C100DBA Practice Test 5 — 25 Questions

Question 1

A MongoDB DBA is asked to ensure that the mongod process on a Linux server automatically restarts if it crashes. The server uses systemd. Which command should the DBA run to enable this behavior?

Accepted Answer

Edit the mongod.service unit file to include Restart=always, then run systemctl daemon-reload and systemctl restart mongod. To enable automatic restart of mongod on failure when using systemd, you must modify the service unit file to include Restart=always (or Restart=on-failure). After editing, run systemctl daemon-reload to reload the configuration, and then restart the service. The systemctl enable command only ensures start at boot, not restart on crash. The other commands are either manual restarts or do not provide automatic restart functionality. (Explanation length: 65 words)

Answer

systemctl enable mongod

Answer

Run mongod --fork --logpath /var/log/mongodb/mongod.log

Answer

systemctl restart mongod

Question 2

A MongoDB 6.0 sharded cluster has a collection with a ranged shard key on the field 'customerId'. The balancer has been running normally, but the operations team notices that one shard consistently holds significantly more chunks than the others. They want to understand which internal metadata collection the mongos uses to determine chunk distribution and to verify the current chunk-to-shard mapping. Which collection should they query?

Accepted Answer

config.chunks. Chunk distribution is tracked in the config database, specifically in config.chunks, which maps each chunk's key range to its host shard. To verify which shard holds more chunks, an administrator queries config.chunks. The other collections store different metadata: shard list, collection definitions, or version info, none of which include per-chunk placement.

Answer

config.shards

Answer

config.collections

Answer

admin.system.version

Question 3

You are a MongoDB DBA at a retail company. The 'orders' collection has a document with _id: 100 and status: 'pending'. You need to atomically update the status to 'shipped' only if the current status is 'pending'. Which update operation should you use?

Accepted Answer

db.orders.updateOne({_id: 100, status: 'pending'}, {$set: {status: 'shipped'}}). The correct approach is to include the current status in the query filter so the update only applies when the status is 'pending'. This leverages MongoDB's atomic single-document updates, ensuring the condition and modification occur as one indivisible operation. Using a filter without the status condition would update regardless of the current value, risking data integrity.

Answer

db.orders.replaceOne({_id: 100}, {status: 'shipped'})

Answer

db.orders.updateOne({_id: 100}, {$set: {status: 'shipped'}})

Answer

db.orders.findAndModify({query: {_id: 100}, update: {$set: {status: 'shipped'}}})

Question 4

An administrator notices that the balancer in a MongoDB sharded cluster is repeatedly migrating chunks between two shards, causing performance degradation. The administrator wants to temporarily stop the balancer to investigate. Which command should be used on mongos?

Accepted Answer

sh.stopBalancer(). To temporarily stop the MongoDB balancer, an administrator should run sh.stopBalancer() on mongos. This command disables the balancer and waits for any in-progress migrations to finish, ensuring a clean stop. It is the standard helper for this purpose. The balancer can be restarted with sh.startBalancer().

Answer

db.runCommand({balancerStop: 1}) on the admin database

Answer

sh.disableBalancer()

Answer

sh.setBalancerState(false)

Question 5

A social media company is using MongoDB to store user posts and comments. The application needs to retrieve a post along with its most recent comments in a single query. Which data modeling approach best aligns with MongoDB's philosophy to optimize this access pattern?

Accepted Answer

Embed a subset of recent comments within the post document and store older comments separately.. The hybrid approach of embedding recent comments while storing older ones separately optimizes the common access pattern of fetching a post with its latest comments. It leverages MongoDB's flexible schema to keep frequently accessed data together, reducing query latency, while avoiding unbounded document growth. This reflects the core philosophy of modeling based on application access patterns rather than rigid normalization.

Answer

Store comments in a separate collection and use $lookup to join them when needed.

Answer

Store comments in a separate collection and use a manual application-level join to combine them with the post.

Answer

Embed all comments within the post document to ensure atomic updates and fast reads.

Question 6

A support team reports that a MongoDB 6.0 replica set is experiencing slow queries on a collection where almost every document has a unique value for the field status. A developer suggests creating a hashed index on status to speed up equality lookups. Which outcome should the DBA expect?

