PCDE Migrate data solutions Practice Question
A team is migrating a 10 TB PostgreSQL database to AlloyDB using DMS with continuous CDC. The migration starts, but the CDC phase is falling behind, with lag increasing over time. The source is a busy production database with high write throughput. What is the most effective action to reduce lag?
⚠ Common exam trap
In Google Cloud exams, a common trap is to think that scaling DMS worker nodes (Option C) always fixes CDC lag, but the bottleneck in high-write environments is typically the source's ability to decode WAL, not DMS's processing capacity.
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 source database's resources (CPU/memory) to handle logical replication load.
Increasing the source database's resources (CPU/memory) directly addresses the root cause of CDC lag in a high-write-throughput PostgreSQL environment. DMS logical replication relies on the source's ability to decode WAL (Write-Ahead Log) records quickly; if the source is CPU- or memory-bound, it cannot keep up with the rate of changes, causing lag to grow. Scaling up the source reduces the bottleneck in WAL generation and decoding, allowing DMS to consume changes faster.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Reduce the batch size for the CDC phase.
Why it's wrong here
Not a configurable parameter in DMS.
- ✓
Increase the source database's resources (CPU/memory) to handle logical replication load.
Why this is correct
The source may be under-resourced to publish changes fast enough.
- ✗
Increase the number of DMS worker nodes.
Why it's wrong here
DMS worker nodes are auto-scaled; not directly configurable.
- ✗
Add more indexes to the target AlloyDB tables.
Why it's wrong here
Indexes slow down writes, making lag worse.
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Same concept, more angles
1 more way this is tested on PCDE
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 using DMS to migrate a MySQL database to Cloud SQL with continuous replication. During the CDC phase, the replication lag is increasing and not catching up. The source database is heavily used for OLTP workloads. Which action would most likely reduce the replication lag?
medium- A.Increase the size of the Cloud SQL destination instance.
- B.Increase the binary log retention period on the source.
- C.Enable parallel replication on DMS.
- ✓ D.Reduce the number of concurrent write operations on the source database.
Why D: Increasing the source database's binary log retention period does not reduce lag; it retains more logs. Reducing concurrent writes on source directly slows the rate of change, allowing DMS to catch up.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This PCDE practice question is part of Courseiva's free Google Cloud 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 PCDE exam.