C100DBA Application Administration Practice Test 3 — 15 Questions

Question 1

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 2

Which MongoDB feature allows an application to receive real-time updates when data changes in a collection?

Accepted Answer

Change Streams. Change Streams provide an event-driven way for applications to react to data modifications in real time without polling the database. By using the aggregation framework, applications can filter for specific events like inserts, updates, or deletes. This is vital for modern microservices architectures, as it allows downstream systems to keep their caches or search indexes synchronized with the primary data store immediately after a write occurs.

Answer

Capped collections

Answer

Database Profiler

Answer

GridFS

Question 3

An application uses a compound index on `{ 'status': 1, 'priority': -1 }`. Which query will efficiently use this index?

Accepted Answer

db.tasks.find({ 'status': 'open' }). Compound indexes are ordered. A query will only use the index if the query filter matches the prefix of the indexed fields. Here, the index starts with 'status'. Therefore, queries filtering on 'status' and optionally 'priority' will be efficient. Understanding index prefixing is crucial for DBAs to ensure that developers write efficient queries that fully leverage the index structures, avoiding unnecessary collection scans and keeping performance stable as the data grows.

Answer

db.tasks.find({ 'priority': 5 })

Answer

db.tasks.find({ 'created': '2023-01-01' })

Answer

db.tasks.find({ 'status': 'open', 'type': 'urgent' })

Question 4

What is the primary function of the 'wtimeout' setting in write concern?

Accepted Answer

It specifies the duration to wait for replica set acknowledgment.. The 'wtimeout' parameter prevents a write operation from blocking indefinitely if the specified replica set members cannot acknowledge the write. This is a critical safety mechanism for application availability. If a network partition occurs or a node goes offline, the application will receive an error after the timeout, allowing it to handle the failure gracefully instead of hanging the application thread and exhausting the connection pool, which could lead to cascading service outages.

Answer

It sets the maximum time a query can run before being killed.

Answer

It limits the number of write retries in the driver.

Answer

It defines the connection timeout for new sessions.

Question 5

Which read concern provides the strongest consistency guarantee by ensuring the data returned has been acknowledged by a majority of the replica set?

Accepted Answer

majority. The 'majority' read concern guarantees that the data read has been committed to a majority of the replica set members, ensuring that the read will not be rolled back after a failover. This is essential for applications requiring strong consistency, such as financial systems. DBAs must guide developers on when to trade off performance for this level of consistency to ensure the application behaves correctly under all possible cluster failure scenarios.

Answer

local

Answer

available

Answer

snapshot

Question 6

An application needs to perform a series of reads and writes across multiple documents in two collections atomically. The deployment is a replica set running MongoDB 4.2. Which feature should the application use?

Accepted Answer

Multi-document transactions. Multi-document transactions in MongoDB 4.2 on replica sets provide ACID guarantees across multiple documents and collections. They are the only feature listed that ensures a series of reads and writes across two collections are applied atomically. The other options are storage or streaming features that do not offer transactional atomicity.

Answer

Change streams with resume tokens

Answer

Capped collections with tailable cursors

Answer

GridFS for storing large documents

Question 7

A development team is building a Node.js application that connects to a MongoDB replica set. They want the driver to automatically retry certain write operations once if they fail due to a transient network error, without requiring custom retry logic in their code. Which connection string option should they enable?

Accepted Answer

retryWrites=true. The retryWrites=true connection string option instructs the MongoDB driver to automatically retry supported write operations once if they fail with a retryable error. This offloads retry logic from the application code and is the standard way to handle transient network issues in replica sets. Other options affect write concern, read routing, or connection timeouts, but none provide automatic write retries.

Answer

w=majority

Answer

readPreference=primaryPreferred

Answer

connectTimeoutMS=30000

Question 8

A financial application must ensure that once a transaction is committed, the data will never be rolled back, even if the primary node crashes immediately after the write is acknowledged. Which write concern should be used?

Accepted Answer

{ w: "majority" }. The requirement is that committed data must never be rolled back, even if the primary crashes immediately after acknowledgment. Majority write concern guarantees that the write is acknowledged by a majority of voting members, so any newly elected primary will have the write. This prevents rollback during failover. Other write concerns either provide weaker guarantees or no acknowledgment at all.

Answer

{ w: 0 }

Answer

{ w: 2 }

Answer

{ w: 1 }

Question 9

A MongoDB DBA is configuring a new application to connect to a sharded cluster. The application requires that reads are distributed across shards and that the driver automatically routes queries to the appropriate shard based on the shard key. Which component must be included in the connection string to enable this routing?

