C100DBA Server Administration Practice Test 3 — 15 Questions

Question 1

A database is experiencing high latency. You run 'mongotop 5'. What specific insight does this provide compared to 'mongostat' for server administration?

Accepted Answer

Time spent by each collection on read and write operations.. While mongostat gives a high-level view of server-wide operations and memory, mongotop provides a granular look at where the storage engine is spending its time. It breaks down read and write time by collection, allowing the DBA to identify which specific table is causing the most load on the system.

Answer

A list of the longest-running queries currently in the system.

Answer

The amount of disk space consumed by each database on the server.

Answer

The memory distribution between the data and index caches.

Question 2

Refer to the exhibit. The MongoDB process has crashed due to a storage engine error. What is the most immediate administrative action required to restore service?

Accepted Answer

Expand the disk volume and restart the mongod process.. The error 'No space left on device' indicates that the disk partition hosting the 'dbPath' directory is completely full. This prevents WiredTiger from writing data, metadata, or logs, causing an immediate shutdown to prevent data corruption. Administrators must free up space on the disk or extend the partition before restarting the service, as the database cannot recover or accept new writes without sufficient storage capacity to manage its internal files.

Answer

Run 'db.repairDatabase()' via the shell to free up space.

Answer

Clear the 'journal' directory files to recover space.

Answer

Modify the 'storage.wiredTiger.engineConfig.cacheSizeGB' setting in the config file.

Question 3

Which configuration file setting is used to specify the network interface and port that a mongod instance listens on?

Accepted Answer

net. The 'net' section in the YAML configuration file provides granular control over how the MongoDB instance communicates over the network. Configuring the 'bindIp' and 'port' is a fundamental step in securing and reaching the database. Properly setting these parameters ensures that the server is accessible only on the intended interfaces, which is a critical security practice to prevent unauthorized access from untrusted networks or public interfaces.

Answer

systemLog

Answer

storage

Answer

processManagement

Question 4

When adding a new node to an existing replica set, what is the primary factor that determines how quickly the new member will be able to begin participating in elections?

Accepted Answer

The amount of data in the dataset and network bandwidth.. The speed at which a new replica set member becomes 'secondary' and ready to participate depends on the amount of data it must synchronize from an existing member. Initial sync involves cloning data and applying the oplog. Understanding this process is vital for capacity planning, as adding a large member can put significant I/O and network pressure on the current primary or secondaries, potentially impacting performance for existing application traffic during the synchronization process.

Answer

The memory limit set in the configuration file.

Answer

The number of users defined in the admin database.

Answer

The version of the MongoDB shell used.

Question 5

An administrator wants to audit all authentication attempts on a MongoDB Enterprise instance. Which feature should be configured to track these events?

Accepted Answer

Set up an 'auditLog' destination in the YAML configuration.. The auditing system in MongoDB Enterprise is essential for compliance and security monitoring. By configuring the audit log to capture events related to authentication and authorization, administrators gain visibility into who is accessing the database and what actions they are performing. This is crucial for forensic analysis, detecting unauthorized access attempts, and satisfying regulatory requirements that mandate detailed record-keeping for all database interactions within a production environment.

Answer

Enable 'logLevel' to 5 in the configuration.

Answer

Use the 'mongostat' utility.

Answer

Enable 'net.ssl.mode' to 'requireSSL'.

Question 6

Which command is used to verify the current replica set configuration and the state of all members?

Accepted Answer

rs.status(). Checking the replica set status is a routine administrative task to ensure high availability and data consistency. The 'rs.status()' command provides a comprehensive view of the cluster, including the state of every member, the current primary, and the sync lag. This command is the primary diagnostic tool for verifying that the replica set is healthy and that all nodes are correctly communicating with each other.

Answer

db.serverStatus()

Answer

rs.conf()

Answer

db.isMaster()

Question 7

What is the consequence of having a replica set with only two nodes and no arbiter?

Accepted Answer

The replica set will be unable to elect a primary if one node fails.. A replica set requires a majority of nodes to elect a primary. In a two-node set, a failure of one node makes it impossible to reach a majority, as one out of two is not greater than half. This design limitation is critical because it leads to the loss of a primary and the inability to elect a new one, resulting in a read-only database and complete outage for write operations.

Answer

The set will automatically double the voting power of the primary.

Answer

The replica set will function normally, but writes will be slow.

Answer

The primary will become read-only while the secondary remains idle.

Question 8

Refer to the exhibit. What is the implication of setting the priority of n2 to 0?

Accepted Answer

n2 will never become the primary node.. A priority of 0 designates a member as a 'passive' node. It can never become the primary, even if all other nodes are down. This configuration is often used for nodes in different data centers to prevent them from taking over as primary, which could cause latency issues for applications. Understanding this is vital for controlling election behavior and ensuring that the primary node is always located in a preferred, low-latency environment for the application workload.

Answer

n2 will be excluded from the voting process entirely.

Answer

n2 will stop replicating data from the primary.

Answer

n2 will be removed from the replica set automatically.

Question 9

Which TWO of the following are valid reasons to use an arbiter in a MongoDB replica set?

