C100DBA Sharding Practice Test 2 — 15 Questions

Question 1

Which THREE components are required to form a functional MongoDB sharded cluster?

Accepted Answer

One or more shards to store the data.. A MongoDB sharded cluster consists of three main components: Shards, Config Servers, and Query Routers (mongos). Shards store the actual data, Config Servers store the cluster's metadata and configuration, and Query Routers act as an interface for applications, directing requests to the appropriate shards. Each plays a distinct and vital role in the cluster's operation.

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A dedicated arbiter node for each shard.

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A central MongoDB Ops Manager instance.

Question 2

A collection has a 'status' field with only three possible values: 'active', 'pending', and 'closed'. Why is 'status' a poor choice for a shard key by itself?

Accepted Answer

Low cardinality prevents the creation of many chunks.. A good shard key needs high cardinality to allow for a large number of chunks. If a shard key only has three possible values, the cluster can have at most three chunks for that collection. This limits the total number of shards that can participate in storing and processing the data, leading to scalability bottlenecks and imbalance.

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The field is not a unique identifier for the documents.

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The field type must be an ObjectID for performance reasons.

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The balancer cannot migrate chunks based on string values.

Question 3

An application executes a query on a sharded collection that does not include the shard key. What is the impact on the cluster performance?

Accepted Answer

The mongos will perform a 'scatter-gather' operation across all shards.. When a query does not include the shard key, the mongos router cannot determine which shard contains the requested data. As a result, it must broadcast the query to every shard in the cluster. This 'scatter-gather' operation increases resource consumption across all nodes and can lead to significant latency and scalability issues as the cluster grows.

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The query will be rejected by the mongos as an invalid operation.

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The query will only be sent to the primary shard of the database.

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The cluster will automatically create a new index to support the query.

Question 4

A multi-document transaction is being executed across multiple shards. Which component is responsible for coordinating the transaction and ensuring the all-or-nothing property?

Accepted Answer

The mongos router that initiated the transaction.. Multi-document transactions in a sharded cluster are coordinated by the mongos router where the transaction was initiated. The mongos acts as the transaction coordinator, using a two-phase commit protocol to ensure consistency across the participant shards. This allows applications to maintain ACID guarantees even when data is distributed across a horizontally scaled environment.

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The primary node of the config server replica set.

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The shard that contains the first document accessed in the transaction.

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A specialized 'Transaction Manager' node that must be configured.

Question 5

A sharded cluster has a three-member replica set for its config servers. If two of the three config servers go offline, what is the immediate impact on the cluster's operations?

Accepted Answer

Chunks cannot be split or migrated between shards.. Config servers store the metadata and configuration settings for the entire sharded cluster. They are deployed as a replica set to ensure high availability. If the replica set loses its majority, it can no longer process writes to the metadata, which has significant implications for administrative tasks and the dynamic behavior of the cluster.

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The cluster becomes read-only for all application data.

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The mongos instances will crash and must be restarted.

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All shards will automatically step down their primary members.

Question 6

An engineering team must shard a high-volume collection tracking global financial transactions. The chosen shard key is based on a timestamp field containing the exact millisecond of each transaction. Why will this specific shard key choice severely degrade cluster write performance over time?

Accepted Answer

Monotonically increasing timestamp values direct all concurrent write operations exclusively to a single chunk located on the current maximum shard, creating a severe insertion bottleneck.. Timestamp shard keys cause write amplification on a single shard because monotonically increasing values always route insertions to the highest chunk in the cluster. This violates key sharding principles by preventing horizontal write scalability. Recognizing insert bottlenecks helps DBAs design balanced, high-throughput architectures using hashed keys or compound prefixes to distribute incoming write operations effectively across multiple cluster shards.

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The MongoDB balancer will immediately purge all historical chunks older than twenty-four hours to conserve disk space, resulting in accidental data loss for compliance records.

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Queries filtering by date ranges will fail because the query router cannot project timestamps across distributed shards without a secondary hashed index on the object identifier.

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MongoDB will automatically convert the millisecond integers into floating-point numbers, causing precision mismatch errors during chunk migration coordination phases.

Question 7

An administrator needs to distribute a high-throughput collection evenly across a sharded cluster using a hashed shard key. What is the primary characteristic of MongoDB's hashed sharding strategy that makes it effective for preventing write bottlenecks?

Accepted Answer

It computes an MD5 hash of the field value to distribute documents pseudo-randomly across all available chunks and shards, preventing single-shard write bottlenecks.. Hashed sharding applies an MD5 hash function to the specified field value before routing documents. This transforms sequential or correlated inputs into pseudo-random hash values, ensuring that writes scatter uniformly across all available cluster shards. Understanding hashed sharding mechanisms enables administrators to design resilient write-heavy architectures that prevent the single-shard bottlenecks common with monotonic keys.

