C100DBA Practice Test 16 — 15 Questions

Question 1

When using 'readConcern: majority', what is the specific guarantee provided to the client?

Accepted Answer

It guarantees that the data read has been committed to a majority of nodes.. The 'majority' read concern guarantees that the data returned has been acknowledged by a majority of the replica set members. This ensures that the data is durable and will not be rolled back in the event of a primary failover. It provides a higher level of consistency for read operations, which is critical for applications that require strict data integrity and visibility.

Answer

It guarantees that the read will never return stale data.

Answer

It forces the read to always come from the primary node.

Answer

It increases the speed of queries by ignoring consistency.

Question 2

A security audit requires that sensitive document data contained in query parameters must not be written to the MongoDB system logs. Which server configuration setting should be enabled?

Accepted Answer

security.redactClientLogData: true. Security administration involves protecting data at rest, in transit, and in logs. Log redaction is a feature that prevents sensitive information, like the contents of a 'find' filter or an 'insert' document, from appearing in the server logs. This is essential for compliance with regulations like GDPR, HIPAA, or PCI-DSS.

Answer

security.authorization: enabled

Answer

operationProfiling.mode: off

Answer

storage.wiredTiger.collectionConfig.blockCompressor: zlib

Question 3

Refer to the exhibit. An aggregation pipeline is failing with an error. How can you modify the pipeline to allow it to process the large dataset?

Accepted Answer

Set 'allowDiskUse' to true in the aggregation options.. The error indicates that the group operation is exceeding the 100MB RAM limit. By setting 'allowDiskUse: true', you permit the MongoDB server to use temporary files for the aggregation process. This is a common requirement for complex analytics where the data working set is larger than the available RAM, ensuring that long-running operations can complete successfully without crashing due to memory exhaustion.

Answer

Increase the RAM allocated to the mongod process configuration.

Answer

Remove the $group stage and perform the grouping in application code.

Answer

Reduce the batch size in the cursor options.

Question 4

What occurs when a replica set member experiences a 'rollback'?

Accepted Answer

The node discards data to align with the new primary and saves it to rollback files.. Rollback happens when a node was previously a primary, performed writes that were never replicated to the majority, and then rejoined the set after a new primary was elected. To maintain consistency, the old primary must discard those unacknowledged writes to match the new primary's state. These discarded writes are saved to a local BSON file for manual inspection and potential recovery by administrators.

Answer

The node automatically merges its divergent data with the primary.

Answer

The node is permanently removed from the replica set until manually re-added.

Answer

The node initiates a global election to force a return to its old state.

Question 5

A DBA executes 'mongodump --oplog' against a primary node in a replica set. What is the specific purpose of the --oplog flag in this administrative context?

Accepted Answer

It captures changes during the dump for point-in-time consistency.. When backing up a live database, data changes while the backup is running. The '--oplog' flag tells mongodump to capture all operations that occur during the dump process. This allows the DBA to restore the database to a consistent state by replaying those operations during the 'mongorestore' process, ensuring data integrity.

Answer

It exports the entire oplog collection into a separate BSON file.

Answer

It speeds up the dump process by reading from the oplog instead of collections.

Answer

It automatically compresses the output using the oplog's compression ratio.

Question 6

A sharded cluster has a three-member replica set for its config servers. If two of the three config servers go offline, what is the immediate impact on the cluster's operations?

Accepted Answer

Chunks cannot be split or migrated between shards.. Config servers store the metadata and configuration settings for the entire sharded cluster. They are deployed as a replica set to ensure high availability. If the replica set loses its majority, it can no longer process writes to the metadata, which has significant implications for administrative tasks and the dynamic behavior of the cluster.

Answer

The cluster becomes read-only for all application data.

Answer

The mongos instances will crash and must be restarted.

Answer

All shards will automatically step down their primary members.

Question 7

Refer to the exhibit. Which index is most effective for this query?

Accepted Answer

{ tags: 1 }. The $all operator is used to find documents where the field contains all the specified elements. When indexing an array field, MongoDB creates a multikey index. A simple index on 'tags' is sufficient here because the multikey index structure inherently supports finding documents that contain specific array elements, enabling the query engine to efficiently narrow down the result set without scanning all documents in the collection.

Answer

{ tags: 'hashed' }

Answer

{ tags: 1, _id: 1 }

Answer

No index is required for array fields.

Question 8

When using the find() method, which cursor modifier is used to skip a specific number of documents before returning results?

Accepted Answer

.skip(). The skip() method is the standard cursor modifier used to bypass a set number of initial documents in a result set. It is essential for implementing pagination in applications, often used in conjunction with limit() to fetch specific pages of data. Understanding its impact on performance is vital, as skipping large numbers of documents can be expensive for the database engine to process.

