C100DBA Practice Test 13 — 15 Questions

Question 1

A MongoDB sharded cluster has a collection with a hashed shard key on the field 'userId'. A query is executed that includes an equality condition on userId. How does mongos route this query?

Accepted Answer

It computes the hash of the userId value and routes the query to the single shard that owns the chunk containing that hash value.. For equality queries on a hashed shard key field, mongos computes the hash of the value and uses its cached chunk metadata to route the query directly to the shard owning that hash range. This enables efficient targeted queries. Scatter-gather only occurs when the query lacks an equality condition on the hashed field.

Answer

It broadcasts the query to all shards because hashed shard keys do not support targeted queries.

Answer

It routes the query to the primary shard of the cluster because hashed keys always map to the primary shard for reads.

Answer

It uses the shard key index to identify the shard but must contact the config servers to resolve the chunk location before routing.

Question 2

You are administering a MongoDB 6.0 replica set with three members: a primary and two secondaries. You need to perform a rolling maintenance on the secondaries, which involves restarting each secondary one at a time. Which two actions must you take to ensure that the replica set remains available for writes during the maintenance? (Choose two.)

Accepted Answer

Restart the secondaries one at a time, waiting for each to fully rejoin and catch up before proceeding to the next.. To keep a replica set writable during rolling maintenance on secondaries, you must ensure the primary retains a majority of voting members. In a three-member set, this means keeping the primary and at least one secondary online. Restarting secondaries one at a time and waiting for each to catch up before moving to the next maintains majority and data consistency. This approach allows the primary to continue accepting writes without interruption, provided no other failures occur.

Answer

Disable authentication on the replica set to simplify the restart process.

Answer

Set the priority of the primary to 0 to prevent it from being re-elected during the maintenance.

Answer

Use rs.stepDown() to force the primary to step down before restarting any secondary.

Question 3

A MongoDB DBA observes that a query with filter { status: "active", created_at: { $gte: ISODate("2024-01-01") } } and sort { created_at: -1 } is performing a collection scan. The collection has an index { status: 1, created_at: -1 }. Which of the following best explains why the index is not being used efficiently?

Accepted Answer

The index { status: 1, created_at: -1 } should support both the filter and sort; the collection scan indicates a different issue such as index not being built or query planner choosing a different plan.. The compound index { status: 1, created_at: -1 } is designed to support queries that filter on status with equality and sort on created_at. The index prefix (status) is an equality match, allowing the index to be used for both filtering and sorting. A collection scan indicates that the index might not exist, be invalid, or the query planner chose a different plan due to factors like low selectivity or stale statistics.

Answer

The index prefix is not equality on the leading field, so the sort cannot be satisfied by the index.

Answer

The index is a partial index that excludes documents with status "active", so it cannot be used.

Answer

The index does not include the sort field as the first field, so the sort cannot use the index.

Question 4

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 5

A global e-commerce platform uses MongoDB with a replica set spanning three data centers. The company wants to ensure that a write is acknowledged by a majority of voting members before it is considered successful, even if one data center experiences a network partition. Which write concern should be configured to meet this requirement?

Accepted Answer

w: majority. w: majority ensures that a write is acknowledged by a majority of voting members in the replica set, providing strong durability guarantees even in the event of a network partition affecting one data center. This write concern is essential for applications that cannot tolerate data loss and need to maintain consistency across geographically distributed nodes.

Answer

w: 1

Answer

w: 2

Answer

w: 0

Question 6

How can you prevent a specific node from becoming a primary in a replica set?

Accepted Answer

Set the node's 'priority' field to 0.. Setting the 'priority' value to 0 is the standard way to ensure a node never becomes primary. This is commonly done for nodes used for analytical tasks or as disaster recovery targets. It ensures these nodes act solely as passive secondaries, allowing them to serve read requests without ever interfering with the primary election process or causing unexpected failovers.

Answer

Set the node's 'votes' field to 0.

Answer

Set the node's 'hidden' field to true.

Answer

Remove the node from the replica set configuration.

Question 7

A developer needs to retrieve a subset of fields from a collection. Which TWO methods achieve this using projection?

Accepted Answer

db.collection.find(query, projection). Projection allows developers to minimize network bandwidth and memory usage by returning only necessary fields from documents. Both find() and findOne() support a second argument that defines the projection document. This is a fundamental optimization technique in MongoDB to prevent the 'select *' anti-pattern, ensuring that large fields like binary data or embedded arrays are not fetched when they are not required by the application layer.

Answer

db.collection.aggregate(pipeline, projection)

Answer

db.collection.findAndModify(query, projection)

Answer

db.collection.update(query, projection)

Question 8

Which operator is best used to add an element to an array only if it does not already exist?

Accepted Answer

$addToSet. The $addToSet operator is the standard MongoDB tool for array manipulation when uniqueness is required. It ensures that the specified value is added to the array only if it is not currently present. This prevents duplicate entries without requiring the application to perform a read-then-write check, making it an atomic and highly performant operation for managing sets within document-based data structures.

