C100DBA Practice Test 9 — 10 Questions

Question 1

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?

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

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 2

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 3

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 4

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 5

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 6

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 7

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 8

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 9

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 10

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