C100DBA Practice Test 3 — 25 Questions

Question 1

A logistics company uses MongoDB to track shipments. Each shipment document includes a status field and an array of location updates. The company wants to ensure that when a shipment's status changes to "delivered," the corresponding location update is also recorded atomically. Which MongoDB feature best supports this requirement while adhering to the philosophy of document atomicity?

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

Accepted Answer

Single-document atomic operations. Single-document atomic operations allow multiple field updates within one document to be atomic, which is ideal when related data is embedded. By storing location updates within the shipment document, the company can atomically change the status and append a new location in one operation, ensuring consistency without multi-document transactions. This leverages MongoDB's core philosophy of designing documents to match access patterns and using document-level atomicity.

Answer

Write concern "majority"

Answer

Two-phase commit

Answer

Multi-document ACID transactions

Question 2

A DBA is troubleshooting a replica set where the secondary is consistently lagging behind the primary by several hours. The DBA notices that the secondary's optime is far behind the primary's optime, and the secondary's oplog window is only 1 hour. The primary has a heavy write workload. Which action should the DBA take to reduce the replication lag and prevent the secondary from falling off the oplog?

Accepted Answer

Scale the secondary vertically by adding more CPU, RAM, or faster disk I/O.. Replication lag is typically caused by the secondary's inability to apply oplog entries as quickly as the primary generates them. Improving the secondary's hardware resources—CPU, RAM, and especially disk I/O—increases its oplog application throughput, directly reducing lag. Increasing oplog size only buys time, and write concern does not affect replication speed.

Answer

Convert the secondary to an arbiter to reduce its workload.

Answer

Increase the write concern on the primary to ensure the secondary acknowledges writes.

Answer

Increase the size of the oplog on the primary and secondaries.

Question 3

You are the DBA for a MongoDB 6.0 replica set with three voting members: rs0-primary (priority 2), rs0-secondary-1 (priority 1), and rs0-secondary-2 (priority 1). The primary fails and is unavailable for an extended period. You need to ensure that rs0-secondary-1 is elected as the new primary instead of rs0-secondary-2. Which action should you take before the primary fails?

Accepted Answer

Set rs0-secondary-1's priority to 2 and rs0-secondary-2's priority to 0.5 using rs.reconfig().. Election priority determines which eligible secondary is most likely to win an election. By setting rs0-secondary-1's priority higher than rs0-secondary-2's, you make it the preferred candidate. Lowering rs0-secondary-2's priority to a non-zero value keeps it as a voting member but reduces its election chances. This approach is the standard way to control primary election outcomes in a MongoDB replica set without making a member unelectable.

Answer

Set rs0-secondary-1's hidden property to true and rs0-secondary-2's hidden property to false using rs.reconfig().

Answer

Set rs0-secondary-1's slaveDelay to 0 and rs0-secondary-2's slaveDelay to 3600 using rs.reconfig().

Answer

Set rs0-secondary-1's votes to 1 and rs0-secondary-2's votes to 0 using rs.reconfig().

Question 4

A sharded cluster has a collection sharded on `{ location: 1 }`. A query is executed with the filter `{ location: { $in: ["NY", "CA"] } }`. How does MongoDB route this query?

Accepted Answer

The query is routed to the shards that own chunks covering the values "NY" and "CA".. MongoDB's query router, mongos, can target queries that include the shard key, even with `$in`. It looks up the chunk ranges for the specified values and routes the query only to the shards that own those chunks. This is known as a targeted query and is efficient because it avoids contacting unnecessary shards.

Answer

The query is sent to all shards because `$in` cannot be targeted.

Answer

The query is routed to a random shard, and that shard forwards the request to the others as needed.

Answer

The query is routed to the primary shard for the database because `$in` is not a targeted operation.

Question 5

You are a MongoDB DBA and need to retrieve all documents from the 'products' collection where the 'price' field is greater than 100 and the 'category' is 'electronics'. Which query accomplishes this?

