C100DBA Practice Test 7 — 10 Questions

Question 1

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 2

Which read concern provides the strongest consistency guarantee by ensuring the data returned has been acknowledged by a majority of the replica set?

Accepted Answer

majority. The 'majority' read concern guarantees that the data read has been committed to a majority of the replica set members, ensuring that the read will not be rolled back after a failover. This is essential for applications requiring strong consistency, such as financial systems. DBAs must guide developers on when to trade off performance for this level of consistency to ensure the application behaves correctly under all possible cluster failure scenarios.

Answer

local

Answer

available

Answer

snapshot

Question 3

In a three-node replica set with default write concern w:1, what happens to an unacknowledged write if the primary crashes immediately after receiving the write command but before replicating it to secondaries?

Accepted Answer

The write is lost, and the new primary will not contain the data.. With w:1, the primary acknowledges the write as soon as it is applied to its local oplog. If the primary fails before replication, the new primary elected from the remaining nodes will not contain this write. This demonstrates the critical trade-off between performance and durability. Clients must use w:'majority' to ensure data consistency during failover events, as single-node acknowledgments are insufficient to guarantee persistence against primary loss.

Answer

The write is automatically rolled back and re-applied when the old primary recovers.

Answer

The write remains in the local storage of the crashed node and becomes visible upon restart.

Answer

The replica set triggers an automatic consistency recovery to force the write to other nodes.

Question 4

When designing a compound shard key in MongoDB, what is the architectural significance of placing a high-cardinality field as the prefix compared to placing a low-cardinality field first?

Accepted Answer

A high-cardinality prefix ensures fine-grained chunk boundaries and even data distribution, whereas a low-cardinality prefix causes massive chunks that cannot be split effectively.. The shard key prefix dictates how MongoDB organizes and partitions chunk ranges across the cluster. A high-cardinality prefix ensures fine-grained chunk boundaries and prevents massive, unmanageable chunks from forming around repetitive values, which often happens when low-cardinality fields like boolean flags or status codes occupy the leading position.

Answer

A high-cardinality prefix allows the query router to bypass authentication checks on secondary shards, improving overall cluster query response times.

Answer

A low-cardinality prefix enables the WiredTiger storage engine to cache entire indexes in RAM, eliminating disk I/O bottlenecks entirely.

Answer

A low-cardinality prefix automatically triggers hashed shard evaluation, bypassing the need for explicit hashing functions during collection creation.

Question 5

A multi-document transaction is being executed across multiple shards. Which component is responsible for coordinating the transaction and ensuring the all-or-nothing property?

Accepted Answer

The mongos router that initiated the transaction.. Multi-document transactions in a sharded cluster are coordinated by the mongos router where the transaction was initiated. The mongos acts as the transaction coordinator, using a two-phase commit protocol to ensure consistency across the participant shards. This allows applications to maintain ACID guarantees even when data is distributed across a horizontally scaled environment.

Answer

The primary node of the config server replica set.

Answer

The shard that contains the first document accessed in the transaction.

Answer

A specialized 'Transaction Manager' node that must be configured.

Question 6

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()

Question 7

A logistics company is designing a MongoDB database for a real-time shipment tracking system. The operations team needs to frequently update shipment statuses and query the latest events. The development team is debating the trade-offs of embedding versus referencing for the shipment events. Which two considerations align with MongoDB's philosophy for this scenario? (Choose two.)

Accepted Answer

Embedding events within the shipment document ensures atomic updates and fast reads for the event history.. The correct considerations are that embedding provides atomic updates and fast reads for the event history, while referencing allows unlimited growth and avoids the 16 MB limit. These reflect MongoDB's flexible modeling philosophy: choose embedding when data is accessed together and atomicity is needed, but opt for referencing when data grows unbounded or is accessed independently. The decision hinges on access patterns and data lifecycle.

Answer

Embedding events is always preferred because it eliminates the need for multi-document transactions.

Answer

Referencing events requires using $lookup for every read, which is as fast as an embedded document read.

Answer

Embedding events ensures that the shipment document remains small and efficient for writes.

Question 8

A DBA is reviewing update operations that use the upsert option. Which TWO statements about upsert behavior in MongoDB are correct? (Choose two.)

Accepted Answer

If no document matches the filter, upsert inserts a new document based on the filter and the update operators.. Upsert inserts a new document when the filter matches nothing, building it from equality conditions in the filter plus the update operators. Non-equality operators do not contribute fields, so the inserted document may lack the filtered field. A unique index is recommended to prevent duplicate inserts from concurrent upserts, but it is not mandatory. Array operators such as $push are allowed in upserts.

Answer

Upsert cannot be used with update operators that modify array elements, such as $push.

Answer

If multiple documents match the filter, upsert updates all of them and inserts no new document.

Answer

Upsert always requires a unique index on the filter fields to avoid duplicate inserts.

Question 9

A financial services company runs a sharded MongoDB cluster with three shards. They need to shard a new collection that stores transaction records. The collection will be queried primarily by 'accountId' and 'transactionDate'. The team wants to minimize scatter-gather queries and ensure even data distribution. Which two actions should they take when choosing and implementing the shard key? (Choose two.)

Accepted Answer

Ensure the shard key has high cardinality and low frequency to avoid jumbo chunks and hot spots.. The compound shard key with 'accountId' first supports targeted queries on account and range queries on date, while also providing high cardinality and low frequency. This combination reduces scatter-gather and avoids hot spots. Hashed or random keys improve distribution but break query targeting, and a monotonically increasing key creates write hot spots.

Answer

Choose a shard key that is monotonically increasing, such as an auto-incrementing transaction ID, to simplify range queries.

Answer

Use a random shard key generated by the application to guarantee even distribution across shards.

Answer

Use a hashed shard key on 'accountId' to ensure even distribution, even though it prevents efficient range queries on 'transactionDate'.

Question 10

A MongoDB DBA is configuring a new sharded cluster. The application requires that queries on the sharded collection include the shard key to avoid scatter-gather. The DBA decides to use a ranged shard key on the field 'region'. After sharding, the DBA notices that queries filtering only on 'region' are performing well, but queries that also filter on 'city' are still scanning all shards. What is the most likely cause?

Accepted Answer

The shard key does not include 'city', so queries with both 'region' and 'city' cannot be targeted to a single shard.. Query targeting in a sharded cluster depends on the shard key. If the shard key is only 'region', queries that filter on 'region' can be routed to specific shards. Adding 'city' does not change the routing because 'city' is not part of the shard key. To target on both fields, the shard key would need to be compound, including both 'region' and 'city'.

Answer

The 'city' field is not indexed, causing the query to scan all shards.

Answer

The balancer has not yet distributed chunks evenly, so some shards have more data and cause the query to scan them.

Answer

The shard key 'region' has low cardinality, so queries cannot be targeted efficiently.