You must predict replica set behavior under failover, partition, and write concern scenarios. The single most important thing: a primary needs majority connectivity to remain primary, and w:1 writes acknowledged but not yet replicated can be rolled back on crash.
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Domain overview
This domain covers MongoDB replica set behavior: how primaries, secondaries, and arbiters interact; how elections and failover work under network partitions; and how write concern and read preference determine durability, consistency, and availability. Questions are scenario-based, asking you to predict outcomes of node failures, partitions, and member configurations.
Exam objectives
Election and failover behavior when a primary loses majority connectivity in a replica set
Member roles: priority 0, hidden, delayed, and arbiter nodes and their election influence
Write concern semantics including w:1, w:majority, and j journaling acknowledgment
Read preference modes such as primary, secondary, and nearest for distributed read routing
Assuming a partitioned primary keeps accepting writes; without majority it steps down and writes fail or roll back.
Confusing priority 0 with hidden; priority 0 can still vote and serve reads unless also hidden.
Believing w:1 guarantees durability; an unacknowledged write can be lost if the primary crashes before replication.
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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?
2Refer to the exhibit. What is the primary function of the node with _id 2 in this replica set?
3When 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?
4In a replica set, what is the 'oplog' and why is it important?
5What occurs when a replica set member experiences a 'rollback'?
6Which THREE factors can significantly impact the time taken for a new primary election?
7What happens to the replica set if a majority of voting members are permanently lost?
8How can you prevent a specific node from becoming a primary in a replica set?
9Which command is used to check the current replication lag status on a secondary node?
10In 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?
11Which member type in a MongoDB replica set is best suited for a geographically distant data center to provide read-only access without influencing elections?
12When 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?
13Which THREE actions occur when a MongoDB secondary node performs an initial sync?
14What is the primary purpose of an Arbiter in a MongoDB replica set?
15What is the primary function of the oplog in a MongoDB replica set?
16Refer to the exhibit. If the primary node fails, which node is most likely to be elected the next primary based on optime?
17What is the result of setting the 'votes' configuration to 0 for a specific node in a replica set?
18When using 'readConcern: majority', what is the specific guarantee provided to the client?
19A 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?
20A 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?
21You 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?
22You 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?
23You 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?
24A 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?
25You 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.)
26You 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.)
27You 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?
28A MongoDB DBA is investigating replication lag on a secondary member named rs0-2. The DBA runs rs.status() and notices that the 'optimeDate' of rs0-2 is several hours behind the primary's 'optimeDate'. The DBA wants to quickly determine the exact time difference. Which field in the rs.status() output should the DBA compare to calculate the replication lag?
You must predict replica set behavior under failover, partition, and write concern scenarios. The single most important thing: a primary needs majority connectivity to remain primary, and w:1 writes acknowledged but not yet replicated can be rolled back on crash.
The Courseiva C100DBA question bank contains 28 questions in the Replication domain. Click any question to see the full explanation and answer breakdown.
Start with a 10-question focused session to identify your baseline accuracy in this domain. Read every explanation — even for questions you answer correctly — to understand the reasoning. Once you score consistently above 80%, move to a 20–30 question session to confirm depth before moving to the next domain.
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