C100DBA Practice Test 2 — 30 Questions

Question 1

An application frequently queries a large collection by 'user_id' and 'status', sorting the results by 'timestamp'. Which index provides the most efficient execution plan?

Accepted Answer

{ user_id: 1, status: 1, timestamp: 1 }. To optimize this query, the ESR (Equality, Sort, Range) rule must be applied. The 'user_id' and 'status' fields are equality filters, while 'timestamp' is used for sorting. An index of {user_id: 1, status: 1, timestamp: 1} allows MongoDB to satisfy the query filters and provide the sort order directly from the index, avoiding a blocking sort operation in memory which is critical for performance on large collections.

Answer

{ timestamp: 1, user_id: 1, status: 1 }

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{ user_id: 1, timestamp: 1, status: 1 }

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{ user_id: 1, status: 1 }

Question 2

Which method should you use to update multiple documents in a collection and insert a new document if no matches are found?

Accepted Answer

db.collection.updateMany({filter}, {update}, {upsert: true}). The updateMany method modifies all documents matching the filter. When combined with the 'upsert' option set to true, it ensures that if no documents match the query criteria, a single new document is created containing the filter and update criteria. This pattern is critical for idempotent data ingestion pipelines where you must guarantee existence of records without performing separate check-then-insert logic, thus preventing race conditions.

Answer

db.collection.updateOne()

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db.collection.replaceOne()

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db.collection.save()

Question 3

Which read preference allows the application to read from the closest member of the replica set to minimize network latency?

Accepted Answer

nearest. The 'nearest' read preference is specifically designed to reduce latency by targeting the node with the lowest network round-trip time. This is highly effective for global deployments where applications are distributed across regions. Understanding read preferences is fundamental for application administration, as it allows developers to influence traffic distribution and resource utilization across the replica set, ensuring the application remains responsive even when the primary node is geographically distant from the client.

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primaryPreferred

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secondary

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secondaryPreferred

Question 4

When performing a findOne operation, what does MongoDB return if no document matches the query criteria?

Accepted Answer

null. The findOne method is designed to return a single document or null. Returning null is the standard way for the driver and shell to indicate that the search criteria did not produce a match. This is important for application control flow, as developers must always verify that a document was actually retrieved before attempting to access its fields to avoid null pointer exceptions in their code.

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An empty document ({})

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An error code

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An empty cursor

Question 5

What is the purpose of the 'OpLog' in a MongoDB Replica Set?

Accepted Answer

It provides a mechanism for asynchronous data replication.. The OpLog (Operations Log) is a capped collection that records all modifications to the data in the database. It is the core component that allows secondary nodes to replicate data from the primary node. By replaying the operations found in the OpLog, secondaries maintain an identical state to the primary. Understanding the OpLog is essential for diagnosing replication lag and ensuring the system is effectively synchronized across all nodes in the cluster.

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It stores user credentials for database authentication.

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It is used for storing index definitions and metadata.

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It acts as a permanent, immutable backup of all data.

Question 6

Which member type in a MongoDB replica set is best suited for a geographically distant data center to provide read-only access without influencing elections?

Accepted Answer

Priority 0 Secondary. A priority 0 secondary is ideal for this scenario. By setting the priority to zero, the node is ineligible to become primary, preventing it from participating in elections. This is critical for nodes in remote locations where high latency could disrupt election timing or cause unnecessary re-elections, while still providing low-latency read access to users in that specific geographical region.

Answer

Arbiter

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Hidden Member

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Delayed Secondary

Question 7

What is the consequence of having a replica set with only two nodes and no arbiter?

Accepted Answer

The replica set will be unable to elect a primary if one node fails.. A replica set requires a majority of nodes to elect a primary. In a two-node set, a failure of one node makes it impossible to reach a majority, as one out of two is not greater than half. This design limitation is critical because it leads to the loss of a primary and the inability to elect a new one, resulting in a read-only database and complete outage for write operations.

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The set will automatically double the voting power of the primary.

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The replica set will function normally, but writes will be slow.

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The primary will become read-only while the secondary remains idle.

