Fixing AWS DMS CDC Lag by Increasing Parallel Apply Threads
A company is migrating a 10 TB Oracle database to Amazon Aurora PostgreSQL. The migration uses AWS DMS with CDC. After the full load, the CDC phase is falling behind by several minutes. The source Oracle database generates 500 MB of redo logs per minute. Which action will most likely improve CDC performance?
Quick Answer
The answer is to increase the number of parallel apply threads on the DMS task. This is correct because when fixing DMS CDC lag, the bottleneck is often the apply phase, where DMS writes changes to the target Aurora PostgreSQL database. By increasing parallel apply threads, you allow DMS to commit multiple transactions concurrently, which directly accelerates the apply rate to match the 500 MB per minute of source redo logs. On the AWS Certified Database Specialty DBS-C01 exam, this question tests your understanding of DMS task tuning under high-volume CDC loads; a common trap is to confuse source-side tuning (like redo log size) with target-side throughput. Remember, redo logs are for Oracle, not for DMS speed. Memory tip: “Parallel applies push past the lag” — when CDC falls behind, think threads, not logs.
⚠ Common exam trap
Watch out — candidates often confuse batch size with parallelism, assuming reducing batch size will speed up apply, when in fact it increases overhead and slows down CDC, while the correct solution is to increase parallelism to match the high redo log generation rate.
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
✓
Increase the number of parallel apply threads on the DMS task
Increasing the number of parallel apply threads on the DMS task allows the target Aurora PostgreSQL database to apply changes concurrently, reducing the bottleneck caused by sequential apply. With 500 MB/min of redo logs, the single-threaded default apply cannot keep pace, so parallel apply directly addresses the lag by distributing the load across multiple threads.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Disable CDC and perform a full load only
Why it's wrong here
This would cause downtime and data loss.
- ✓
Increase the number of parallel apply threads on the DMS task
Why this is correct
More parallel apply threads can increase throughput for applying changes to the target.
- ✗
Reduce the batch size in the DMS task settings
Why it's wrong here
Reducing batch size would decrease throughput, not improve it.
- ✗
Increase the source Oracle redo log size
Why it's wrong here
This does not directly affect DMS's ability to capture changes.
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Same concept, more angles
1 more way this is tested on DBS-C01
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company is migrating a 5 TB Oracle database to Amazon Aurora MySQL using AWS DMS. The full load completes successfully, but the CDC phase is slow and the target Aurora cluster is consuming high CPU. The DMS instance type is dms.c4.2xlarge. Which change will most likely improve CDC performance?
hard- A.Increase the Aurora cluster's max_connections parameter
- ✓ B.Increase the number of parallel apply threads in the DMS task
- C.Increase the DMS task's target storage allocation
- D.Change the Aurora cluster to a larger instance class
Why B: Increasing the number of parallel apply threads in the DMS task allows the CDC phase to apply multiple transactions concurrently to the target Aurora cluster, reducing the backlog of changes. The high CPU on the target suggests the apply process is the bottleneck, not the source or network, and DMS's parallel apply threads directly address this by enabling multi-threaded writes, which improves throughput without requiring larger instances.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.