C100DBA Replication Practice Test 2 — 15 Questions

Question 1

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?

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

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

Answer

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 2

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 3

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.

Answer

Set the new member as hidden to prevent client reads.

Answer

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

Question 4

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.

Answer

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 5

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.

Answer

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

Answer

It acts as a load balancer for incoming read requests.

Question 6

When a secondary node is lagging behind the primary, what is the most significant risk to the replica set if the lag time exceeds the length of the oplog?

Accepted Answer

The secondary will enter a 'RECOVERING' state and require a full resync.. If a secondary falls behind the primary beyond the duration of the oplog, the secondary must perform an initial sync. This involves cloning the entire dataset, which is resource-intensive and can significantly degrade performance on the primary. Managing oplog size is a critical operational task to ensure that transient network issues do not force expensive full data copies across the network.

Answer

The secondary will trigger an automatic election to become primary.

Answer

The primary will automatically reduce its write concern to accommodate the lag.

Answer

The replica set will automatically increase the oplog size to compensate.

Question 7

In a replica set, what is the 'oplog' and why is it important?

Accepted Answer

A capped collection that records all write operations for replication.. The oplog (operations log) is a capped collection that stores all write operations applied to the database. It is the fundamental mechanism for replication in MongoDB. Secondaries read from the primary's oplog and apply the same operations locally to stay in sync. Without the oplog, there would be no way to propagate data changes reliably or recover from temporary periods of secondary node disconnection.

Answer

A full backup of all data for disaster recovery.

Answer

A log of all read operations for performance auditing.

Answer

A temporary cache for incoming client write requests.

Question 8

What occurs when a replica set member experiences a 'rollback'?

Accepted Answer

The node discards data to align with the new primary and saves it to rollback files.. Rollback happens when a node was previously a primary, performed writes that were never replicated to the majority, and then rejoined the set after a new primary was elected. To maintain consistency, the old primary must discard those unacknowledged writes to match the new primary's state. These discarded writes are saved to a local BSON file for manual inspection and potential recovery by administrators.

Answer

The node automatically merges its divergent data with the primary.

Answer

The node is permanently removed from the replica set until manually re-added.

Answer

The node initiates a global election to force a return to its old state.

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

What happens to the replica set if a majority of voting members are permanently lost?

Accepted Answer

The set stops accepting writes to maintain consistency.. A replica set requires a majority of its voting members to be online to elect a new primary and perform write operations with majority concern. If the majority is lost, the remaining nodes enter a read-only state, as they cannot reach consensus. This is a critical failure scenario that requires immediate administrative intervention, such as reconfiguring the set or restoring the failed nodes.

Answer

The set automatically switches to a single-node mode to remain functional.

Answer

The secondary nodes elect a new primary regardless of the missing majority.

Answer

The primary node continues to accept writes for a short period.

Question 11

How can you prevent a specific node from becoming a primary in a replica set?

Accepted Answer

Set the node's 'priority' field to 0.. Setting the 'priority' value to 0 is the standard way to ensure a node never becomes primary. This is commonly done for nodes used for analytical tasks or as disaster recovery targets. It ensures these nodes act solely as passive secondaries, allowing them to serve read requests without ever interfering with the primary election process or causing unexpected failovers.

Answer

Set the node's 'votes' field to 0.

Answer

Set the node's 'hidden' field to true.

Answer

Remove the node from the replica set configuration.

Question 12

Which command is used to check the current replication lag status on a secondary node?

Accepted Answer

rs.printSecondaryReplicationInfo(). The rs.printSecondaryReplicationInfo() command provides a convenient summary of how far behind each secondary is compared to the primary. This is a vital tool for monitoring the health of the replica set and identifying nodes that may be having trouble keeping up with the write volume, which could signal underlying network or hardware performance bottlenecks.

Answer

rs.status()

Answer

rs.printReplicationInfo()

Answer

db.stats()

Question 13

In a three-node replica set with default write concerns, what occurs when the primary node experiences a network partition and cannot communicate with the majority of the set?

Accepted Answer

The primary steps down to secondary status, and the set elects a new primary.. When a primary loses connectivity to the majority of the replica set, it steps down to secondary status to prevent split-brain scenarios. This process ensures data consistency by requiring a majority of nodes to acknowledge writes. The remaining nodes will initiate an election to choose a new primary, maintaining the availability and integrity of the cluster during the temporary partition event.

Answer

The primary continues accepting writes until the partition is resolved.

Answer

The primary immediately crashes and shuts down the mongod process.

Answer

The secondary nodes force a rollback on the primary node.

Question 14

Which member type in a MongoDB replica set is best suited for a geographically distant data center to provide read-only access without influencing elections?

Accepted Answer

Priority 0 Secondary. A priority 0 secondary is ideal for this scenario. By setting the priority to zero, the node is ineligible to become primary, preventing it from participating in elections. This is critical for nodes in remote locations where high latency could disrupt election timing or cause unnecessary re-elections, while still providing low-latency read access to users in that specific geographical region.

Answer

Arbiter

Answer

Hidden Member

Answer

Delayed Secondary

Question 15

When adding a new member to an existing replica set, what is the most common reason for the initial sync process to fail if the data set is extremely large?

Accepted Answer

The oplog size on the primary is smaller than the duration of the sync.. Initial sync often fails for large data sets because the oplog on the primary node is too short. If the syncing node takes longer to copy the initial data than the time it takes for the primary to overwrite the necessary oplog entries, the syncing node will fall too far behind and be forced to restart the process, causing an infinite loop of sync failures.

Answer

The primary node lacks sufficient RAM for the new member.

Answer

The new member's hardware is not identical to the primary.

Answer

The replica set name does not match on the new member.