Question 8

Which operator would you use to remove an element from an array field in a document?

Accepted Answer

$pull. The $pull operator is the primary tool for removing items from arrays based on a specified query condition. It is highly effective because it allows you to remove one or many matching elements in a single operation without needing to pull the entire document into application memory, modify the array, and push it back. This promotes efficient resource usage and data consistency.

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$remove

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$pop

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$unset

Question 9

Refer to the exhibit. What is the primary function of the node with _id 2 in this replica set?

Accepted Answer

It provides a vote during elections to help maintain a majority.. An arbiter exists solely to participate in elections and break ties. It does not replicate data or serve read requests, making it a low-resource way to achieve a majority vote in a set. This is crucial for environments where a full-data secondary is cost-prohibitive but high availability is still required to maintain quorum during network partitions.

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It stores a copy of the data and can serve read queries.

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It performs automatic failover by hosting a redundant copy of the oplog.

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It acts as a load balancer for incoming read requests.

Question 10

Refer to the exhibit. Which MongoDB philosophy is represented by this document structure?

Accepted Answer

Data locality for optimized retrieval patterns.. This document structure demonstrates the principle of embedding data. Instead of creating a separate collection for sensor readings and joining them via foreign keys, the readings are nested directly within the document. This approach minimizes the need for multi-table joins, which is a key tenet of MongoDB's design to keep data that is accessed together stored together, resulting in faster read performance and simplified application logic for time-series or hierarchical datasets.

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Normalization for maximum data consistency.

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Enforcement of a rigid schema using array types.

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The use of an Oplog to track document changes.

Question 11

Which THREE components are mandatory architectural parts of every fully functioning MongoDB sharded cluster deployment? Choose 3 answers.

Accepted Answer

A config server replica set that stores all cluster metadata, chunk ranges, and routing configuration data.. A sharded cluster relies on three core functional components: config servers to maintain cluster metadata, mongos query routers to direct client requests, and shard replicas to store partitioned data safely. Knowing these architecture requirements helps administrators design resilient, highly available production clusters with proper redundancy at every layer of the infrastructure.

Answer

A dedicated primary balancer coordinator node running outside the config server replica set to manage migrations.

Answer

An independent Apache ZooKeeper quorum cluster to coordinate distributed locking across all participating mongod nodes.

Question 12

An application serves users in Europe and North America. The DBA wants to ensure that European user data stays on shards located in Europe to comply with data residency laws. Which feature should be used?

Accepted Answer

Zones and Zone Ranges.. Zones (formerly known as tag-aware sharding) allow administrators to associate specific shard key ranges with particular shards. By tagging shards with geographical identifiers and defining zones for ranges of the shard key, the balancer will automatically move data to the appropriate physical location. This is essential for meeting regulatory requirements and reducing latency.

Answer

Replica set tags for read preferences.

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Hashed sharding with a geo-spatial index.

Answer

The movePrimary command for each database.

Question 13

Which command is used to verify the current replica set configuration and the state of all members?

Accepted Answer

rs.status(). Checking the replica set status is a routine administrative task to ensure high availability and data consistency. The 'rs.status()' command provides a comprehensive view of the cluster, including the state of every member, the current primary, and the sync lag. This command is the primary diagnostic tool for verifying that the replica set is healthy and that all nodes are correctly communicating with each other.

Answer

db.serverStatus()

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rs.conf()

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db.isMaster()

Question 14

Refer to the exhibit. The explain output shows a SORT stage wrapping an IXSCAN stage. What is the performance implication of this execution plan?

Accepted Answer

The query engine performed an in-memory sort because the index did not support the requested sort order.. When a SORT stage wraps an IXSCAN stage, it means the index used for filtering did not satisfy the sort requirement. The query engine had to load the matching documents into memory to sort them, risking memory limit exceptions on large result sets.

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The query is fully optimized because it leverages an index scan for filtering records.

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The query executed a full collection scan across all cluster nodes.

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The storage engine automatically converted the query into a covered query.

Question 15

Which cursor method should you use to sort documents by a field named 'age' in descending order?

Accepted Answer

.sort({age: -1}). The sort() method is used to define the order of results returned by a query. Passing {age: -1} ensures that the documents are ordered from oldest to youngest. Mastering sorting is essential for returning data in a meaningful way to users, such as showing the most recent posts or the highest-rated products first, which are common requirements for almost all web and mobile applications.

Answer

.sort({age: 1})

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.orderBy({age: 'desc'})

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.order({age: 'desc'})

Question 16

How does the 'Replica Set' architecture influence client application design?

Accepted Answer

It enables read distribution across multiple nodes.. Replica sets require the driver to be aware of the topology to perform automatic discovery and routing. Since a replica set has a primary node for writes and multiple secondaries for reads, the application must manage read preferences and connection strings correctly. Understanding this architecture is crucial for writing resilient applications that can handle failover events without losing connection or experiencing data consistency issues during node promotions.

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Applications must manually hardcode all secondary IP addresses.

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It mandates that all reads must be 'strong' consistency.

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It forces the application to implement custom failover logic.

Question 17

A financial services firm is migrating its reporting application from a relational database to MongoDB. The lead architect argues that MongoDB's schema-less design will eliminate the need for data modeling. Which statement best describes MongoDB's philosophy regarding schema design?

Accepted Answer

Schema design is still essential but is driven by application access patterns rather than normalization rules.. MongoDB's schema flexibility does not eliminate the need for data modeling; instead, it shifts the focus to designing documents that align with how the application reads and writes data. Embedding and referencing are chosen based on access patterns, not normalization rules. This approach optimizes performance and scalability while still requiring careful planning to avoid pitfalls like unbounded document growth.

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Schema design is optional because MongoDB automatically infers and enforces relationships between collections.

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Schema design should follow third normal form to minimize data duplication and ensure consistency.

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Schema design is unnecessary because MongoDB validates all documents against a collection schema by default.

Question 18

A media company stores articles with nested comments and tags. The development team wants to retrieve an article along with its comments in a single database round trip. Which MongoDB philosophy supports this design?

Accepted Answer

Embedding related data within a single document. Embedding related data such as comments and tags within the article document allows the application to retrieve everything in one query. This reduces round trips and latency, directly supporting the team's goal of a single database call to render an article page.

Answer

Using a relational foreign key constraint between articles and comments

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Storing comments as a comma-separated string in a single field

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Normalizing comments into a separate collection with manual joins

Question 19

A healthcare analytics team stores patient records in MongoDB. Each record includes a nested array of vital sign readings, and the team frequently queries for patients whose latest blood pressure reading exceeds a threshold. They decide to store each reading as a separate document in a readings collection and reference the patient. Which MongoDB design philosophy does this decision contradict?

Accepted Answer

Data that is accessed together should be stored together.. The team's choice to reference readings instead of embedding them conflicts with the core MongoDB principle that related data accessed together should be stored together. Embedding the readings array within the patient document would allow a single query to retrieve the latest blood pressure without additional lookups, improving performance and simplifying application logic. This principle guides schema design for locality and efficiency.

Answer

Use server-side JavaScript for all data validation.

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Store all data in a single collection to simplify sharding.

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Normalize all data to the third normal form to eliminate redundancy.

Question 20

A social media platform uses MongoDB to store user posts. Each post document contains an array of comments. The platform's engineers notice that some posts have tens of thousands of comments, causing documents to approach the 16 MB size limit and degrading read performance. They want to redesign the schema to handle this growth while preserving the ability to retrieve a post with its most recent comments efficiently. Which approach best aligns with MongoDB's schema design philosophy?

Accepted Answer

Embed only the most recent comments in the post document and store older comments in a separate collection.. The hybrid approach of embedding recent comments and storing older ones separately aligns with MongoDB's philosophy of designing for the common access pattern while managing document growth. By keeping the post document small, reads remain fast, and the application can still retrieve older comments via a separate query when needed. This pattern is recommended for scenarios with unbounded arrays, balancing performance and scalability.

Answer

Store each comment as a separate document in the same collection and use a parent reference to link them to the post.

Answer

Increase the document size limit by configuring the storage engine to allow larger BSON documents.

Answer

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