C100DBA Practice Test 15 — 15 Questions

Question 1

What is the result of setting the 'votes' configuration to 0 for a specific node in a replica set?

Accepted Answer

The node will not participate in primary elections.. Setting 'votes' to 0 makes the node a non-voting member. It will still replicate data and can serve read queries, but it will never participate in elections for a new primary. This is useful for scaling read capacity without increasing the quorum size, which would otherwise require adding more nodes to maintain a majority in the election process.

Answer

The node will no longer replicate data from the primary.

Answer

The node will be removed from the replica set configuration.

Answer

The node will automatically become the primary.

Question 2

Which THREE metrics provided by the 'mongostat' utility are most helpful for identifying a performance bottleneck caused by a high volume of concurrent write operations?

Accepted Answer

insert. Monitoring server health via mongostat is a daily task for a DBA. High write volumes are best identified by looking at the actual operation counts (insert, update, delete) and the state of the write queue. If the 'qw' (queue write) count is high, it indicates that the storage engine cannot keep up with the incoming write requests.

Answer

getmore

Answer

vsize

Question 3

What is the primary function of the oplog in a MongoDB replica set?

Accepted Answer

To record all operations for asynchronous replication.. The oplog, or operation log, is a capped collection that stores all write operations as they are applied to the database. Secondaries read these entries and apply them to their own datasets. This asynchronous replication process ensures that all members of the replica set eventually reach the same state, providing both data redundancy and high availability across the cluster.

Answer

To store user credentials and authentication data.

Answer

To cache frequently accessed query results in memory.

Answer

To perform automatic backups for disaster recovery.

Question 4

During a peak load period, the 'page faults' metric in mongostat increases significantly. What is the most likely administrative cause related to server resources?

Accepted Answer

The working set exceeds the available physical RAM.. Page faults occur when the mongod process attempts to access data that is not currently in the physical RAM (resident memory). While some page faults are normal, a sudden spike usually indicates that the 'working set' (the data and indexes accessed most frequently) has grown larger than the available RAM, forcing frequent disk reads.

Answer

The number of concurrent connections has reached the 'maxConns' limit.

Answer

The CPU is throttled due to excessive BSON serialization.

Answer

The journaling frequency is set too high for the disk subsystem.

Question 5

Why does MongoDB support both embedding and referencing in data modeling?

Accepted Answer

To manage data size limits and complex relationships.. Embedding is ideal for performance when data is accessed together, but referencing is necessary for handling unbounded data, complex relationships, or avoiding document size limits (16MB). By offering both, MongoDB provides the flexibility to choose the right strategy based on the specific cardinality and access frequency of the data, allowing developers to balance performance and storage constraints effectively without forcing a one-size-fits-all architectural approach.

Answer

To support legacy relational database imports.

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To force developers to use ACID transactions.

Answer

To eliminate the need for indexes in the database.

Question 6

A developer reports slow queries on a specific collection. You decide to enable the database profiler to capture only queries taking longer than 200ms. Which command configuration achieves this?

Accepted Answer

db.setProfilingLevel(1, { slowms: 200 }). The database profiler is an essential administrative tool for diagnosing performance issues. Profiling level 1 is used to capture 'slow' operations without the overhead of capturing every single request (level 2). Setting the 'slowms' threshold to 200 allows the DBA to filter out expected noise and focus on the problematic operations.

Answer

db.setProfilingLevel(2, { slowms: 200 })

Answer

mongod --profile 200 --slowms 1

Answer

db.adminCommand({ profile: 0, slowms: 200 })

Question 7

Refer to the exhibit. An administrator attempts to recover a corrupted standalone mongod instance using the --repair flag. Based on the error log output, what is the most appropriate next step for the administrator?

Accepted Answer

Restore the data files from the most recent known-good backup.. The --repair flag is a last-resort tool that attempts to salvage data from corrupted files, but it cannot fix severe filesystem or metadata corruption where blocks are missing or unreadable. The 'Short read' error indicates physical or logical corruption that WiredTiger cannot bypass. In this scenario, restoring from a known-good backup is the only reliable way to ensure data integrity.