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It compresses all chunk migration payloads using cryptographic hashing algorithms to minimize network bandwidth consumption during background balancing operations.

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It automatically converts multi-key array fields into single scalar values to prevent indexing errors on distributed sharded collections.

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It locks the target shard during write operations to guarantee strict serializable isolation levels across distributed cluster environments.

Question 8

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.

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

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

Question 9

An administrator notices that the MongoDB balancer is actively migrating chunks during peak business hours, causing noticeable application latency spikes. What is the standard administrative approach to mitigate this performance impact?

Accepted Answer

Configure a balancer window by updating the settings collection in the config database to restrict migrations to specific off-peak hours.. Configuring a balancer window restricts chunk migrations to off-peak hours, preventing background balancing traffic from contending with production application workloads. Mastering balancer scheduling controls allows DBAs to maintain cluster health and optimal data distribution without disrupting user experience during critical business hours.

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Permanently terminate the mongos process responsible for orchestrating chunk migrations across the cluster nodes.

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Execute the dropDatabase command on every shard to purge unbalanced chunks instantly and reset the cluster.

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Disable journaling across all shard replica set members to accelerate background data migration speeds.

Question 10

Refer to the exhibit. An administrator inspects chunk metadata for a sharded collection using a compound shard key: { region: 1, account_id: 1 }. What does the presence of MinKey and MaxKey signify in this specific chunk range definition?

Accepted Answer

They represent absolute lower and upper bounds for the account_id field within the EMEA region, ensuring all accounts for that region reside in this chunk.. MinKey and MaxKey represent lower and upper boundary markers in MongoDB that evaluate smaller or larger than any other BSON data type. In this compound chunk range, they ensure that all documents matching the 'EMEA' region regardless of their account_id value fall within this specific chunk. Understanding BSON boundary types is essential for troubleshooting custom chunk splits and zone sharding configurations.

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They indicate that the chunk is corrupted and requires immediate repair using the validate command on the config server.

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They specify encryption keys used by the WiredTiger storage engine to secure data chunks at rest on shard01.

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They are temporary placeholder values assigned by the balancer during active chunk splits that will automatically expire after twenty-four hours.

Question 11

An organization wants to restrict customer data for European Union residents to a specific set of physical servers located in Frankfurt, complying with data residency regulations. Which MongoDB sharding feature should the DBA implement to achieve this?

Accepted Answer

Zone sharding, which associates specific shard key ranges with designated groups of shards to enforce data residency rules.. Zone sharding allows administrators to associate specific shard key ranges with defined sets of shards called zones. By tagging European servers into an 'EU' zone and associating the region shard key range with that zone, MongoDB guarantees that EU customer data stays exclusively on those designated physical shards. This is critical for regulatory compliance and data sovereignty requirements.

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Dynamic range compression, which encrypts and confines specific database collections to localized storage volumes on individual nodes.

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Global cluster replication, which synchronizes all database documents simultaneously across every shard in every continent.

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Read preference tagging, which routes read operations to secondary members in specific data centers without altering write locations.

Question 12

When designing a compound shard key in MongoDB, what is the architectural significance of placing a high-cardinality field as the prefix compared to placing a low-cardinality field first?

Accepted Answer

A high-cardinality prefix ensures fine-grained chunk boundaries and even data distribution, whereas a low-cardinality prefix causes massive chunks that cannot be split effectively.. The shard key prefix dictates how MongoDB organizes and partitions chunk ranges across the cluster. A high-cardinality prefix ensures fine-grained chunk boundaries and prevents massive, unmanageable chunks from forming around repetitive values, which often happens when low-cardinality fields like boolean flags or status codes occupy the leading position.

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A high-cardinality prefix allows the query router to bypass authentication checks on secondary shards, improving overall cluster query response times.

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A low-cardinality prefix enables the WiredTiger storage engine to cache entire indexes in RAM, eliminating disk I/O bottlenecks entirely.

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A low-cardinality prefix automatically triggers hashed shard evaluation, bypassing the need for explicit hashing functions during collection creation.

Question 13

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.

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

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

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

Question 14

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.

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

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

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

Question 15

Which TWO statements accurately describe the behavior and management of orphaned documents in a MongoDB sharded cluster? Choose 2 answers.

Accepted Answer

Orphaned documents are documents that remain on a shard after a chunk has been migrated away to another node in the cluster.. Orphaned documents are leftover data chunks remaining on a source shard after a chunk migration fails to complete cleanly or before a background cleanup process runs. Understanding how MongoDB handles orphaned documents helps DBAs troubleshoot storage utilization issues and run cleanup tasks safely without risking valid data.

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Orphaned documents are automatically indexed by the query router and included in all scatter-gather query results by default.

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The cluster balancer deletes orphaned documents simultaneously across all shards during every active migration window.

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Orphaned documents prevent the config server from performing routine replica set elections until they are manually purged.