Accepted Answer

To break a tie in elections for replica sets with an even number of nodes.. Arbiters are lightweight processes that participate in elections but do not hold data. They are a cost-effective way to achieve a majority for elections in small replica sets. Understanding when to use an arbiter is crucial for designing a cost-efficient and highly available architecture, especially when deploying in multiple zones where adding a full data-bearing node might be cost-prohibitive or unnecessary due to data redundancy requirements.

Answer

To increase the read throughput of the replica set.

Answer

To maintain a copy of the data for disaster recovery.

Answer

To act as a hidden node for analytical queries.

Question 10

You are monitoring a MongoDB replica set and notice that the replication lag on a secondary is increasing steadily. You suspect that the secondary is unable to keep up with the write load. Which administrative action should you take first to diagnose the issue?

Accepted Answer

Check the output of rs.printSlaveReplicationInfo() to see the lag and the time of the last oplog entry.. The first step in diagnosing replication lag is to quantify it and check the secondary's progress. rs.printSlaveReplicationInfo() provides the lag in seconds and the timestamp of the last oplog entry applied, helping you understand the severity and whether the secondary is making progress. Other actions like restarting or resizing the oplog are premature without diagnosis.

Answer

Restart the secondary to see if the lag clears.

Answer

Run db.currentOp() on the secondary to see long-running operations.

Answer

Increase the oplog size on the primary to allow more time for the secondary to catch up.

Question 11

A MongoDB replica set has three members: a primary and two secondaries. The DBA needs to perform maintenance on the primary node without causing an election that could lead to data loss. The maintenance requires stopping the mongod process for 30 minutes. Which step should the DBA take first to ensure the primary steps down gracefully and a secondary takes over?

Accepted Answer

Run `rs.stepDown(600)` on the primary.. To gracefully transfer primacy before maintenance, the DBA should use `rs.stepDown()`, which forces the primary to become a secondary and triggers an election. The optional argument specifies how long the stepped-down member remains ineligible for election, preventing it from immediately reclaiming primacy. This allows a secondary to take over and provides a stable primary during the maintenance window.

Answer

Run `db.shutdownServer()` on the primary.

Answer

Run `rs.freeze(600)` on the primary.

Answer

Run `rs.remove()` to remove the primary from the replica set.

Question 12

A MongoDB DBA is asked to ensure that the mongod process on a Linux server automatically restarts if it crashes. The server uses systemd. Which command should the DBA run to enable this behavior?

Accepted Answer

Edit the mongod.service unit file to include Restart=always, then run systemctl daemon-reload and systemctl restart mongod. To enable automatic restart of mongod on failure when using systemd, you must modify the service unit file to include Restart=always (or Restart=on-failure). After editing, run systemctl daemon-reload to reload the configuration, and then restart the service. The systemctl enable command only ensures start at boot, not restart on crash. The other commands are either manual restarts or do not provide automatic restart functionality. (Explanation length: 65 words)

Answer

systemctl enable mongod

Answer

Run mongod --fork --logpath /var/log/mongodb/mongod.log

Answer

systemctl restart mongod

Question 13

A DBA is configuring a replica set with three members: one primary and two secondaries. The DBA wants to ensure that a write concern of { w: 'majority' } is acknowledged only after the write has been applied to a majority of voting members and has been written to the on-disk journal on those members. Which write concern should the DBA use?

Accepted Answer

{ w: 'majority', j: true }. The requirement is for a write concern that ensures acknowledgment after a majority of voting members have applied the write and written it to the on-disk journal. The write concern { w: 'majority', j: true } achieves both: w: 'majority' dynamically requires a majority of voting members, and j: true forces journaling on those members before acknowledgment. Using a fixed w: 2 may not adapt to membership changes, and omitting j: true does not guarantee journaling. (Explanation length: 65 words)

Answer

{ w: 'majority' }

Answer

{ w: 2, j: true }

Answer

{ w: 'majority', j: false }

Question 14

A DBA needs to grant a new application service account the ability to read and write data in any database except the admin database on a MongoDB 6.0 replica set. The account should not be able to perform administrative actions such as managing users or shutting down the server. Which built-in role should the DBA assign?

Accepted Answer

readWriteAnyDatabase. The readWriteAnyDatabase built-in role provides read and write access to all non-system databases without granting administrative privileges. It is the least-privilege role that satisfies the requirement. The other roles either grant excessive privileges, are scoped to a single database, or do not provide the needed access across all application databases.

Answer

root

Answer

dbOwner on each application database

Answer

readWrite on the admin database

Question 15

A MongoDB 6.0 replica set is experiencing performance issues during peak hours. The DBA notices that the primary is spending significant time on disk I/O. The DBA wants to identify which collections are generating the most disk reads and writes. Which administrative command should the DBA use?

Accepted Answer

mongotop. mongotop is designed to provide per-collection read and write statistics, showing how much time each collection spends on these operations. This directly answers the need to find collections generating the most disk I/O. The other tools either provide server-wide metrics, point-in-time operation lists, or static collection statistics that do not reflect active I/O.

Answer

db.currentOp()

Answer

db.serverStatus()

Answer

db.collection.stats()