C100DEV Drivers, Tools, Transactions, and Search Practice Test 3 — 15 Questions

Question 1

An application uses a multi-document transaction to update inventory and process an order. What happens if the transaction exceeds the default transaction lifetime limit of 60 seconds?

Accepted Answer

The server automatically aborts the transaction and returns an error to the client.. Transactions in MongoDB are designed to be short-lived to prevent performance degradation and memory pressure. If a transaction stays open longer than the transactionLifetimeLimitSeconds configuration, the reaper process marks it as aborted. This ensures that locking contention is minimized, as prolonged transactions hold locks on documents, preventing other operations from proceeding and potentially leading to high latency or deadlocks across the cluster.

Answer

The transaction automatically commits partial changes to ensure data availability.

Answer

The client receives a warning, but the transaction continues until completion.

Answer

The database increases the transaction lifetime limit dynamically to prevent failure.

Question 2

Which tool is primarily recommended for performing binary backups and restores of a MongoDB deployment?

Accepted Answer

mongodump. mongodump and mongorestore are standard utilities for creating and restoring binary backups of MongoDB databases. Unlike mongosh which is for interaction, or Compass for visualization, these tools operate directly on the BSON data representation. Understanding the correct backup utility is essential for disaster recovery and data migration, ensuring that the integrity of the data is maintained during the backup process across different MongoDB versions.

Answer

mongosh

Answer

Compass

Answer

mongostat

Question 3

Refer to the exhibit. An application receives this error during a multi-document transaction. What is the correct approach for the application developer?

Accepted Answer

Use the driver's retry logic to perform the entire transaction again.. The error label 'TransientTransactionError' indicates that the operation failed due to a temporary state, such as a network glitch or a transient deadlock. In MongoDB, these errors are expected and should be handled by the application through a retry mechanism. Using the driver's built-in 'withTransaction' helper function is the recommended practice, as it automatically implements the necessary retry logic for these types of transient, non-permanent failures.

Answer

Terminate the application immediately and alert the database administrator.

Answer

Retry the specific failed write operation only, outside of the transaction.

Answer

Hard-code a try-catch block to skip the transaction and log the error.

Question 4

Which MongoDB tool should be used to perform a point-in-time backup of a MongoDB cluster while the database is running?

Accepted Answer

mongodump. mongodump is the standard command-line utility for creating binary exports of MongoDB data. When used with the --oplog option, it allows for point-in-time recovery by capturing the operations log during the backup process. This is essential for maintaining data integrity in production environments where downtime must be avoided, allowing administrators to restore the cluster to a specific state as needed by business requirements.

Answer

mongoimport

Answer

mongostat

Answer

mongorestore

Question 5

What is the primary purpose of the 'causal consistency' feature provided by MongoDB sessions?

Accepted Answer

To ensure that a client reads the results of its own writes.. Causal consistency ensures that operations within a session appear in an order that respects the causal relationship between them, even when reading from secondary nodes. This is vital in distributed systems to prevent 'time-travel' anomalies, such as reading a stale version of a document immediately after an update. By using the operation time, MongoDB guarantees that a user sees their own writes immediately, maintaining a logical progression of data state.

Answer

To force all reads and writes to occur on the primary node only.

Answer

To improve overall write throughput by batching operations.

Answer

To automatically shard data based on access patterns.

Question 6

In the context of the MongoDB Node.js driver, what happens if an application fails to provide a write concern?

Accepted Answer

The driver applies the default write concern (usually 'w: 1').. If no write concern is explicitly defined, the driver uses the default write concern configured at the client or database level. If no default is configured, the driver falls back to 'w: 1', meaning the operation is considered successful once the primary node receives and acknowledges the write. This is the standard behavior to balance performance with a baseline level of data durability in most production applications.

Answer

The write will never be acknowledged by the server.

Answer

The server automatically sets the write concern to 'majority'.

Answer

The write will be rejected by the MongoDB server.

Question 7

Which TWO of the following are common use cases for using the 'mongosh' shell in a production environment?

Accepted Answer

Querying and troubleshooting live collection data.. mongosh is the modern shell for MongoDB, offering a rich environment for database administration and scripting. Beyond simple CRUD, it is essential for checking cluster health, managing indexes, and performing emergency administrative tasks directly on the server. Because it provides full access to the MongoDB driver API, it is an indispensable tool for developers and DBAs to troubleshoot production issues and perform ad-hoc data investigations effectively.

Answer

Developing the entirety of a high-concurrency Node.js application.

Answer

Replacing the driver for all production database traffic.

Answer

Automatically scaling the number of nodes in an Atlas cluster.

Question 8

What is the primary function of the MongoDB connection pool in a driver?

Accepted Answer

To reduce the latency of opening new connections.. A connection pool maintains a cache of open connections to the database server. Reopening a connection for every request is expensive due to the overhead of TCP handshakes and authentication. By reusing existing connections, the driver significantly reduces latency and resource consumption, allowing the application to handle high volumes of traffic more efficiently and providing a more stable connection to the database layer.

Answer

To encrypt data in transit between the client and the server.

Answer

To automatically shard data based on the client's request.

Answer

To store query results in memory for faster retrieval.

Question 9

A developer uses the MongoDB Node.js driver to run a multi-document transaction that updates an accounts collection and inserts a ledger entry. The transaction commits successfully, but a second application instance immediately reads the ledger entry through a session that is not causally linked to the transaction session and receives stale data. The developer wants the second instance to observe the transaction's committed writes without changing the transaction itself. Which approach should the developer use?