Accepted Answer

db.products.find({price: {$gt: 100}, category: 'electronics'}). The implicit AND by combining field conditions in a single query document is the standard way to match documents where multiple criteria must be true. Using $and explicitly is also valid but redundant. $or would match either condition, and .filter() is not a MongoDB method.

Answer

db.products.find({$or: [{price: {$gt: 100}}, {category: 'electronics'}]})

Answer

db.products.find({price: {$gt: 100}}).filter({category: 'electronics'})

Answer

db.products.find({$and: [{price: {$gt: 100}}, {category: 'electronics'}]})

Question 6

A MongoDB 6.0 replica set has a collection called orders with the default read concern. Users occasionally see an order document that still shows status "pending" even though a separate application thread just updated it to "shipped" and received an acknowledgment. The update used default write concern and targeted a single document by _id. Which read concern should you set on the read operation to guarantee the read returns data acknowledged by a majority of replica set members?

Accepted Answer

readConcern: "majority". The symptom is a stale read after an acknowledged single-document update in a replica set. Majority read concern ensures the read only returns data that a majority of members have acknowledged, so the newly committed "shipped" status becomes visible. Local and available provide no such guarantee, and snapshot is aimed at multi-document consistency rather than this single-document visibility problem.

Answer

readConcern: "snapshot"

Answer

readConcern: "available"

Answer

readConcern: "local"

Question 7

Refer to the exhibit. What philosophy regarding client connectivity does this log entry reflect?

Accepted Answer

MongoDB prioritizes universal language and driver support.. The log entry indicates that MongoDB accepts standard TCP connections from client applications. This reflects the philosophy of providing a simple, standard-based interface for developers. By using standard socket-based communication, MongoDB allows drivers to be implemented in virtually any programming language, enabling widespread compatibility and easy integration into diverse application ecosystems without requiring custom protocols or proprietary communication middleware for database interaction.

Answer

MongoDB uses a proprietary, non-standard transport protocol.

Answer

All clients must be authenticated via a central service.

Answer

The database requires persistent, long-lived connections.

Question 8

Which THREE of the following are core pillars of the MongoDB architecture?

Accepted Answer

Horizontal scalability through sharding.. MongoDB's architecture is built on the principles of horizontal scalability, high availability, and developer-focused data modeling. These pillars allow MongoDB to handle massive scale and fluctuating workloads efficiently. By integrating sharding for capacity, replication for reliability, and flexible schemas for agility, the system addresses the common limitations of static relational databases, offering a comprehensive platform for modern, distributed cloud-native applications that require both speed and robustness in dynamic environments.

Answer

Strict, static schema enforcement at the storage layer.

Answer

Global dependency on a single centralized primary node.

Question 9

An administrator needs to create a restricted user for a third-party monitoring tool. The user must be able to perform backups and monitor server status but cannot read actual document data. Which TWO built-in roles should be combined?

Accepted Answer

backup. MongoDB's Role-Based Access Control (RBAC) allows for granular permission management. The 'backup' role provides the necessary privileges to use tools like mongodump without granting general read access to all collections. The 'clusterMonitor' role allows the user to run commands like 'serverStatus' and 'top', which are essential for monitoring performance.

Answer

readAnyDatabase

Answer

dbAdmin

Answer

hostManager

Question 10

An application needs to update a user's balance. Which method ensures the operation is atomic for a single document?

Accepted Answer

Use the $inc operator with the updateOne method.. Atomic updates are a cornerstone of MongoDB's document model. Using update operators like $inc ensures that the update is performed in a single, atomic step at the document level. This prevents race conditions where two threads might attempt to update the same balance simultaneously, ensuring that data integrity is maintained without the need for manual locking mechanisms in the application layer.

Answer

Read the document, update the value in application code, and save the document.

Answer

Wrap the operation in a multi-document transaction every time.

Answer

Use a global application-level mutex lock before updating.

Question 11

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 12

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 13

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 14

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 15

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 16

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 17

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 18

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 19

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 20

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.