C100DBA Practice Test 4 — 25 Questions

Question 1

A DBA notices that a query on the products collection uses an index scan but returns only a small fraction of documents. The index is { category: 1, price: 1 }. The query filters on category and price with a range condition on price. Which statement best describes why the index might still be efficient despite scanning many index entries?

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

Accepted Answer

The index scan is efficient because it can use the index bounds to limit the scan to the relevant category and price range, reducing the number of documents fetched.. The index { category: 1, price: 1 } supports efficient filtering when the query has an equality predicate on category and a range predicate on price. MongoDB can use index bounds to seek to the correct category and scan only the relevant price range, minimizing the number of documents fetched. This makes the index scan efficient despite examining many index entries.

Answer

The index scan is efficient because MongoDB automatically converts it to a collection scan when the selectivity is low.

Answer

The index scan is efficient because it avoids fetching documents entirely due to the index being covered.

Answer

The index scan is efficient because it uses the index only for sorting and not for filtering.

Question 2

A collection 'events' has a compound index { tenantId: 1, createdAt: -1 }. A developer runs db.events.find({ tenantId: "acme", createdAt: { $gte: ISODate("2024-01-01") } }).sort({ createdAt: -1 }).limit(50). The explain output shows the index is used but the query still performs a large in-memory sort. Which action resolves the sort performance issue?

Accepted Answer

Ensure the index is { tenantId: 1, createdAt: -1 } and verify the query does not use a collation or type mismatch that prevents index-provided sort.. A compound index can provide both the equality match and the sort when the equality fields form the prefix and the sort field follows. With { tenantId: 1, createdAt: -1 }, the query's equality on tenantId and sort on createdAt should be served without an in-memory sort. A blocking SORT in explain therefore points to a mismatch such as collation, type, or query shape rather than a missing index.

Answer

Add a sort key to the index: { tenantId: 1, createdAt: -1, _id: 1 }.

Answer

Reverse the compound index to { createdAt: -1, tenantId: 1 }.

Answer

Create a separate single-field index on createdAt.

Question 3

You are a DBA for a MongoDB replica set. A secondary node has been down for several days and is far behind the primary. The oplog on the primary has wrapped, and the secondary's last oplog entry is no longer present. What must you do to bring the secondary back into sync?

Accepted Answer

Perform a full resync by stopping the secondary, deleting its data files, and restarting it.. If a secondary's last oplog entry is no longer in the primary's oplog, the secondary cannot catch up incrementally. A full resync is required, which means deleting the secondary's data files and restarting it to perform an initial sync. This process copies all data from the primary or another sync source.

Answer

Increase the oplog size on the primary to prevent wrapping and then restart the secondary.

Answer

Restart the secondary with the --resync option to force a full resync.

Answer

Run rs.syncFrom() on the secondary to point it to a different sync source.

Question 4

A DBA is designing indexes for a collection that stores user activity logs. The collection has fields: user_id, action, timestamp, and metadata (an embedded document). The most common queries are: (1) find all actions for a given user_id sorted by timestamp descending; (2) find all users who performed a specific action within a time range. Which TWO indexes would best support these queries? (Choose two.)

Accepted Answer

{ user_id: 1, timestamp: -1 }. The two indexes that best support the queries are { user_id: 1, timestamp: -1 } for query (1) and { action: 1, timestamp: 1 } for query (2). The first provides an equality match on user_id and a sort on timestamp, while the second provides an equality match on action and a range on timestamp. These indexes align with the equality-sort and equality-range patterns, ensuring efficient index usage.

Answer

{ metadata: 1 }

Answer

{ user_id: 1, action: 1, timestamp: -1 }

Answer

{ timestamp: 1 }

Question 5

You are updating a document in the `products` collection. The document has an array field `tags`. You need to add the tag "sale" to the array only if it does not already exist, and you want to ensure that no duplicate tags are added even if multiple updates run concurrently. Which update operator should you use?

