C100DBA Practice Test 2 — 10 Questions

Question 1

Which design principle explains why MongoDB uses a document-oriented model instead of a rigid tabular schema?

Accepted Answer

Aligning data storage with object-oriented application models.. MongoDB utilizes a document model to align data structures with how applications represent objects, promoting developer productivity and flexibility. By allowing nested structures, developers avoid complex joins required in relational systems, which significantly improves read performance for related data. This philosophy enables schemas to evolve organically as application requirements change, supporting agile development cycles where modifying a table's structure across millions of rows would otherwise impose significant operational overhead or downtime.

Answer

Normalization of data to reduce storage footprints.

Answer

Strict enforcement of data types via ACID compliance.

Answer

Ensuring all related data is stored in separate collections.

Question 2

During routine cluster maintenance, an administrator needs to remove an underperforming shard from a production MongoDB sharded cluster safely. What is the mandatory first step the administrator must execute?

Accepted Answer

Run the removeShard command on the admin database via a mongos connection to initiate the automated chunk draining process.. Removing a shard safely begins with executing the removeShard command, which initiates the draining process. The cluster balancer automatically migrates all chunks residing on the target shard to other available shards in the cluster, ensuring no user data is lost before the node is decommissioned.

Answer

Execute the dropDatabase command on every secondary member of the shard replica set to clear local storage blocks instantly.

Answer

Manually shut down all mongod processes associated with the target shard before notifying the config server replica set.

Answer

Run the reshardCollection command to merge all orphaned documents into a single backup file on the primary config server.

Question 3

Which TWO of the following are essential for maintaining performance in a sharded cluster?

Accepted Answer

Selecting a shard key with high cardinality.. A well-chosen shard key is the most critical factor for even data distribution, while monitoring chunks ensures that data remains balanced across the cluster. If the shard key is poorly chosen, some shards may become bottlenecks (hotspots), while others remain underutilized. Proper chunk management prevents excessive migration and ensures that query routers (mongos) efficiently route requests to the appropriate shards, maintaining overall system stability and throughput.

Answer

Hardcoding the shard primary in the application connection string.

Answer

Disabling the balancer during all business hours.

Answer

Using the same shard key for every collection in the database.

Question 4

Which design principle should guide the decision between embedding and referencing?

Accepted Answer

Model data based on the application's read and write patterns.. The primary principle for data modeling in MongoDB is the 'access pattern.' You should model data to match how your application queries it. If data is almost always retrieved together, it should be embedded. If the data is accessed independently or is extremely large, it should be referenced. This ensures that the system remains performant by minimizing the number of disk reads and network round-trips required to serve requests.

Answer

Always embed to prevent any possibility of data duplication.

Answer

Always reference to keep the database size as small as possible.

Answer

Use embedding for all data to ensure ACID compliance.

Question 5

When designing an application, what is the best practice for handling connection pools?

Accepted Answer

Maintain a long-lived, singleton instance of the MongoClient.. Maintaining a single, long-lived connection pool instance in the application is the most efficient way to manage database connections. Repeatedly creating and destroying connections is extremely expensive, introducing unnecessary latency and resource overhead on both the application server and the database. Proper pool management is a core application administration task that ensures the database handles traffic gracefully without exhausting its maximum configured connection limit due to unnecessary connection churn from the client side.

Answer

Create a new connection for every query.

Answer

Close the connection after every single write operation.

Answer

Set the connection pool size to zero.

Question 6

An application frequently queries a large collection using two fields: 'category' (equality) and 'timestamp' (range). Which index strategy provides the most efficient execution plan?

Accepted Answer

{ category: 1, timestamp: 1 }. The ESR (Equality, Sort, Range) rule dictates that equality fields should come first in a compound index, followed by sort fields, and finally range fields. By placing 'category' first, MongoDB eliminates most irrelevant documents immediately. The 'timestamp' field follows, allowing the engine to leverage the index for range filtering. This minimizes memory usage and prevents costly full collection scans, which is critical for maintaining performance as data volume scales.

Answer

{ timestamp: 1, category: 1 }

Answer

{ category: 1 }

Answer

{ timestamp: 1 }

Question 7

An application issues a find query against a sharded collection without including the shard key in the query filter. How does the mongos query router handle this operation?

Accepted Answer

It performs a scatter-gather query by sending the request to all shards in the cluster, merging the results before returning them to the client.. When a query lacks the shard key, the mongos query router cannot determine which specific shard holds the target document. Consequently, it executes a scatter-gather query, forwarding the request to every shard in the cluster, collecting the responses, and merging them before returning the result to the client.

Answer

It rejects the query immediately with an invalid operation exception to protect cluster performance from unindexed scans.

Answer

It routes the query exclusively to the primary shard of the database, bypassing all other shards in the infrastructure.

Answer

It broadcasts the query only to the config server primary, which evaluates the predicate against cached metadata documents.

Question 8

Which capability of MongoDB's document model most significantly distinguishes it from key-value stores?

Accepted Answer

The ability to index and query nested document fields.. While key-value stores are limited to looking up data by a single key, MongoDB allows for deep inspection and querying of the document content. MongoDB indexes fields within the document, enables complex aggregation pipelines, and supports ad-hoc queries on any field. This versatility transforms the database from a simple lookup engine into a powerful analytical and transactional system, allowing it to handle diverse workloads that require rich data exploration and complex filtering.

Answer

The ability to store binary data as BSON.

Answer

The use of a primary key for document retrieval.

Answer

The support for horizontal scaling across clusters.

Question 9

Refer to the exhibit. An administrator attempts to execute the shardCollection command but receives the displayed error message. What is the root cause of this failure?

Accepted Answer

The collection lacks a supporting index whose key pattern matches or starts with the fields specified in the shard key definition.. MongoDB requires an explicit index whose prefix matches the designated shard key before sharding a collection. The error indicates that no such index exists on the collection, preventing the query router from establishing the necessary routing boundaries. Creating the correct index prior to running shardCollection resolves this issue.

Answer

The target database has not been enabled for sharding using the enableSharding administrative command.

Answer

The specified shard key contains an unsupported array field that violates BSON multi-key index constraints.

Answer

The config server replica set is currently undergoing an election and cannot process metadata write operations.

Question 10

A sharded cluster experiences poor query performance because a targeted query targeting a specific shard key still results in a scatter-gather operation across all shards. What is the root cause?

Accepted Answer

The query predicate completely omitted the shard key or its valid prefix fields.. To target a specific shard directly, a query must include the exact shard key or a valid prefix of the shard key in its filter predicate. Omitting the shard key forces the mongos router to query every shard in the cluster to assemble the final result set.

Answer

The chunk migration threshold was set too low, causing frequent balancing cycles across shards.

Answer

The collection was created as unhashed, preventing the mongos router from hashing query inputs.

Answer

The balancer process was paused, leaving uneven document distribution across cluster shards.