The answer is to increase the instance size of the readers to improve apply throughput. This is correct because replica lag during batch processing occurs when the reader nodes cannot keep up with the rate of writes being applied from the oplog; larger reader instances provide more CPU and memory resources to process and apply those writes faster, directly reducing the lag without altering the writer’s batch workload. On the AWS Certified Database Specialty DBS-C01 exam, this scenario tests your understanding of how DocumentDB replication works—specifically that readers apply changes asynchronously from the oplog, and that scaling readers is the targeted fix for apply-side bottlenecks. A common trap is to assume increasing the writer’s size helps, but that only addresses write throughput, not the reader’s apply capacity. Memory tip: “Readers need resources to replay—scale them, not the writer, to slay the lag.”
DBS-C01 Monitoring and Troubleshooting Practice Question
This DBS-C01 practice question tests your understanding of monitoring and troubleshooting. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 with one writer and two readers. The application writes a large amount of data during batch processing, and after a batch completes, the writer's CPU is high, and the readers have significant replica lag. The team wants to reduce replica lag without affecting the batch performance. What should they do?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Increase the instance size of the readers to improve apply throughput
Option D is correct because the replica lag is due to the readers applying writes from the oplog; increasing the reader instance size gives them more CPU and memory to apply writes faster. Option A is incorrect because increasing the writer size does not directly help readers. Option B is incorrect because decreasing the batch size reduces writes but the team wants to maintain performance. Option C is incorrect because changing the storage type does not affect replica lag.
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.
✗
Change the storage type to Provisioned IOPS on all instances
Why it's wrong here
DocumentDB uses a shared storage volume; IOPS affect both writer and readers.
✓
Increase the instance size of the readers to improve apply throughput
Why this is correct
Larger readers can apply oplog entries faster, reducing lag.
Related concept
Static NAT maps one inside address to one outside address.
✗
Reduce the batch size to lower the write rate
Why it's wrong here
This would affect batch processing performance.
✗
Increase the instance size of the writer to handle the batch faster
Why it's wrong here
The writer is already fast enough; the bottleneck is on the readers.
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 media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
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: Increase the instance size of the readers to improve apply throughput — Option D is correct because the replica lag is due to the readers applying writes from the oplog; increasing the reader instance size gives them more CPU and memory to apply writes faster. Option A is incorrect because increasing the writer size does not directly help readers. Option B is incorrect because decreasing the batch size reduces writes but the team wants to maintain performance. Option C is incorrect because changing the storage type does not affect replica lag.
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.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
This DBS-C01 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the DBS-C01 exam.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.