C100DBA Replication Practice Test 3 — 15 Questions

Question 1

Which THREE actions occur when a MongoDB secondary node performs an initial sync?

Accepted Answer

It copies all documents from each collection on the primary.. During initial sync, the node first clones all databases from the primary, then applies all changes recorded in the oplog that occurred during the cloning phase, and finally builds all necessary indexes for the collections. This multi-stage process ensures the new member reaches a consistent state with the primary without requiring an outage or downtime for the existing cluster members.

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It immediately becomes the primary to speed up the process.

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It automatically purges all data from the primary node.

Question 2

What is the primary purpose of an Arbiter in a MongoDB replica set?

Accepted Answer

To participate in elections and help achieve a majority.. An arbiter's sole purpose is to participate in elections to help reach a majority vote without storing any data. By not storing data, it remains lightweight and requires minimal hardware resources, providing a cost-effective way to fulfill the voting requirements of a replica set in scenarios where an extra full-data node is not needed for capacity or redundancy.

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To serve as a standby node for automatic failover.

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To provide read-only access to the database.

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To replicate data from the primary to secondary nodes.

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.

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To store user credentials and authentication data.

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To cache frequently accessed query results in memory.

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To perform automatic backups for disaster recovery.

Question 4

Refer to the exhibit. If the primary node fails, which node is most likely to be elected the next primary based on optime?

Accepted Answer

Member[2] will likely be elected because it is more up-to-date.. Elections prioritize the node with the most up-to-date data. The optime (operation time) represents the latest operation applied by the node. Member[2] is the most advanced secondary with a timestamp of 95, compared to Member[1] at 90. Therefore, Member[2] has the highest probability of winning the election, ensuring the new primary has the most current data available in the set.

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Member[1] will always be elected.

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No node can be elected because the timestamps are all different.

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The election will trigger a full re-sync for all nodes.

Question 5

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.

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The node will no longer replicate data from the primary.

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The node will be removed from the replica set configuration.

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The node will automatically become the primary.

Question 6

When using 'readConcern: majority', what is the specific guarantee provided to the client?

Accepted Answer

It guarantees that the data read has been committed to a majority of nodes.. The 'majority' read concern guarantees that the data returned has been acknowledged by a majority of the replica set members. This ensures that the data is durable and will not be rolled back in the event of a primary failover. It provides a higher level of consistency for read operations, which is critical for applications that require strict data integrity and visibility.

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It guarantees that the read will never return stale data.

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It forces the read to always come from the primary node.

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It increases the speed of queries by ignoring consistency.

Question 7

A three-member replica set has default settings, and all three members are healthy. A DBA needs to perform an index build on the primary during business hours. Which concern should the DBA consider regarding replica set behavior?

Accepted Answer

The index build will be applied to all data-bearing members, and the primary waits for secondaries to finish before committing the index.. In MongoDB 4.4 and later, index builds on replica sets are coordinated: the primary initiates the build, secondaries build concurrently, and the primary waits for all data-bearing secondaries to finish before committing. This ensures index consistency across the set. The operation can be resource-intensive and may impact replication lag temporarily, but it does not roll back or step down due to lag.

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The index build will cause the primary to step down if any secondary has a replication lag greater than 10 seconds.

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The index build will only be performed on the primary, and secondaries will build the index lazily during their next election.

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The index build will be rolled back on the secondaries if any secondary falls behind.

Question 8

A DBA is troubleshooting a replica set where the secondary is consistently lagging behind the primary by several hours. The DBA notices that the secondary's optime is far behind the primary's optime, and the secondary's oplog window is only 1 hour. The primary has a heavy write workload. Which action should the DBA take to reduce the replication lag and prevent the secondary from falling off the oplog?

Accepted Answer

Scale the secondary vertically by adding more CPU, RAM, or faster disk I/O.. Replication lag is typically caused by the secondary's inability to apply oplog entries as quickly as the primary generates them. Improving the secondary's hardware resources—CPU, RAM, and especially disk I/O—increases its oplog application throughput, directly reducing lag. Increasing oplog size only buys time, and write concern does not affect replication speed.

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Convert the secondary to an arbiter to reduce its workload.

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Increase the write concern on the primary to ensure the secondary acknowledges writes.

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Increase the size of the oplog on the primary and secondaries.

Question 9

You are the DBA for a three-member MongoDB replica set. The primary becomes unreachable due to a network partition, and two secondaries remain in communication. After the election timeout, one secondary is elected primary. What is the required condition for a new primary to be elected?

Accepted Answer

A majority of voting members must be available and able to communicate.. MongoDB replica sets require a majority of voting members to elect a primary. In a three-member set, two members must be available. The elected primary must have the highest optime among the majority to ensure data consistency. This mechanism prevents split-brain scenarios and ensures that only one primary can accept writes at a time.

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Any secondary with the highest priority automatically becomes primary without needing a majority.

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The arbiter, if present, must vote and can be the sole voter to elect a primary.