Accepted Answer

A hashed index will not improve equality query performance and is primarily intended for even data distribution in sharded clusters.. Hashed indexes are optimized for even distribution of values, which is critical for sharding but not for improving equality query performance on a single replica set. A standard B-tree index on a high-cardinality field like status already provides efficient equality lookups and supports range queries. Therefore, a hashed index would not be the right choice to speed up these queries; it is better suited for shard keys that need uniform distribution.

Answer

Hashed indexes provide no benefit for equality queries on high-cardinality fields and should not be used.

Answer

A hashed index will improve equality query performance but will prevent the use of range queries on status.

Answer

Hashed indexes are ideal for equality lookups and will outperform a standard B-tree index for this query pattern.

Question 7

A healthcare provider is building a patient portal that must remain available even if a data center loses power. The operations team wants the database to continue accepting writes during a single-region outage. Which MongoDB philosophy directly enables this resilience?

Accepted Answer

Automatic failover through replica sets with majority write concern. Replica sets provide automatic failover by electing a new primary when the current one fails, and majority write concern ensures that acknowledged writes are durable across a majority of members. Together, they allow the portal to keep accepting writes during a single-region outage without manual intervention.

Answer

Relying on periodic logical backups to a cold standby server

Answer

Deploying a standalone mongod instance in each region with application-level conflict resolution

Answer

Storing all patient data on a single replicated node with manual failover

Question 8

A DBA is preparing to shard a collection in a MongoDB 6.0 cluster. The collection currently has no indexes other than the default _id index. The chosen shard key is { userId: 1 }. What must the DBA do before running shardCollection?

Accepted Answer

Create an index on userId, because a sharded collection requires a supporting index on the shard key.. Before sharding a collection, MongoDB requires an index whose leading fields match the shard key. Without it, the shardCollection command returns an error. Creating an index on userId satisfies this requirement and enables the cluster to manage chunk boundaries and route queries. Emptying the collection or disabling the balancer is unnecessary and would not satisfy the actual prerequisite.

Answer

Convert the shard key to a hashed key first, because ranged shard keys cannot be applied to existing collections.

Answer

Set the collection to unsharded status and disable the balancer for the duration of the operation.

Answer

Move the collection to the primary shard and ensure it is empty, because only empty collections can be sharded.

Question 9

You are a DBA for a MongoDB replica set. A developer asks you to explain what the oplog is and why it is important. What is the correct description?

Accepted Answer

The oplog is a capped collection that records all write operations and is used by secondaries to replicate data from the primary.. The oplog is a capped collection in the local database that records all write operations. Secondaries replicate by reading the oplog and applying the operations. This mechanism ensures that all members eventually have the same data, enabling high availability and failover.

Answer

The oplog is a temporary file used during initial sync to transfer data from the primary to a new secondary.

Answer

The oplog is a cache that stores frequently accessed documents to improve read performance.

Answer

The oplog is a log file on disk that records all queries and read operations for auditing purposes.

Question 10

A sharded cluster stores customer documents in the customers collection, sharded on the customerId field. An application runs an updateOne with the filter { email: "user@example.com" } and the update { $set: { tier: "gold" } }. The email field is not the shard key and has no index. What is the most likely outcome of this operation?

Accepted Answer

The mongos broadcasts the update to all shards, and each shard performs a collection scan to evaluate the filter.. In a sharded collection, routing decisions depend on the shard key. Because the filter uses email, which is neither the shard key nor indexed, mongos cannot target a shard and must broadcast to all shards. Each shard then scans its local documents to evaluate the filter, making the update a scatter-gather operation. The update is not rejected, and no global email index exists.

Answer

The update is routed only to the shard that owns the email range because mongos maintains a global index on email.

Answer

The update fails immediately because the filter does not include the shard key.

Answer

The update succeeds only on the primary shard and is silently ignored on other shards.

Question 11

You are monitoring a MongoDB replica set and notice that the replication lag on a secondary is increasing steadily. You suspect that the secondary is unable to keep up with the write load. Which administrative action should you take first to diagnose the issue?