Accepted Answer

$addToSet. $addToSet is designed to add a value to an array only if it is not already present, making it the correct operator for ensuring uniqueness. It performs this check atomically, so concurrent updates will not introduce duplicates. $push would append duplicates, $set would replace the array, and $addToSet with $each is overkill for a single value.

Answer

$addToSet with $each

Answer

$push

Answer

$set

Question 6

A MongoDB 6.0 sharded cluster has a collection sharded with a hashed shard key on 'userId'. The operations team needs to run an aggregation that groups documents by 'userId' and calculates the total transaction amount. They want to know whether the aggregation can be optimized to run only on the shards that contain the relevant 'userId' values. Which statement describes the correct behavior?

Accepted Answer

The aggregation can target specific shards if the $match stage includes an equality condition on 'userId'.. For a hashed shard key, mongos can target a specific shard when the query includes an equality condition on the shard key field. This is because the hash of the value determines the chunk and thus the shard. Other stages or options do not affect targeting. Therefore, an aggregation with a $match on 'userId' with an equality can be optimized to run on a single shard.

Answer

The aggregation can target specific shards only if it includes a $sort stage on 'userId' before the $group stage.

Answer

The aggregation will always run on all shards because hashed shard keys prevent any query targeting.

Answer

The aggregation must be run with the allowDiskUse option to target specific shards.

Question 7

A collection 'logs' receives continuous inserts and is queried by both timestamp ranges and a rarely used severity field. A DBA creates six indexes to cover every query variant, and now insert throughput has dropped sharply while index sizes dominate the working set. Which action best restores insert throughput while preserving the important query paths?

Accepted Answer

Drop redundant indexes and consolidate overlapping ones into compound indexes that serve multiple query shapes.. Every index adds maintenance work to each insert, update, and delete, so a collection with many overlapping indexes suffers write amplification and cache pressure. Auditing index usage, dropping unused or redundant indexes, and consolidating overlapping prefixes into compound indexes reduces per-write maintenance while keeping the timestamp range and severity access paths covered. Write concern and cache tuning do not remove index maintenance cost.

Answer

Convert all indexes to hashed indexes to reduce index size.

Answer

Set the collection's write concern to w:0 to avoid index maintenance waits.

Answer

Increase the WiredTiger cache size so more indexes stay resident.

Question 8

A DBA is reviewing a slow query on the `orders` collection. The query is: `db.orders.find( { status: "shipped", total: { $gt: 100 } } )`. The collection has an index on `{ status: 1, total: 1 }`. Which statement best describes how MongoDB will use this index to satisfy the query?

Accepted Answer

The index will be used to filter both `status` and `total` because the index prefix matches the equality condition and the range condition.. A compound index with an equality field followed by a range field is ideal for queries that filter on both. MongoDB will use the index to seek to the equality value and then scan the range on the second field. This minimizes document examinations and is the expected behavior for the given index and query.

Answer

The index will only be used for `status`, and `total` will be filtered in memory because range conditions cannot use an index.

Answer

The index will be used, but only if the query includes a sort on `total` to enforce index order.

Answer

The index will not be used because the query includes a range condition on `total`.

Question 9

A healthcare startup is evaluating MongoDB for a patient management system. The CTO asks how MongoDB ensures data durability and consistency across multiple servers without manual intervention. Which feature best describes MongoDB's built-in mechanism for automatic failover and data redundancy?

Accepted Answer

Replica sets. Replica sets are MongoDB's foundation for high availability, providing automatic failover and data redundancy across multiple nodes. When the primary becomes unavailable, the remaining nodes elect a new primary, allowing the system to continue operating. This built-in mechanism requires no manual intervention and is essential for production deployments that demand continuous uptime and data durability.

Answer

Write concern

Answer

Journaling

Answer

Sharding

Question 10

An application uses a MongoDB driver that supports retryable writes. A write operation is sent to the primary, which applies the write but crashes before sending the acknowledgment. The driver receives a network error. What will the driver do by default?

