C100DBA Replication — Timed 20 Questions

Question 1

Which TWO of the following conditions must be met for a replica set member to be eligible to become a primary?

Accepted Answer

The member must have a priority value greater than zero.. Election eligibility is strictly governed by the member's current state and its visibility to the majority of the replica set. A member must be caught up with the oplog to ensure minimal rollback risk and must maintain connectivity to a majority of the set to prevent split-brain scenarios. Understanding these criteria is essential for troubleshooting why a specific node fails to transition to primary during failover events.

Answer

The member must be the node with the highest amount of storage allocated.

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The member must have at least 50% of the total disk space available.

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The member must be running a different OS than the current primary.

Question 2

Which setting should be used in a connection string to ensure that an application only reads from a node that has data at least as fresh as the primary?

Accepted Answer

causalConsistency=true. Causal consistency ensures that read operations reflect the effects of all preceding write operations within the same session. By enabling causal consistency and using appropriate read concerns, applications guarantee that their view of the data is consistent and never reverts to an older state, preventing the common issue of reading stale data after an immediate write operation.

Answer

readPreference=nearest

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readConcern=majority

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writeConcern=majority

Question 3

Which THREE of the following are valid methods to add a new member to an existing replica set?

Accepted Answer

Use the rs.add() method in the mongosh shell.. Adding nodes is a standard administrative task that must be done carefully to avoid disrupting existing replication. Whether using the shell, updating the configuration object, or automating via infrastructure code, the core logic is the same: the configuration object must be updated and pushed to the current primary to propagate the change to the rest of the nodes.

Answer

Manually edit the data files in the /data/db directory.

Answer

Use the rs.initiate() method on the new node.

Question 4

Refer to the exhibit. If node 0 (the current primary) goes offline, which node will be elected as the new primary?

Accepted Answer

Node 1. Replica set elections are influenced by the priority value. Nodes with higher priority are preferred candidates. When the current primary fails, the node with the highest priority among the available nodes will win the election. If multiple nodes have the same highest priority, the one with the most recent oplog data wins, ensuring that the most capable and up-to-date node takes over.

Answer

Node 2

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Both Node 1 and Node 2 will share the primary role.

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The cluster will remain without a primary until Node 0 is repaired.

Question 5

What is the primary purpose of the 'vote' field in the replica set configuration?

Accepted Answer

To specify if the node should participate in the election process.. The 'votes' field allows administrators to control how many votes a member contributes to an election. By default, all members have 1 vote. Setting this to 0 can be useful in specific configurations, such as preventing a geographically distant node from impacting election outcomes while still allowing it to maintain a copy of the data for local read scaling.

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To determine the node's priority in an election.

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To define the read preference of the client connection.

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To set the maximum number of concurrent connections for the node.

Question 6

Which of the following best describes the 'hidden' member status in a replica set?

Accepted Answer

It does not receive client read requests from the driver.. A hidden member remains part of the replica set and keeps its data synced with the primary, but it is invisible to standard client connections. This is ideal for hosting nodes dedicated to specific tasks, such as long-running analytical queries or backups, without them impacting the production traffic routed by the drivers. It allows for infrastructure separation within the same set.

Answer

It stops replicating data from the primary to save bandwidth.

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It is invisible to the primary and does not participate in any replication.

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It is prohibited from participating in replica set elections.

Question 7

Which TWO of the following configurations are required to ensure that a replica set can survive the loss of a single node without manual intervention?

Accepted Answer

A minimum of three voting nodes in the replica set.. A replica set must have at least three voting nodes to survive the loss of one member. With three nodes, a majority is two, allowing an election to proceed if one node fails. Additionally, all nodes must have a priority greater than zero to be eligible for election, ensuring that the remaining nodes can successfully promote a new primary node.

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All nodes configured with priority 0.

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Enabling the 'delayed' attribute on all members.

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Setting all nodes to 'hidden' status.

Question 8

Refer to the exhibit. If the primary node crashes, what is the expected outcome of the election process?

Accepted Answer

One of the remaining nodes will be elected primary.. The election process will succeed because all nodes are fully synchronized, as indicated by identical optime values. The remaining secondary nodes will hold an election, and since both have the latest data, one will be elected primary based on the internal priority configuration. This illustrates that a healthy, synchronized set transitions seamlessly, minimizing downtime while ensuring all nodes agree on the state of the data before electing a successor.