Accepted Answer

Check the output of rs.printSlaveReplicationInfo() to see the lag and the time of the last oplog entry.. The first step in diagnosing replication lag is to quantify it and check the secondary's progress. rs.printSlaveReplicationInfo() provides the lag in seconds and the timestamp of the last oplog entry applied, helping you understand the severity and whether the secondary is making progress. Other actions like restarting or resizing the oplog are premature without diagnosis.

Answer

Restart the secondary to see if the lag clears.

Answer

Run db.currentOp() on the secondary to see long-running operations.

Answer

Increase the oplog size on the primary to allow more time for the secondary to catch up.

Question 12

A MongoDB 6.0 sharded cluster uses a shard key of { region: 1, customerId: 1 } for a global order collection. Most queries filter on customerId only, without region. The operations team observes that nearly every read is a scatter-gather across all shards. What is the most accurate explanation for this behavior?

Accepted Answer

The shard key fields are in the wrong order: customerId must precede region so that queries filtering only on customerId can target a subset of shards.. Query targeting on a sharded collection depends on whether the predicate includes a prefix of the shard key. With { region: 1, customerId: 1 }, the leading field is region, so a query filtering only on customerId matches no prefix and mongos broadcasts to all shards. Placing customerId first allows equality queries on customerId to be routed to a single shard, resolving the scatter-gather pattern.

Answer

The collection uses hashed sharding, which prevents any targeted reads regardless of which shard key fields appear in the query predicate.

Answer

The balancer is disabled, so chunks for the customerId values are not distributed and queries cannot be routed to a single shard.

Answer

The shard key is compound, and MongoDB cannot route queries unless every field in the shard key is present with an equality match.

Question 13

A database administrator is configuring a MongoDB replica set. The application requires that writes are acknowledged by a majority of voting members before being considered successful, and that the write is also written to the on-disk journal before acknowledgment. Which write concern specification achieves this?

Accepted Answer

{ w: "majority", j: true }. The write concern { w: "majority", j: true } ensures that the write is acknowledged by a majority of voting members and that it is written to the on-disk journal before acknowledgment. This provides both durability and high availability. The other options either lack journaling or lack majority acknowledgment, or both.

Answer

{ w: 1, j: true }

Answer

{ w: "majority", j: false, wtimeout: 5000 }

Answer

{ w: "majority", j: false }

Question 14

A three-member replica set has default settings, and all three members are healthy. A DBA needs to perform an index build on the primary during business hours. Which concern should the DBA consider regarding replica set behavior?

Accepted Answer

The index build will be applied to all data-bearing members, and the primary waits for secondaries to finish before committing the index.. In MongoDB 4.4 and later, index builds on replica sets are coordinated: the primary initiates the build, secondaries build concurrently, and the primary waits for all data-bearing secondaries to finish before committing. This ensures index consistency across the set. The operation can be resource-intensive and may impact replication lag temporarily, but it does not roll back or step down due to lag.

Answer

The index build will cause the primary to step down if any secondary has a replication lag greater than 10 seconds.

Answer

The index build will only be performed on the primary, and secondaries will build the index lazily during their next election.

Answer

The index build will be rolled back on the secondaries if any secondary falls behind.

Question 15

A DBA is investigating a performance issue. A query on the `logs` collection uses the following filter: `{ $or: [ { level: "error" }, { level: "warning" } ] }`. There is an index on `{ level: 1 }`. Which statement correctly describes how MongoDB executes this query?

Accepted Answer

MongoDB will use the index on `level` to perform two separate index scans and then merge the results.. For `$or` queries where each clause can be satisfied by an index, MongoDB performs separate index scans for each clause and then merges the results. This is more efficient than a collection scan. The index on `level` is suitable for both equality conditions, so the query will leverage it twice.

Answer

MongoDB will use the index only if the `$or` clauses are combined with `$and`.

Answer

MongoDB will perform a collection scan because `$or` queries cannot use indexes.

Answer

MongoDB will use the index on `level` but only for the first clause in the `$or` array.

Question 16

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?

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 17

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 18

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 19

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 20

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