is troubleshooting slow queries in Amazon RDS for MySQL. The 'Threads_running' status variable is consistently above 200. The application uses connection pooling. Which metric should be monitored to identify the root cause?

Accepted Answer

Innodb_row_lock_current_waits. InnoDB row lock waits directly indicate contention for row-level locks, which is a common cause of high Threads_running when many queries are blocked waiting for locks held by other transactions. Monitoring Innodb_row_lock_current_waits reveals the number of transactions currently waiting for a row lock, pinpointing lock contention as the root cause of slow queries and elevated running threads.

Answer

Queries_per_second

Answer

Slow_queries

Answer

Threads_connected

Question 6

A company is migrating an on-premises Oracle database to Amazon Aurora PostgreSQL. The database contains a large table that is partitioned by date. The company wants to minimize downtime during migration and ensure compatibility. Which TWO actions should the company take?

Accepted Answer

Use AWS Database Migration Service (DMS) with ongoing replication to migrate the data.. AWS DMS with ongoing replication (change data capture) allows continuous synchronization from the source Oracle database to the target Aurora PostgreSQL, minimizing downtime by keeping the target nearly up-to-date until the final cutover. This approach supports large partitioned tables and reduces the migration window compared to a full load only.

Answer

Migrate the database using a full load only and accept downtime.

Answer

Use AWS Schema Conversion Tool (SCT) to convert the schema automatically without any manual adjustments.

Answer

Replace the Oracle database with Amazon RDS for MySQL.

Question 7

A database specialist needs to monitor the number of deadlocks occurring in an Amazon RDS for SQL Server DB instance. Which CloudWatch metric should be used?

Accepted Answer

Deadlocks. The 'Deadlocks' metric in Amazon RDS for SQL Server specifically counts the number of deadlocks that occur on the DB instance. Option A is incorrect because 'BlockedTransactions' tracks transactions that are blocked, not deadlocks. Option C is incorrect because 'DatabaseConnections' is a count of active connections, not related to deadlocks. Option D is incorrect because 'LockWaits' measures the number of lock waits, which is a different contention event.

Answer

BlockedTransactions

Answer

DatabaseConnections

Answer

LockWaits

Question 8

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?

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 9

Which TWO factors should be considered when choosing between Amazon DynamoDB and Amazon RDS for MySQL for a new e-commerce application with variable traffic patterns?

Accepted Answer

Operational overhead: DynamoDB is serverless, RDS requires patching and provisioning.. DynamoDB is a fully managed serverless service that eliminates the need for patching, provisioning, or managing servers, whereas Amazon RDS for MySQL requires manual patching, scaling, and instance provisioning. This operational overhead difference is critical for variable traffic patterns, as DynamoDB automatically handles infrastructure management, reducing administrative burden.

Answer

Ability to perform complex joins and aggregations: DynamoDB supports SQL-like queries.

Answer

Support for ACID transactions in both databases.

Answer

Encryption at rest is only available for RDS.

Question 10

A company is migrating a PostgreSQL database to Amazon RDS for PostgreSQL. They want to automate the deployment of the RDS instance using infrastructure as code. Which AWS service should they use?

Accepted Answer

AWS CloudFormation.. AWS CloudFormation is the correct service for automating the deployment of an RDS for PostgreSQL instance using infrastructure as code. It allows you to define the entire RDS instance configuration—including DB instance class, storage, backup retention, and security groups—in a declarative JSON or YAML template, which can be version-controlled and reused for consistent, repeatable deployments.

Answer

Amazon EC2 Auto Scaling.

Answer

AWS Management Console.

Answer

AWS Command Line Interface (CLI).