C100DBA Practice Test 12 — 15 Questions

Question 1

A development team is building a real-time analytics dashboard that reads from a MongoDB replica set. The dashboard can tolerate slightly stale data but must never show data that could be rolled back. The team also wants to minimize read latency. Which read concern should they use?

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

Accepted Answer

read concern majority. Read concern majority guarantees that the data returned has been acknowledged by a majority of replica set members and is rollback-safe, while still allowing reads from secondaries to reduce latency. Since the dashboard tolerates slight staleness, this is the ideal balance between consistency and performance.

Answer

read concern linearizable

Answer

read concern snapshot

Answer

read concern local

Question 2

A MongoDB DBA is configuring a new application to connect to a sharded cluster. The application requires that reads are distributed across shards and that the driver automatically routes queries to the appropriate shard based on the shard key. Which component must be included in the connection string to enable this routing?

Accepted Answer

The mongos router hostname and port.. In a sharded cluster, applications must connect to mongos routers, which are responsible for routing queries to the appropriate shards based on the shard key. The connection string should include the hostnames and ports of mongos instances. Connecting directly to config servers or shards bypasses the routing layer and is not supported for application traffic.

Answer

The config server replica set hostnames and ports.

Answer

The replica set name of the shards.

Answer

The shard replica set hostnames and ports.

Question 3

You are the DBA for a three-member MongoDB replica set. The primary becomes unreachable due to a network partition, and two secondaries remain in communication. After the election timeout, one secondary is elected primary. What is the required condition for a new primary to be elected?

Accepted Answer

A majority of voting members must be available and able to communicate.. MongoDB replica sets require a majority of voting members to elect a primary. In a three-member set, two members must be available. The elected primary must have the highest optime among the majority to ensure data consistency. This mechanism prevents split-brain scenarios and ensures that only one primary can accept writes at a time.

Answer

Any secondary with the highest priority automatically becomes primary without needing a majority.

Answer

The arbiter, if present, must vote and can be the sole voter to elect a primary.

Answer

The secondary with the most recent oplog entry is elected primary even if it is the only member available.

Question 4

A DBA is adding a new secondary to an existing replica set. The data set is 2 TB, and the network bandwidth between the new member and the primary is 100 Mbps. The DBA wants to minimize the impact on the primary during the initial sync. Which approach should the DBA take?

Accepted Answer

Seed the new member by copying the data files from an existing secondary while it is offline, then start it with the same replica set configuration.. Seeding a new member by copying data files from an existing secondary (taken offline) is an efficient way to add a member without burdening the primary with a full initial sync over a slow network. This method leverages local copy speed and avoids consuming primary resources, which is critical when network bandwidth is limited.

Answer

Configure the new member as a hidden member to reduce its replication load.

Answer

Increase the oplog size on the primary to accommodate the initial sync.

Answer

Set the new member's priority to 0 during the initial sync to prevent it from becoming primary.

Question 5

A development team is building a mobile application backend using MongoDB. They want to ensure that the application can handle rapid changes to the data model without requiring database downtime or complex migrations. Which MongoDB feature directly supports this requirement?

Accepted Answer

Dynamic schema. The dynamic schema is a fundamental MongoDB feature that permits documents within a collection to have varying fields. This flexibility enables developers to iterate quickly, adding or changing fields without performing database migrations or causing downtime. For a mobile application with evolving requirements, this means the backend can adapt seamlessly, aligning with MongoDB's philosophy of developer agility and schema flexibility.

Answer

Horizontal scaling via sharding

Answer

Change streams

Answer

Multi-document ACID transactions

Question 6

A financial services team is designing a real-time fraud detection system. They need a database that can store alerts with flexible, evolving fields such as device fingerprint, geolocation, and transaction metadata, and that can scale horizontally across many nodes. Which MongoDB philosophy best supports this requirement?

Accepted Answer

A document-oriented model with dynamic schemas. MongoDB's document-oriented model with dynamic schemas lets each fraud alert carry different fields without migrations, while built-in sharding enables horizontal scale across nodes. Together, these features directly address the need for flexible, evolving data capture and distributed scalability in a real-time system.

Answer

Using a single-node in-memory cache with no persistence

Answer

A rigid normalized schema enforced by the database engine

Answer

Storing all alert data as large binary blobs outside the database

Question 7

A financial application must ensure that once a transaction is committed, the data will never be rolled back, even if the primary node crashes immediately after the write is acknowledged. Which write concern should be used?

Accepted Answer

{ w: "majority" }. The requirement is that committed data must never be rolled back, even if the primary crashes immediately after acknowledgment. Majority write concern guarantees that the write is acknowledged by a majority of voting members, so any newly elected primary will have the write. This prevents rollback during failover. Other write concerns either provide weaker guarantees or no acknowledgment at all.

Answer

{ w: 0 }

Answer

{ w: 2 }

Answer

{ w: 1 }

Question 8

A MongoDB DBA is managing a sharded cluster where the collection 'orders' is sharded on the field 'orderDate' using ranged sharding. The DBA needs to add a new shard to the cluster to handle increasing data volume. After adding the shard, the balancer begins migrating chunks. During this migration, the DBA runs a query that includes 'orderDate' and notices increased latency. What is the most likely reason for the increased latency?

Accepted Answer

The balancer migration process consumes resources on the shards and may cause temporary performance degradation.. Adding a shard triggers the balancer to migrate chunks to distribute data. This migration process involves copying data and updating metadata, which consumes resources on the source and destination shards. As a result, concurrent queries may experience increased latency until migrations finish. The other options describe unrelated issues or misconceptions about indexing and hot spots.