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The secondary with the most recent oplog entry is elected primary even if it is the only member available.

Question 10

You are the DBA for a MongoDB 6.0 replica set with three voting members: rs0-primary (priority 2), rs0-secondary-1 (priority 1), and rs0-secondary-2 (priority 1). The primary fails and is unavailable for an extended period. You need to ensure that rs0-secondary-1 is elected as the new primary instead of rs0-secondary-2. Which action should you take before the primary fails?

Accepted Answer

Set rs0-secondary-1's priority to 2 and rs0-secondary-2's priority to 0.5 using rs.reconfig().. Election priority determines which eligible secondary is most likely to win an election. By setting rs0-secondary-1's priority higher than rs0-secondary-2's, you make it the preferred candidate. Lowering rs0-secondary-2's priority to a non-zero value keeps it as a voting member but reduces its election chances. This approach is the standard way to control primary election outcomes in a MongoDB replica set without making a member unelectable.

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Set rs0-secondary-1's hidden property to true and rs0-secondary-2's hidden property to false using rs.reconfig().

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Set rs0-secondary-1's slaveDelay to 0 and rs0-secondary-2's slaveDelay to 3600 using rs.reconfig().

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Set rs0-secondary-1's votes to 1 and rs0-secondary-2's votes to 0 using rs.reconfig().

Question 11

You are a DBA for a MongoDB replica set. A secondary node has been down for several days and is far behind the primary. The oplog on the primary has wrapped, and the secondary's last oplog entry is no longer present. What must you do to bring the secondary back into sync?

Accepted Answer

Perform a full resync by stopping the secondary, deleting its data files, and restarting it.. If a secondary's last oplog entry is no longer in the primary's oplog, the secondary cannot catch up incrementally. A full resync is required, which means deleting the secondary's data files and restarting it to perform an initial sync. This process copies all data from the primary or another sync source.

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Increase the oplog size on the primary to prevent wrapping and then restart the secondary.

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Restart the secondary with the --resync option to force a full resync.

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Run rs.syncFrom() on the secondary to point it to a different sync source.

Question 12

A DBA is adding a new secondary to an existing replica set. The data set is 2 TB, and the network bandwidth between the new member and the primary is 100 Mbps. The DBA wants to minimize the impact on the primary during the initial sync. Which approach should the DBA take?

Accepted Answer

Seed the new member by copying the data files from an existing secondary while it is offline, then start it with the same replica set configuration.. Seeding a new member by copying data files from an existing secondary (taken offline) is an efficient way to add a member without burdening the primary with a full initial sync over a slow network. This method leverages local copy speed and avoids consuming primary resources, which is critical when network bandwidth is limited.

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Configure the new member as a hidden member to reduce its replication load.

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Increase the oplog size on the primary to accommodate the initial sync.

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Set the new member's priority to 0 during the initial sync to prevent it from becoming primary.

Question 13

You are configuring a MongoDB replica set for a global application. You need to add a member that will never become primary and will be invisible to client applications. Which two steps must you take when adding this member? (Choose two.)

Accepted Answer

Set its priority to 0.. To create a member that never becomes primary and is invisible to clients, you must set both priority to 0 and hidden to true. Priority 0 ensures it cannot be elected primary, while hidden true prevents client visibility and use as a sync source. These two settings together meet the requirements.

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Set its votes to 0.

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Set its slaveDelay to a large value.

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Set its buildIndexes to false.

Question 14

You are administering a MongoDB 6.0 replica set with three members: a primary and two secondaries. You need to perform a rolling maintenance on the secondaries, which involves restarting each secondary one at a time. Which two actions must you take to ensure that the replica set remains available for writes during the maintenance? (Choose two.)

Accepted Answer

Restart the secondaries one at a time, waiting for each to fully rejoin and catch up before proceeding to the next.. To keep a replica set writable during rolling maintenance on secondaries, you must ensure the primary retains a majority of voting members. In a three-member set, this means keeping the primary and at least one secondary online. Restarting secondaries one at a time and waiting for each to catch up before moving to the next maintains majority and data consistency. This approach allows the primary to continue accepting writes without interruption, provided no other failures occur.

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Disable authentication on the replica set to simplify the restart process.

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Set the priority of the primary to 0 to prevent it from being re-elected during the maintenance.

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Use rs.stepDown() to force the primary to step down before restarting any secondary.

Question 15

You are a DBA for a MongoDB replica set. A developer asks you to explain what the oplog is and why it is important. What is the correct description?

Accepted Answer

The oplog is a capped collection that records all write operations and is used by secondaries to replicate data from the primary.. The oplog is a capped collection in the local database that records all write operations. Secondaries replicate by reading the oplog and applying the operations. This mechanism ensures that all members eventually have the same data, enabling high availability and failover.

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The oplog is a temporary file used during initial sync to transfer data from the primary to a new secondary.

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The oplog is a cache that stores frequently accessed documents to improve read performance.

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The oplog is a log file on disk that records all queries and read operations for auditing purposes.