The answer is to switch from semi-synchronous to asynchronous replication, increase the replica’s machine type, and enable parallel replication threads. Semi-synchronous replication forces the primary to wait for at least one replica to acknowledge each transaction before committing, which directly causes high replication lag under heavy write loads. By switching to asynchronous replication, the primary commits immediately without waiting, while a larger replica machine type provides more CPU and memory to process incoming changes faster, and parallel replication applies multiple transactions concurrently instead of serially. On the Google Professional Cloud Database Engineer exam, this question tests your understanding of MySQL replication modes and Cloud SQL scaling options—a common trap is assuming synchronous replication is always better for consistency, but for lag reduction, asynchronous is the practical choice. