C100DBA Application Administration Practice Test 2 — 15 Questions

Question 1

An application uses a MongoDB deployment with a replica set and occasionally performs write operations that include a unique index constraint. The application team reports that during a network partition, some write operations fail with duplicate key errors even though the data does not actually duplicate existing documents. Which write concern setting could be causing this behavior?

Choose an answer to begin — your selection is scored in the full session.

Accepted Answer

w=majority with retryWrites=true. When retryWrites=true is enabled, the driver may retry a write operation that it believes failed due to a network error, but the original write might have actually succeeded on the primary. The retry attempt then fails with a duplicate key error because the unique index constraint is violated by the already-inserted document. This is expected behavior and does not indicate data duplication. The write concern itself does not cause the duplicate key error; it is the combination with retryWrites.

Answer

w=1 with j=false

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w=0

Answer

w=1 with wtimeout=1000

Question 2

A database administrator is configuring a MongoDB replica set for a new application that requires high availability and the ability to perform reads from secondary nodes to reduce load on the primary. The application must also ensure that reads from secondaries do not return data that could be rolled back. Which two read preference and read concern combinations should be used? (Choose two.)

Accepted Answer

readPreference: "nearest", readConcern: "majority". The application needs to read from secondaries to reduce primary load while ensuring that reads are not rollback-able. "majority" read concern provides that guarantee. It can be combined with read preferences that allow secondary reads, such as "secondary" and "nearest". The combination with "secondary" directly reads from secondaries, while "nearest" may read from secondaries if they are closer. Both satisfy the requirements.

Answer

readPreference: "primary", readConcern: "majority"

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readPreference: "secondary", readConcern: "linearizable"

Answer

readPreference: "secondaryPreferred", readConcern: "local"

Question 3

A MongoDB application uses a replica set and requires that reads never return data that has not been acknowledged by a majority of replica set members. The application also needs to minimize read latency by reading from secondaries when possible. Which two read concern and read preference combinations satisfy these requirements? (Choose two.)

Accepted Answer

readConcern: majority with readPreference: primary. The two valid combinations are majority read concern with either secondary or primary read preference. Both ensure that reads return data acknowledged by a majority of replica set members, preventing rollback. The secondary read preference also minimizes latency. The other options either lack majority acknowledgment or use an incompatible read preference.

Answer

readConcern: local with readPreference: secondary

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readConcern: linearizable with readPreference: secondary

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readConcern: available with readPreference: secondary

Question 4

An application needs to update a user's balance. Which method ensures the operation is atomic for a single document?

Accepted Answer

Use the $inc operator with the updateOne method.. Atomic updates are a cornerstone of MongoDB's document model. Using update operators like $inc ensures that the update is performed in a single, atomic step at the document level. This prevents race conditions where two threads might attempt to update the same balance simultaneously, ensuring that data integrity is maintained without the need for manual locking mechanisms in the application layer.

Answer

Read the document, update the value in application code, and save the document.

Answer

Wrap the operation in a multi-document transaction every time.

Answer

Use a global application-level mutex lock before updating.

Question 5

Refer to the exhibit. An aggregation pipeline is failing with an error. How can you modify the pipeline to allow it to process the large dataset?

Accepted Answer

Set 'allowDiskUse' to true in the aggregation options.. The error indicates that the group operation is exceeding the 100MB RAM limit. By setting 'allowDiskUse: true', you permit the MongoDB server to use temporary files for the aggregation process. This is a common requirement for complex analytics where the data working set is larger than the available RAM, ensuring that long-running operations can complete successfully without crashing due to memory exhaustion.

Answer

Increase the RAM allocated to the mongod process configuration.

Answer

Remove the $group stage and perform the grouping in application code.

Answer

Reduce the batch size in the cursor options.

Question 6

Which connection string parameter should be used to ensure that an application only reads data from nodes that are considered current and consistent?

Accepted Answer

readPreference=primary. Read preference settings dictate where the application directs its read operations. Using 'primary' ensures that the application always reads the most recent data, avoiding the latency issues inherent in replication lag. This is critical for applications that require strong consistency and cannot tolerate reading stale data, such as financial transactions or inventory management systems, where data accuracy is paramount.

Answer

readPreference=secondary

Answer

readPreference=nearest

Answer

readPreference=secondaryPreferred

Question 7

An application uses a time-series collection. What is the benefit of using the TTL (Time-To-Live) index feature?

Accepted Answer

It allows the database to remove documents based on a timestamp field.. TTL indexes provide an automated mechanism for cleaning up expired data. By setting a TTL index on a date field, MongoDB periodically removes documents that exceed the defined age, which is essential for managing storage and keeping the database size under control in time-series workloads. This reduces administrative manual cleanup tasks and ensures the system maintains a consistent performance level by pruning old, irrelevant data automatically.

Answer

It automatically creates a new collection for every time period.

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It ensures data is compressed automatically to save disk space.

Answer

It prevents duplicate documents from being inserted into the collection.

Question 8

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.

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Disabling the balancer during all business hours.

Answer

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

Question 9

What is the purpose of the 'write concern' in MongoDB?

Accepted Answer

To define the durability and replication requirements for a write.. Write concern determines the level of acknowledgement requested from the database for write operations. It is a critical setting for balancing performance against data durability. By choosing the right write concern, developers can ensure that their application meets its specific data safety requirements, whether that means waiting for confirmation from a single node or guaranteeing that the write has been replicated across a majority of the cluster nodes.

Answer

To specify which primary node the write should be directed to.

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To encrypt the data being written to the database.

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To automatically compress the data before it reaches the disk.

Question 10

An application is performing many updates to a document. Which strategy is most effective to avoid fragmentation in the data files?

Accepted Answer

Add padding to the documents to accommodate future growth.. Document growth is a primary cause of fragmentation in MongoDB collections. When a document grows and no longer fits in its current location on disk, it must be moved to a new location. By pre-allocating space or using an embedding strategy that keeps documents within a predictable size range, administrators can reduce the frequency of document moves, thereby minimizing disk fragmentation and maintaining long-term database performance.

Answer

Use the 'compact' command during peak traffic hours.

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Disable the WiredTiger cache periodically.

Answer

Perform a collection scan after every update.

Question 11

Which connection string parameter should be used to ensure an application driver automatically discovers all members of a replica set?

Accepted Answer

replicaSet. The replicaSet parameter is essential for drivers to identify the cluster name and initiate the discovery process. When this parameter is specified, the driver uses the provided seed list to perform a 'isMaster' command against the nodes, allowing it to map the entire topology. Understanding this mechanism is vital for ensuring high availability, as the driver needs to dynamically redirect operations to the new primary during automated failover events.

Answer

maxPoolSize

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wtimeoutMS

Answer

readPreference

Question 12

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.

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The database is out of memory.

Question 13

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.

Answer

primaryPreferred

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secondary

Answer

secondaryPreferred

Question 14

An application is using a capped collection for logging. What happens when the collection reaches its configured size limit?

Accepted Answer

The oldest documents are overwritten by new inserts.. Capped collections follow a 'first-in, first-out' pattern where the oldest documents are automatically overwritten to maintain the specified size. This fixed-size behavior ensures that logging collections do not consume infinite disk space, which is critical for system stability. Application administrators must account for this behavior when designing log management, as it prevents the need for manual cleanup scripts while ensuring the database only retains the most recent relevant operational data for the application.

Answer

The collection automatically expands to accommodate new data.

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The database throws a 'CollectionFull' error for new inserts.

Answer

All existing documents are deleted to free up space.

Question 15

Which mechanism ensures an application receives an error if a write is not persisted to the majority of nodes in a replica set?

Accepted Answer

Setting the write concern to 'w: majority'.. The 'majority' write concern provides a guarantee that the write operation has been committed to a majority of the replica set members. This is the industry standard for ensuring data durability in the event of a primary node failover. DBAs must enforce this setting for critical business data to prevent 'lost updates' where an application assumes a write is permanent when it actually exists only on the primary that is about to crash.

Answer

Setting the read concern to 'majority'.

Answer

Enabling the 'journal' option on all members.

Answer

Using the 'majority' read preference.