C100DBA Application Administration Practice Question
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?
⚠ Common exam trap
The trap here is assuming that any acknowledgment from a secondary (such as w:2) is enough to prevent rollback, but only majority write concern ensures that a majority of voting members have the write, which is necessary to prevent rollback during failover.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
{ 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 analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
{ w: 0 }
Why it's wrong here
A write concern of { w: 0 } is unacknowledged, meaning the driver does not wait for any acknowledgment from the server. This offers no durability guarantee and can result in lost writes if the primary fails. It is the opposite of what the financial application requires, as it provides no assurance against rollback.
- ✓
{ w: "majority" }
Why this is correct
A write concern of { w: "majority" } ensures that the write is acknowledged by a majority of voting members in the replica set. This means the write is durable across failover because any new primary must be one of the members that acknowledged the write. Therefore, the write cannot be rolled back, satisfying the application's requirement.
- ✗
{ w: 2 }
Why it's wrong here
A write concern of { w: 2 } requires acknowledgment from the primary and one secondary. While this provides some durability, it does not guarantee that a majority of voting members have acknowledged the write. In a three-node replica set, if the primary and one secondary acknowledge but both fail, the write could still be rolled back. Thus, it is not as robust as majority write concern.
- ✗
{ w: 1 }
Why it's wrong here
A write concern of { w: 1 } acknowledges the write after it is applied to the primary only. If the primary crashes before the write replicates to a secondary, the write can be rolled back when a new primary is elected. This does not meet the requirement that committed data never be rolled back, so it is insufficient for the financial application's durability needs.
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Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official MongoDB exam blueprint
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