C100DBA Practice Test 5 — 10 Questions

Question 1

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 2

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.

Answer

Hashed sharding with a geo-spatial index.

Answer

The movePrimary command for each database.

Question 3

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()

Answer

rs.conf()

Answer

db.isMaster()

Question 4

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.

Answer

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

Answer

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

Answer

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

Question 5

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})

Answer

.orderBy({age: 'desc'})

Answer

.order({age: 'desc'})

Question 6

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.

Answer

Applications must manually hardcode all secondary IP addresses.

Answer

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

Answer

It forces the application to implement custom failover logic.

Question 7

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.

Answer

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

Answer

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

Answer

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

Question 8

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

Answer

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

Answer

Normalizing comments into a separate collection with manual joins

Question 9

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.

Answer

Store all data in a single collection to simplify sharding.

Answer

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

Question 10

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.