Accepted Answer

Retry the write once, and the server will deduplicate it using the transaction number.. Retryable writes automatically retry a write operation once on retryable errors. The driver uses a transaction number and session ID so the server can deduplicate the retry, ensuring the write is applied exactly once. The driver does not retry indefinitely, does not skip retrying, and does not alter the write concern. The correct behavior is a single retry with deduplication.

Answer

Retry the write indefinitely until it succeeds.

Answer

Retry the write but with write concern 0 to avoid another failure.

Answer

Return the error to the application without retrying.

Question 11

You are configuring a MongoDB replica set for a global application. You need to add a member that will never become primary and will be invisible to client applications. Which two steps must you take when adding this member? (Choose two.)

Accepted Answer

Set its priority to 0.. To create a member that never becomes primary and is invisible to clients, you must set both priority to 0 and hidden to true. Priority 0 ensures it cannot be elected primary, while hidden true prevents client visibility and use as a sync source. These two settings together meet the requirements.

Answer

Set its votes to 0.

Answer

Set its slaveDelay to a large value.

Answer

Set its buildIndexes to false.

Question 12

A DBA is troubleshooting a slow query on a large collection. Which two techniques directly reduce the number of documents the query must examine for an equality-plus-range predicate? (Choose two.)

Accepted Answer

Create a compound index with the equality field first and the range field second.. Reducing documents examined comes from narrowing the index scan. A compound index ordered equality-then-range lets the engine seek to the exact key range, and a partial index filtered on the equality predicate stores only relevant entries, shrinking the index. Both directly cut keys and documents examined. Read concern, hint to _id, and page fill settings do not change the access path work for this predicate.

Answer

Add a hint forcing the query to use the _id index.

Answer

Set the query's read concern to 'majority' to reduce examined documents.

Answer

Increase the index's fill factor using collMod to leave more space on pages.

Question 13

A development team is building a Node.js application that connects to a MongoDB replica set. They want the driver to automatically retry certain write operations once if they fail due to a transient network error, without requiring custom retry logic in their code. Which connection string option should they enable?

Accepted Answer

retryWrites=true. The retryWrites=true connection string option instructs the MongoDB driver to automatically retry supported write operations once if they fail with a retryable error. This offloads retry logic from the application code and is the standard way to handle transient network issues in replica sets. Other options affect write concern, read routing, or connection timeouts, but none provide automatic write retries.

Answer

w=majority

Answer

readPreference=primaryPreferred

Answer

connectTimeoutMS=30000

Question 14

A database administrator wants to configure a read preference that directs all read operations to the primary node of a replica set, ensuring the most up-to-date data. Which read preference mode should be used?

Accepted Answer

primary. The requirement is to always read the most up-to-date data, which is only guaranteed when reading from the primary. The "primary" read preference ensures that all reads are directed to the primary node. Other read preferences may read from secondaries, which can have stale data, or may fall back to secondaries if the primary is unavailable, violating the requirement.

Answer

primaryPreferred

Answer

nearest

Answer

secondary

Question 15

An application needs to perform a series of reads and writes across multiple documents in two collections atomically. The deployment is a replica set running MongoDB 4.2. Which feature should the application use?

Accepted Answer

Multi-document transactions. Multi-document transactions in MongoDB 4.2 on replica sets provide ACID guarantees across multiple documents and collections. They are the only feature listed that ensures a series of reads and writes across two collections are applied atomically. The other options are storage or streaming features that do not offer transactional atomicity.

Answer

Change streams with resume tokens

Answer

Capped collections with tailable cursors

Answer

GridFS for storing large documents

Question 16

A global IoT platform ingests sensor readings from millions of devices. The team must choose a database that can store semi-structured data, scale horizontally, and provide high write throughput. Which TWO MongoDB features directly support these requirements? (Choose two.)