Answer

The election will fail because the majority cannot be reached.

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The set will enter a read-only state indefinitely.

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The secondaries will initiate a rollback before electing a primary.

Question 9

Refer to the exhibit. If srv2 goes down, what happens to the primary election?

Accepted Answer

srv1 will be elected as the new primary.. With srv2 (priority 2) offline, the set still has two members. srv1 has priority 1, making it eligible for election. Since srv1 is a valid voting member, it will be elected as the new primary. Even though srv3 has priority 0, the cluster can still function with srv1 because it meets the majority voting requirement and has a priority greater than zero.

Answer

The set becomes read-only because srv2 was the only node allowed to be primary.

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srv3 will be elected as the new primary.

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The election will fail, and the replica set will crash.

Question 10

Which setting controls how long a secondary node will wait before attempting to reconnect to a primary that has become unreachable?

Accepted Answer

electionTimeoutMillis. The `heartbeatIntervalMillis` and `electionTimeoutMillis` settings work together to define cluster health and failure detection. If a node does not receive a heartbeat within the timeout period, it assumes the primary is down. This mechanism is vital for tuning the balance between fast failover and avoiding unnecessary elections in networks with occasional latency spikes or congestion.

Answer

oplogSizeMB

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replSetName

Answer

wiredTigerCacheSizeGB

Question 11

Which command is used to view the current replication configuration and status in a MongoDB shell?

Accepted Answer

rs.status(). The rs.conf() command displays the replica set configuration, while rs.status() provides real-time information about the health and state of each member. These commands are essential tools for any MongoDB administrator, as they allow for quick verification of the cluster's topology, the identity of the current primary, and the sync status of all secondary members in the replica set.

Answer

db.listCollections()

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db.serverStatus()

Answer

rs.printReplicationInfo()

Question 12

A DBA manages a replica set with members in two data centers: Site A (primary and one secondary) and Site B (one secondary). The DBA wants to ensure that if Site A becomes completely unavailable, the remaining member in Site B does not become primary and accept writes, because the application requires strong consistency. Which replica set configuration setting should the DBA adjust?

Accepted Answer

Configure the Site B member as hidden.. Configuring the Site B member as hidden sets its priority to 0 and prevents it from becoming primary, while still allowing it to replicate data and participate in elections as a non-voting member if configured. This ensures that during a Site A outage, the remaining member cannot accept writes, preserving the strong consistency requirement.

Answer

Set the priority of the Site B member to 0.

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Set the write concern for the replica set to majority.

Answer

Set the votes of the Site B member to 0.

Question 13

A DBA is configuring a replica set for a new application that requires high availability and the ability to perform maintenance on secondaries without impacting the primary. The DBA needs to ensure that the replica set can continue to accept writes if the primary becomes unavailable. Which two configuration settings or practices should the DBA implement? (Choose two.)

Accepted Answer

Ensure that the majority of voting members are always available for election.. To ensure a replica set can continue accepting writes after a primary failure, you need at least three data-bearing voting members so a majority can elect a new primary, and you must ensure that a majority of voting members are available. These two practices enable automatic failover and maintain write availability.

Answer

Configure a delayed secondary to allow point-in-time recovery.

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Set the write concern to w:1 to improve write performance during failover.

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Set the priority of all secondaries to 0 to prevent them from becoming primary.

Question 14

You are managing a MongoDB replica set with members in two data centers: Site A (primary and one secondary) and Site B (one secondary). You want to ensure that if Site A fails entirely, the secondary in Site B can become primary and accept writes. What should you do?

Accepted Answer

Add a third data-bearing member in Site B to create a majority in that site.. To allow a secondary in Site B to become primary when Site A fails, the replica set must have a majority of voting members in Site B. Adding a third data-bearing member in Site B achieves this. The two members in Site B then form a majority of the three voting members and can elect a primary, ensuring writes can continue.

Answer

Configure the secondary in Site B with a higher priority than the primary in Site A.

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Add an arbiter in Site B to provide an additional vote.

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Set the secondary in Site B to have a vote of 0 and rely on the primary in Site A to remain available.

Question 15

You are a DBA for a MongoDB 6.0 replica set. You need to add a new member to the replica set, but you want to minimize the impact on the primary's performance during the initial sync. Which approach should you take?