Answer

$push

Answer

$set

Answer

$each

Question 9

Refer to the exhibit. What is the effect of the 'writeConcern' parameter in this operation?

Accepted Answer

It ensures the data is replicated to most nodes before acknowledgment.. The write concern {w: 'majority'} ensures that the operation is only acknowledged after it has been written to a majority of the replica set members. This is essential for data durability and preventing data loss in the event of a primary node failover. It provides a stronger guarantee than the default 'w: 1' by ensuring that data is replicated before the application proceeds.

Answer

It forces the update to occur only on the primary node.

Answer

It only updates documents that have a majority of fields set.

Answer

It makes the update operation read-only.

Question 10

An application uses a compound index on `{ 'status': 1, 'priority': -1 }`. Which query will efficiently use this index?

Accepted Answer

db.tasks.find({ 'status': 'open' }). Compound indexes are ordered. A query will only use the index if the query filter matches the prefix of the indexed fields. Here, the index starts with 'status'. Therefore, queries filtering on 'status' and optionally 'priority' will be efficient. Understanding index prefixing is crucial for DBAs to ensure that developers write efficient queries that fully leverage the index structures, avoiding unnecessary collection scans and keeping performance stable as the data grows.

Answer

db.tasks.find({ 'priority': 5 })

Answer

db.tasks.find({ 'created': '2023-01-01' })

Answer

db.tasks.find({ 'status': 'open', 'type': 'urgent' })

Question 11

What happens to the replica set if a majority of voting members are permanently lost?

Accepted Answer

The set stops accepting writes to maintain consistency.. A replica set requires a majority of its voting members to be online to elect a new primary and perform write operations with majority concern. If the majority is lost, the remaining nodes enter a read-only state, as they cannot reach consensus. This is a critical failure scenario that requires immediate administrative intervention, such as reconfiguring the set or restoring the failed nodes.

Answer

The set automatically switches to a single-node mode to remain functional.

Answer

The secondary nodes elect a new primary regardless of the missing majority.

Answer

The primary node continues to accept writes for a short period.

Question 12

Which command is used to check the current replication lag status on a secondary node?

Accepted Answer

rs.printSecondaryReplicationInfo(). The rs.printSecondaryReplicationInfo() command provides a convenient summary of how far behind each secondary is compared to the primary. This is a vital tool for monitoring the health of the replica set and identifying nodes that may be having trouble keeping up with the write volume, which could signal underlying network or hardware performance bottlenecks.

Answer

rs.status()

Answer

rs.printReplicationInfo()

Answer

db.stats()

Question 13

Refer to the exhibit. What does this output indicate about the query performance?

Accepted Answer

The query requires an index to avoid a full collection scan.. A 'COLLSCAN' indicates that the database is performing a full collection scan, reading every document to satisfy the query. The high number of docs examined relative to the returned documents suggests that the query is inefficient. In a production environment, this is a major performance bottleneck, as the database engine must load entire documents into memory to evaluate the filter, leading to high I/O and latency.

Answer

The query is fully covered by an index.

Answer

The query is performing well because nReturned equals totalDocsExamined.

Answer

The index is being used, but it is not selective enough.

Question 14

An administrator needs to distribute a high-throughput collection evenly across a sharded cluster using a hashed shard key. What is the primary characteristic of MongoDB's hashed sharding strategy that makes it effective for preventing write bottlenecks?

Accepted Answer

It computes an MD5 hash of the field value to distribute documents pseudo-randomly across all available chunks and shards, preventing single-shard write bottlenecks.. Hashed sharding applies an MD5 hash function to the specified field value before routing documents. This transforms sequential or correlated inputs into pseudo-random hash values, ensuring that writes scatter uniformly across all available cluster shards. Understanding hashed sharding mechanisms enables administrators to design resilient write-heavy architectures that prevent the single-shard bottlenecks common with monotonic keys.

Answer

It compresses all chunk migration payloads using cryptographic hashing algorithms to minimize network bandwidth consumption during background balancing operations.

Answer

It automatically converts multi-key array fields into single scalar values to prevent indexing errors on distributed sharded collections.

Answer

It locks the target shard during write operations to guarantee strict serializable isolation levels across distributed cluster environments.

Question 15

Which connection string parameter should be used to ensure an application driver automatically discovers all members of a replica set?

Accepted Answer

replicaSet. The replicaSet parameter is essential for drivers to identify the cluster name and initiate the discovery process. When this parameter is specified, the driver uses the provided seed list to perform a 'isMaster' command against the nodes, allowing it to map the entire topology. Understanding this mechanism is vital for ensuring high availability, as the driver needs to dynamically redirect operations to the new primary during automated failover events.

Answer

maxPoolSize

Answer

wtimeoutMS

Answer

readPreference