Accepted Answer

After the transaction commits, capture the operationTime and clusterTime from the transaction session and pass them in the readConcern and afterClusterTime of the subsequent read session.. Causal consistency requires the second session's read to wait until the cluster time has passed the transaction's commit. The transaction session exposes operationTime and clusterTime, and the subsequent read must include afterClusterTime in its readConcern to enforce that ordering. This ensures the committed ledger entry is visible without modifying the transaction or relying on read preference.

Answer

Start a new session for the read, set readConcern to "snapshot" on the read operation, and run the read without linking it to the transaction session.

Answer

Use the same session that ran the transaction for the subsequent read operation on the second application instance.

Answer

Set the read preference of the second instance to "primaryPreferred" and retry the read until the ledger entry appears.

Question 10

A developer needs to run a multi-document transaction using the MongoDB Node.js driver. The transaction must read from the 'orders' collection and write to the 'inventory' collection. Which sequence of operations correctly implements the transaction?

Accepted Answer

Start a session with client.startSession(), start a transaction with session.startTransaction(), perform reads and writes with the session option, then commit with session.commitTransaction().. A multi-document transaction in the Node.js driver requires a session. The session must be started, the transaction begun, operations executed with the session passed as an option, and then the transaction committed. Omitting any step, such as not passing the session to operations or not explicitly committing, results in operations not being part of the transaction or the transaction remaining open.

Answer

Use a session, start a transaction, perform operations with the session, and rely on automatic commit when the session ends.

Answer

Begin a transaction with db.beginTransaction(), run operations on the db object, and call db.commitTransaction().

Answer

Use client.startTransaction() directly, perform operations without a session, and call transaction.commit().

Question 11

A developer is using the MongoDB Node.js driver to perform a multi-document transaction that updates multiple collections. During testing, the transaction occasionally fails with a TransientTransactionError. What is the most appropriate way to handle this error?

Accepted Answer

Retry the entire transaction from the beginning, including starting a new session and re-executing all operations.. When a transaction fails with TransientTransactionError, the correct approach is to retry the entire transaction. This means aborting the current transaction if it is still open, starting a new session or reusing the session after aborting, and re-executing all operations. Retrying only a single operation or ignoring the error can result in partial updates or constraint violations.

Answer

Increase the transaction lifetime limit using the maxTransactionTimeMS option and continue without retrying.

Answer

Ignore the error and proceed, as MongoDB will automatically retry the transaction.

Answer

Retry only the failed operation within the same transaction, without aborting or restarting the transaction.

Question 12

A developer runs `mongosh` against a replica set and wants to inspect the current database's collections before writing any queries. Which command should be executed?

Accepted Answer

show collections. The shell helper `show collections` is the idiomatic mongosh command for listing collections in the active database. It is concise, requires no JavaScript and directly satisfies the developer's need to inspect the database before writing queries. Other options either query a nonexistent collection, return different metadata or list databases rather than collections.

Answer

db.runCommand({ listDatabases: 1 })

Answer

db.collections.find()

Answer

db.getCollectionInfos()

Question 13

A developer wants to query MongoDB Atlas Search for documents where the 'description' field contains the phrase 'wireless headphones'. Which Atlas Search operator should be used to achieve this?

Accepted Answer

The phrase operator.. To search for an exact phrase in Atlas Search, the phrase operator is the correct choice. It ensures that the terms appear in the specified order and adjacent to each other. Other operators like text, wildcard, or compound serve different purposes and would not guarantee phrase matching.

Answer

The compound operator.

Answer

The wildcard operator.

Answer

The text operator.

Question 14

A developer is writing a Node.js application that must execute a series of document updates across two collections atomically. The application uses the MongoDB Node.js driver and needs to ensure that if any operation fails, all changes are rolled back. Which driver API should the developer use to achieve this?

Accepted Answer

Use the startSession() method on the MongoClient instance and then call session.withTransaction() to run the operations.. Multi-document transactions in MongoDB are managed through client sessions. The Node.js driver's session.withTransaction() method handles the lifecycle, including retries on transient errors and automatic commit or abort. This ensures that updates across multiple collections are atomic. Other methods like bulkWrite or findOneAndUpdate operate on a single collection or document and lack the necessary transactional guarantees.

Answer

Use the insertMany() method with the ordered option set to false to ensure atomicity.

Answer

Use the bulkWrite() method with ordered: true to group updates across collections.

Answer

Use the findOneAndUpdate() method with a session parameter for each update.

Question 15

An application using the MongoDB Node.js driver must ensure that a multi-document transaction is retried correctly when it encounters a transient error. Which two practices are required? (Choose two.)

Accepted Answer

On a commit error with the UnknownTransactionCommitResult label, retry the commit operation.. The Node.js driver exposes error labels that tell the application how to recover. A TransientTransactionError requires restarting the entire transaction, while UnknownTransactionCommitResult requires retrying the commit. These two behaviors are the documented retry pattern for multi-document transactions. Other options either misplace read concern, misunderstand the lifetime limit or weaken durability without addressing retries.

Answer

Increase the transaction lifetime limit beyond 60 seconds to avoid all retries.

Answer

Set the readConcern to 'snapshot' on every individual operation inside the transaction.

Answer

Disable write concern to make commits faster and reduce retries.