Accepted Answer

Add the new member with { priority: 0, votes: 0 } and allow MongoDB to automatically select a sync source, which will prefer a secondary if one is available and up-to-date.. To minimize impact on the primary during initial sync, you should let MongoDB choose a sync source automatically. MongoDB's sync source selection algorithm prefers a secondary that is up-to-date and has low network latency, which reduces the load on the primary. Configuring the new member with priority 0 and votes 0 is also recommended to prevent it from being elected primary or influencing elections before it is fully synchronized. This combination ensures a smooth addition without degrading primary performance.

Answer

Add the new member with { priority: 0, votes: 0 } and set its buildIndexes to false to speed up the initial sync.

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Add the new member with { priority: 0, votes: 0 } and then initiate the initial sync during off-peak hours.

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Add the new member with { priority: 0, votes: 0 } and use a secondary as the sync source by setting the new member's syncSourceHost to that secondary.

Question 16

A three-member MongoDB replica set (rs0) has members mongodb-1 (priority 1, votes 1), mongodb-2 (priority 0.5, votes 1), and mongodb-3 (priority 0, votes 1). mongodb-3 is configured as a hidden member with votes 1. The primary mongodb-1 becomes unresponsive due to a network partition. Which member will be elected as the new primary?

Accepted Answer

mongodb-2. The election process chooses the eligible member with the highest priority. mongodb-2 has priority 0.5 while mongodb-3 has priority 0 but is hidden, making it ineligible. Since a majority of voting members (mongodb-2 and mongodb-3) are available, mongodb-2 becomes the new primary.

Answer

mongodb-3

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No new primary will be elected because the remaining members cannot form a majority.

Answer

mongodb-1 will remain primary until it is manually stepped down.

Question 17

You are monitoring a MongoDB 6.0 replica set and notice that a secondary member is significantly behind the primary. You need to determine the replication lag. Which command should you use to check the replication lag on the secondary?

Accepted Answer

rs.status(). The rs.status() command provides a comprehensive view of the replica set, including each member's state, optime, and heartbeat information. By comparing the optime of the secondary with that of the primary, you can determine the replication lag. This command can be run on any member and is the most direct way to assess lag and overall replication health. It is the standard tool for DBAs to monitor replication status.

Answer

rs.printSlaveReplicationInfo()

Answer

db.serverStatus().repl

Answer

db.printReplicationInfo()

Question 18

A MongoDB DBA is planning to add a new member to an existing replica set. The new member will be in a remote data center with higher network latency. The DBA wants to ensure the new member can be added successfully and does not negatively impact the existing replica set. Which two actions should the DBA take before adding the new member? (Choose two.)

Accepted Answer

Ensure the new member has sufficient disk space and network bandwidth for the initial sync.. Before adding a new member in a remote data center, the DBA should set its priority to 0 to prevent it from becoming primary, which could introduce write latency. Additionally, the DBA must ensure the new member has sufficient disk space and network bandwidth to perform the initial sync without issues.

Answer

Configure the new member with a lower priority than the existing members.

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Set the new member as hidden to prevent client reads.

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Set the new member's votes to 0 to prevent it from participating in elections.

Question 19

In a three-node replica set with default write concern w:1, what happens to an unacknowledged write if the primary crashes immediately after receiving the write command but before replicating it to secondaries?

Accepted Answer

The write is lost, and the new primary will not contain the data.. With w:1, the primary acknowledges the write as soon as it is applied to its local oplog. If the primary fails before replication, the new primary elected from the remaining nodes will not contain this write. This demonstrates the critical trade-off between performance and durability. Clients must use w:'majority' to ensure data consistency during failover events, as single-node acknowledgments are insufficient to guarantee persistence against primary loss.

Answer

The write is automatically rolled back and re-applied when the old primary recovers.

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The write remains in the local storage of the crashed node and becomes visible upon restart.

Answer

The replica set triggers an automatic consistency recovery to force the write to other nodes.

Question 20

Refer to the exhibit. What is the primary function of the node with _id 2 in this replica set?

Accepted Answer

It provides a vote during elections to help maintain a majority.. An arbiter exists solely to participate in elections and break ties. It does not replicate data or serve read requests, making it a low-resource way to achieve a majority vote in a set. This is crucial for environments where a full-data secondary is cost-prohibitive but high availability is still required to maintain quorum during network partitions.

Answer

It stores a copy of the data and can serve read queries.

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It performs automatic failover by hosting a redundant copy of the oplog.

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It acts as a load balancer for incoming read requests.