Accepted Answer

The mongos router hostname and port.. In a sharded cluster, applications must connect to mongos routers, which are responsible for routing queries to the appropriate shards based on the shard key. The connection string should include the hostnames and ports of mongos instances. Connecting directly to config servers or shards bypasses the routing layer and is not supported for application traffic.

Answer

The config server replica set hostnames and ports.

Answer

The replica set name of the shards.

Answer

The shard replica set hostnames and ports.

Question 10

An application uses a MongoDB driver that supports retryable writes. A write operation is sent to the primary, which applies the write but crashes before sending the acknowledgment. The driver receives a network error. What will the driver do by default?

Accepted Answer

Retry the write once, and the server will deduplicate it using the transaction number.. Retryable writes automatically retry a write operation once on retryable errors. The driver uses a transaction number and session ID so the server can deduplicate the retry, ensuring the write is applied exactly once. The driver does not retry indefinitely, does not skip retrying, and does not alter the write concern. The correct behavior is a single retry with deduplication.

Answer

Retry the write indefinitely until it succeeds.

Answer

Retry the write but with write concern 0 to avoid another failure.

Answer

Return the error to the application without retrying.

Question 11

A MongoDB application requires that reads never see data that could be rolled back, even in the event of a replica set failover. The application also needs to read from secondary nodes to distribute load. Which read concern should be used with a secondary read preference?

Accepted Answer

"majority". The application needs to read from secondaries while ensuring that the data returned is not subject to rollback. "majority" read concern guarantees that the data has been acknowledged by a majority of replica set members, making it durable across failover. It is compatible with secondary read preferences, unlike "linearizable", which requires primary reads. Therefore, "majority" is the correct choice.

Answer

"linearizable"

Answer

"available"

Answer

"local"

Question 12

A financial application requires that read operations never return data that could later be rolled back, even if it means reading from a secondary. The application connects to a replica set with readPreference=secondary. Which read concern level should be used to guarantee that the data read has been acknowledged by a majority of replica set members and cannot be rolled back?

Accepted Answer

readConcern: { level: "majority" }. majority read concern guarantees that the data read has been acknowledged by a majority of replica set members, making it durable and not subject to rollback. It can be used with readPreference=secondary, allowing the application to read from secondaries while still ensuring that the data is majority-committed. linearizable is stronger but only works with primary reads.

Answer

readConcern: { level: "local" }

Answer

readConcern: { level: "available" }

Answer

readConcern: { level: "linearizable" }

Question 13

A developer is using the MongoDB Node.js driver and wants to ensure that a write operation is acknowledged by a majority of replica set members and is also written to the on-disk journal before returning success. Which write concern should be specified?

Accepted Answer

{ w: "majority", j: true }. The developer needs both majority acknowledgment and journaling. The write concern { w: "majority", j: true } ensures that a majority of replica set members have applied the write and that it has been written to the on-disk journal on those members. This provides the strongest durability guarantee, preventing data loss even if the primary crashes. Other options either lack majority acknowledgment or journaling.

Answer

{ w: 2, j: true }

Answer

{ w: 1, j: true }

Answer

{ w: "majority", j: false }

Question 14

A database administrator wants to configure a read preference that directs all read operations to the primary node of a replica set, ensuring the most up-to-date data. Which read preference mode should be used?

Accepted Answer

primary. The requirement is to always read the most up-to-date data, which is only guaranteed when reading from the primary. The "primary" read preference ensures that all reads are directed to the primary node. Other read preferences may read from secondaries, which can have stale data, or may fall back to secondaries if the primary is unavailable, violating the requirement.

Answer

primaryPreferred

Answer

nearest

Answer

secondary

Question 15

A database administrator is configuring a MongoDB replica set. The application requires that writes are acknowledged by a majority of voting members before being considered successful, and that the write is also written to the on-disk journal before acknowledgment. Which write concern specification achieves this?

Accepted Answer

{ w: "majority", j: true }. The write concern { w: "majority", j: true } ensures that the write is acknowledged by a majority of voting members and that it is written to the on-disk journal before acknowledgment. This provides both durability and high availability. The other options either lack journaling or lack majority acknowledgment, or both.

Answer

{ w: 1, j: true }

Answer

{ w: "majority", j: false, wtimeout: 5000 }

Answer

{ w: "majority", j: false }