Answer

.offset()

Answer

.limit()

Answer

.next()

Question 9

Which situation is best suited for a Multikey Index?

Accepted Answer

Querying documents based on specific elements inside an array field.. Multikey indexes are used when you need to index a field that contains an array. MongoDB creates an index entry for each element in the array, allowing you to efficiently query for documents containing specific array values. This is essential for applications managing tags, inventory lists, or user attributes stored as arrays, ensuring that queries filtering by these elements perform with high efficiency rather than scanning the entire collection.

Answer

Indexing a collection where every document contains a simple numeric ID.

Answer

Improving the performance of large joins across multiple collections.

Answer

Enforcing unique constraints on non-array fields.

Question 10

What is the consequence of having a high number of 'keysExamined' compared to 'nReturned' in a query?

Accepted Answer

The query is inefficiently scanning the index.. A large discrepancy between 'keysExamined' and 'nReturned' suggests that the index is not very selective or is being used inefficiently. The engine is scanning many index entries that do not match the query criteria, which is a major performance drain. Investigating the selectivity of the index and potentially adding more fields to the compound index can help narrow down the search and improve efficiency.

Answer

The index is too small and needs to be rebuilt.

Answer

The database is performing a cache miss.

Answer

The query will be automatically optimized.

Question 11

What happens when you perform a find() query on a collection where the projection includes both an inclusion and an exclusion for different fields?

Accepted Answer

The query returns an error.. MongoDB enforces strict rules regarding field projection. You cannot mix inclusions and exclusions in the same projection object, with the exception of the _id field. This design choice ensures predictable behavior for the result set. Understanding this limitation is important for developers when they need to fetch specific subsets of data while omitting others, as it prevents common errors during query construction.

Answer

The inclusion takes precedence over the exclusion.

Answer

The exclusion takes precedence over the inclusion.

Answer

It returns the intersection of both fields.

Question 12

When evaluating index performance, what is the impact of a 'partial index'?

Accepted Answer

It allows for faster writes by indexing fewer documents.. Partial indexes allow you to index only a subset of documents that meet a specific filter condition. This reduces index size and the overhead of maintaining the index during writes, as only updates to documents matching the filter trigger an index update. This is highly effective for reducing memory usage when you only need to index documents with specific statuses, such as 'active' or 'pending', rather than the entire collection.

Answer

It forces all documents in the collection to be indexed.

Answer

It automatically makes all queries return faster.

Answer

It is only compatible with unique indexes.

Question 13

Refer to the exhibit. An administrator inspects chunk metadata for a sharded collection using a compound shard key: { region: 1, account_id: 1 }. What does the presence of MinKey and MaxKey signify in this specific chunk range definition?

Accepted Answer

They represent absolute lower and upper bounds for the account_id field within the EMEA region, ensuring all accounts for that region reside in this chunk.. MinKey and MaxKey represent lower and upper boundary markers in MongoDB that evaluate smaller or larger than any other BSON data type. In this compound chunk range, they ensure that all documents matching the 'EMEA' region regardless of their account_id value fall within this specific chunk. Understanding BSON boundary types is essential for troubleshooting custom chunk splits and zone sharding configurations.

Answer

They indicate that the chunk is corrupted and requires immediate repair using the validate command on the config server.

Answer

They specify encryption keys used by the WiredTiger storage engine to secure data chunks at rest on shard01.

Answer

They are temporary placeholder values assigned by the balancer during active chunk splits that will automatically expire after twenty-four hours.

Question 14

Refer to the exhibit. What is the impact of this operation on the document?

Accepted Answer

It deletes the 'qty' field and creates a new 'quantity' field with the same value.. The $rename operator updates the name of a field within a document. It is atomic at the document level. If the field 'qty' exists, it is renamed to 'quantity'. If 'qty' does not exist, the operation does nothing. This is useful for schema migrations or refactoring data models without needing to delete and re-insert the document, which preserves the original _id and other field values.

Answer

It fails because 'qty' is not a field in the document.

Answer

It replaces the entire document with a new one containing only the quantity field.

Answer

It creates a new collection named 'quantity'.

Question 15

What is the primary function of the 'wtimeout' setting in write concern?

Accepted Answer

It specifies the duration to wait for replica set acknowledgment.. The 'wtimeout' parameter prevents a write operation from blocking indefinitely if the specified replica set members cannot acknowledge the write. This is a critical safety mechanism for application availability. If a network partition occurs or a node goes offline, the application will receive an error after the timeout, allowing it to handle the failure gracefully instead of hanging the application thread and exhausting the connection pool, which could lead to cascading service outages.

Answer

It sets the maximum time a query can run before being killed.

Answer

It limits the number of write retries in the driver.

Answer

It defines the connection timeout for new sessions.