Accepted Answer

Sharding with a well-chosen shard key. Sharding distributes data across shards for horizontal scale and high write throughput, while the dynamic schema accommodates heterogeneous sensor payloads without migrations. Together, these features address the platform's need to ingest semi-structured data from millions of devices and scale out as volume grows.

Answer

Mandatory multi-document ACID transactions for every insert

Answer

Strict table-level locking for all write operations

Answer

Automatic sharding based on the first field alphabetically

Question 17

A DBA is troubleshooting a sharded cluster where the balancer is not migrating chunks even though there is an imbalance. The DBA runs `sh.getBalancerState()` and it returns `true`. Which of the following is the most likely reason the balancer is not migrating chunks?

Accepted Answer

The balancer window is configured to a time period outside of the current time.. The balancer only runs during the configured balancing window. Even if `sh.getBalancerState()` returns `true`, migrations will not occur outside that window. The DBA should check the balancer window settings and ensure they align with the desired migration times. Other options would either cause broader failures or are not relevant to the balancer's operation.

Answer

The config servers are in a read-only state.

Answer

The collection has no indexes on the shard key, so the balancer cannot identify chunks to migrate.

Answer

The shard key is hashed, and hashed shard keys prevent the balancer from migrating chunks.

Question 18

A DBA observes that a query with filter { a: 5, b: { $gt: 10 } } and sort { c: 1 } is using a collection scan despite the existence of an index { a: 1, b: 1, c: 1 }. The DBA wants to improve performance. Which action is most likely to allow the index to support both the filter and the sort?

Accepted Answer

Create a new index { a: 1, c: 1, b: 1 } so that the sort field c comes before the range field b.. The index { a: 1, b: 1, c: 1 } cannot support the sort on c because b is a range predicate, which means the index order after the equality on a is determined by b, not c. To support both the filter and the sort, the index should be reordered so that the sort field c follows the equality field a, and the range field b comes last. The index { a: 1, c: 1, b: 1 } allows the equality on a to bound the prefix, then the sort on c to be satisfied by the index order, and finally the range on b to be applied as an index filter.

Answer

Create a partial index on { a: 1, b: 1, c: 1 } with a filter on b > 10, which will allow the sort to use the index.

Answer

Add a separate index on { c: 1 } and rely on the query planner to perform an index intersection with the existing index.

Answer

Modify the query to remove the range condition on b, so the existing index can support the sort on c.

Question 19

A developer is using the MongoDB Node.js driver and wants to ensure that a write operation is acknowledged by a majority of replica set members and is also written to the on-disk journal before returning success. Which write concern should be specified?

Accepted Answer

{ w: "majority", j: true }. The developer needs both majority acknowledgment and journaling. The write concern { w: "majority", j: true } ensures that a majority of replica set members have applied the write and that it has been written to the on-disk journal on those members. This provides the strongest durability guarantee, preventing data loss even if the primary crashes. Other options either lack majority acknowledgment or journaling.

Answer

{ w: 2, j: true }

Answer

{ w: 1, j: true }

Answer

{ w: "majority", j: false }

Question 20

A DBA is configuring a replica set with three members: one primary and two secondaries. The DBA wants to ensure that a write concern of { w: 'majority' } is acknowledged only after the write has been applied to a majority of voting members and has been written to the on-disk journal on those members. Which write concern should the DBA use?

Accepted Answer

{ w: 'majority', j: true }. The requirement is for a write concern that ensures acknowledgment after a majority of voting members have applied the write and written it to the on-disk journal. The write concern { w: 'majority', j: true } achieves both: w: 'majority' dynamically requires a majority of voting members, and j: true forces journaling on those members before acknowledgment. Using a fixed w: 2 may not adapt to membership changes, and omitting j: true does not guarantee journaling. (Explanation length: 65 words)

Answer

{ w: 'majority' }

Answer

{ w: 2, j: true }

Answer

{ w: 'majority', j: false }