Answer

The config servers are overloaded with metadata updates, causing mongos to delay query routing.

Answer

The query must now be routed to the new shard, which has not yet been indexed, causing a full collection scan.

Answer

The shard key 'orderDate' is monotonically increasing, so all new data is going to the new shard, causing a hot spot.

Question 9

A financial services company is evaluating MongoDB for a new application that requires strong consistency for critical transactions and high availability across multiple data centers. The team must choose features that align with MongoDB's philosophy of balancing consistency, availability, and partition tolerance. (Choose two.)

Accepted Answer

Read concern "majority". Read concern "majority" and write concern "majority" together provide strong consistency and durability by ensuring that reads and writes are acknowledged by a majority of replica set members. This combination is essential for critical financial transactions that cannot tolerate stale reads or data loss. These features reflect MongoDB's tunable consistency model, allowing the company to balance consistency and availability according to their requirements.

Answer

Write concern "w: 1"

Answer

Read preference "nearest"

Answer

Read concern "local"

Question 10

A MongoDB DBA is investigating replication lag on a secondary member named rs0-2. The DBA runs rs.status() and notices that the 'optimeDate' of rs0-2 is several hours behind the primary's 'optimeDate'. The DBA wants to quickly determine the exact time difference. Which field in the rs.status() output should the DBA compare to calculate the replication lag?

Accepted Answer

The 'optimeDate' field of each member.. Replication lag is calculated by comparing the 'optimeDate' of the secondary with that of the primary. The 'optimeDate' field shows the timestamp of the last oplog entry applied by each member, so the difference indicates how far behind the secondary is.

Answer

The 'pingMs' field of each member.

Answer

The 'lastHeartbeat' field of each member.

Answer

The 'optime' field of each member.

Question 11

A DBA is reviewing a query that performs a sort on a field named score in descending order. The collection has an index { score: 1 }. The query also includes a filter on a field named active with an equality condition. The DBA observes that the query planner chooses a collection scan and performs an in-memory sort. Which index would allow the query to use an index for both the filter and the sort?

Accepted Answer

{ active: 1, score: -1 }. The query filters on active with equality and sorts by score descending. The optimal index places the equality field first and the sort field second in the matching direction: { active: 1, score: -1 }. This allows MongoDB to use the index for the filter and to provide the descending sort order directly, avoiding an in-memory sort and improving performance.

Answer

{ score: -1, active: 1 }

Answer

{ score: 1, active: 1 }

Answer

{ active: 1, score: 1 }

Question 12

A DBA needs to optimize a query that filters on a field with high cardinality, such as email, and also sorts on a low cardinality field, such as country. The query is { email: "user@example.com" } with sort { country: 1 }. Which index would best support this query?

Accepted Answer

{ email: 1, country: 1 }. The compound index { email: 1, country: 1 } is optimal because it allows an equality match on email, which is highly selective, and then provides sorted results by country. The index order matches the query pattern: equality first, then sort. This avoids an in-memory sort and minimizes the number of documents examined.

Answer

{ email: 1 }

Answer

{ country: 1 }

Answer

{ country: 1, email: 1 }

Question 13

A database administrator is setting up a new sharded cluster and needs to enable sharding on a database named 'sales' before sharding any collections. Which command should be executed on mongos to enable sharding for this database?

Accepted Answer

sh.enableSharding("sales"). To enable sharding on a database in MongoDB, you must run sh.enableSharding() on mongos with the database name. This marks the database as sharding-enabled in the config servers, allowing collections within it to be sharded. The other commands either shard a collection, add a shard, or use incorrect syntax.

Answer

db.runCommand({enableSharding: "sales"}) on the admin database

Answer

sh.addShard("sales")

Answer

sh.shardCollection("sales", {_id: "hashed"})

Question 14

A financial services company shards a transactions collection using a ranged shard key on an incrementing timestamp field. Over several months, the cluster develops a single hot shard that receives almost all writes, while the other shards remain nearly idle. Which change best addresses the root cause while preserving efficient range queries on timestamp?

Accepted Answer

Use a compound shard key that combines a low-cardinality prefix such as a hash of the timestamp bucket with the timestamp, or apply a bucketing strategy to spread inserts.. A monotonically increasing shard key concentrates all inserts in the highest chunk, creating a single hot shard regardless of cluster size. Introducing a hashed or bucketed prefix spreads writes across many chunks and shards, while retaining timestamp as a suffix preserves efficient range queries. This combination solves the imbalance without sacrificing the query pattern the application depends on.

Answer

Enable the balancer and lower the chunk size to 32 MB so chunks migrate more aggressively during peak hours.

Answer

Convert the shard key to a hashed key on timestamp, which spreads writes evenly across shards and keeps range queries efficient.

Answer

Add more shards to the cluster so the hot shard's load is diluted across a larger number of nodes.

Question 15

You need to remove a single document from the `users` collection where the `email` field is "alice@example.com". The collection has a unique index on `email`. Which method should you use?

Accepted Answer

db.users.deleteOne({ email: "alice@example.com" }). deleteOne is the correct method to remove a single document matching a filter. It is the modern replacement for remove() and is designed for deleting one document. Since the email field has a unique index, there is at most one match, making deleteOne both safe and efficient. deleteMany and drop are inappropriate because they either delete multiple documents or the entire collection.

Answer

db.users.drop({ email: "alice@example.com" })

Answer

db.users.deleteMany({ email: "alice@example.com" })

Answer

db.users.remove({ email: "alice@example.com" })