Answer

Run the --repair command again with the --wiredTigerEngineConfig option.

Answer

Delete the mongod.lock file and restart the server normally.

Answer

Manually edit the WiredTiger.wt file to fix the offset error.

Question 8

Which feature of MongoDB allows it to maintain data consistency during primary node failure?

Accepted Answer

Automatic failover via replica set elections.. MongoDB uses replica sets to ensure high availability. When a primary node becomes unavailable, the remaining nodes in the replica set conduct an election to select a new primary. This process is governed by the consensus algorithm, ensuring that only a node with the most up-to-date data can become the new primary, thereby preventing data loss and maintaining the consistency of the cluster during automated failover scenarios.

Answer

Manual shard intervention by the administrator.

Answer

The use of a secondary index for read-only access.

Answer

The mongos router forcing a global write lock.

Question 9

Which THREE factors can significantly impact the time taken for a new primary election?

Accepted Answer

The network latency between replica set members.. The speed of a failover is determined by how quickly nodes detect the primary is down and how quickly they can reach a consensus. Network latency between nodes, the heartbeat interval, and the number of nodes in the set are all critical. Optimizing these factors is necessary for maintaining a high-availability architecture that can recover from outages within the expected time window defined by SLAs.

Answer

The total number of documents in the largest collection.

Answer

The amount of free RAM available on the primary node.

Question 10

Refer to the exhibit. What is the primary cause of the performance issue seen in this query plan?

Accepted Answer

The query requires a full collection scan.. The exhibit shows a 'COLLSCAN', which indicates the database is scanning every document in the collection to satisfy the query. With 500,000 scanned documents returned for a single result, the query is severely inefficient. This demonstrates a lack of a supporting index, leading to high CPU and I/O usage. Identifying these inefficient queries is vital for DBAs to prevent application-wide slowdowns caused by resource exhaustion on the primary database node.

Answer

The query is hitting the primary index limit.

Answer

The query is being blocked by a write lock.

Answer

The database is out of memory.

Question 11

What is the primary benefit of using a Covered Query in MongoDB?

Accepted Answer

It avoids reading the full document from disk.. A covered query is one where all the fields in the query projection exist within the index itself. Because the database can retrieve all necessary data directly from the index tree without needing to fetch the actual document from disk, it drastically reduces I/O. This is one of the most effective ways to optimize read performance for high-frequency queries in large collections.

Answer

It forces the data to stay in the WiredTiger cache.

Answer

It automatically compresses all returned data.

Answer

It eliminates the need for any index on the collection.

Question 12

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.

Answer

The field is not a unique identifier for the documents.

Answer

The field type must be an ObjectID for performance reasons.

Answer

The balancer cannot migrate chunks based on string values.

Question 13

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.

Answer

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

Answer

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

Answer

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

Question 14

How does the 'w: majority' write concern align with the philosophy of data reliability in MongoDB?

Accepted Answer

It ensures the data is durable on a majority of nodes.. The 'w: majority' write concern ensures that a write operation is acknowledged only after it has been replicated to a majority of nodes in the replica set. This philosophy prioritizes data durability and consistency over the absolute lowest latency. By requiring a majority, MongoDB ensures that even if a primary node fails immediately after a write, the data persists on other nodes, minimizing the window for data loss during failover.

Answer

It guarantees that the write is applied to all shards.

Answer

It optimizes for the fastest possible write performance.

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It forces an immediate sync to the storage disk.

Question 15

What happens if you run an update operation with the $set operator on a field that does not yet exist in the document?

Accepted Answer

The field is created with the new value.. MongoDB is schema-flexible, meaning it will create the field if it is missing during an update operation. The $set operator is intelligent enough to add the field with the specified value, which simplifies code by eliminating the need for pre-checks or initialization logic. This is a core strength of the document model that allows for evolving data requirements without requiring explicit database migrations.

Answer

The update fails with an error.

Answer

The update is ignored.

Answer

The entire document is replaced.