The answer is that one of the replicas is down or experiencing high replication lag. When a write concern is set to 'majority' in a DocumentDB cluster with one primary and two replicas, the primary must receive an acknowledgment from at least two nodes (itself plus one replica) before confirming the write. If a replica is unavailable or lagging too far behind, the majority cannot be formed, causing the WriteConcernError. On the AWS Certified Database Specialty DBS-C01 exam, this scenario tests your understanding of how replication and write acknowledgment interact—a common trap is assuming the primary is overloaded or that read preference settings affect writes, but neither is relevant here. Remember that write concern is about durability and acknowledgment, not performance. A useful memory tip is “Majority needs more than half alive”—if one of three nodes is down, you lose the majority, and writes will fail.
DBS-C01 Monitoring and Troubleshooting Practice Question
This DBS-C01 practice question tests your understanding of monitoring and troubleshooting. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A company is running a MongoDB-compatible Amazon DocumentDB cluster. The application team reports that write operations are failing intermittently with a `WriteConcernError` indicating that the write concern could not be satisfied. The cluster has one primary and two replicas. What is the MOST likely cause of this issue?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Clue: "primary"
Why it matters: Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.
One of the replicas is down or experiencing high replication lag
If a replica is down, the write concern 'majority' cannot be satisfied because only the primary and one replica are available, but majority may require two replicas depending on configuration.
B
The cluster does not have enough replica instances to satisfy the write concern
Why wrong: With 3 nodes, a majority write concern requires 2 nodes, which is possible if all are healthy.
C
The application is using an incorrect read preference
Why wrong: Read preference affects reads, not writes.
D
The primary instance is overloaded with read requests
Why wrong: Read load does not affect write concern.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
One of the replicas is down or experiencing high replication lag
Option D is correct because if the write concern is set to 'majority' and a replica is down or lagging, the write cannot be acknowledged. Option A is wrong because the primary is not overloaded; the issue is write concern. Option B is wrong because read preference does not affect writes. Option C is wrong because the number of replicas is sufficient for majority if all are healthy.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
One of the replicas is down or experiencing high replication lag
Why this is correct
If a replica is down, the write concern 'majority' cannot be satisfied because only the primary and one replica are available, but majority may require two replicas depending on configuration.
Clue confirmation
The clue words "most likely", "primary" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
✗
The cluster does not have enough replica instances to satisfy the write concern
Why it's wrong here
With 3 nodes, a majority write concern requires 2 nodes, which is possible if all are healthy.
✗
The application is using an incorrect read preference
Why it's wrong here
Read preference affects reads, not writes.
✗
The primary instance is overloaded with read requests
Why it's wrong here
Read load does not affect write concern.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
A cloud solutions architect for a retail company is evaluating services for a new workload. The correct answer here reflects best practice for the specific scenario described — not a general cloud recommendation. NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated. Cloud exam questions reward reading the constraint carefully: the same technology can be right or wrong depending on the use case.
What to study next
Got this wrong? Here's your next step.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related DBS-C01 NAT questions on configuration and troubleshooting.
Monitoring and Troubleshooting — This question tests Monitoring and Troubleshooting — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: One of the replicas is down or experiencing high replication lag — Option D is correct because if the write concern is set to 'majority' and a replica is down or lagging, the write cannot be acknowledged. Option A is wrong because the primary is not overloaded; the issue is write concern. Option B is wrong because read preference does not affect writes. Option C is wrong because the number of replicas is sufficient for majority if all are healthy.
What should I do if I get this DBS-C01 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related DBS-C01 NAT questions on configuration and troubleshooting.
Are there clue words in this question I should notice?
Yes — watch for: "most likely", "primary". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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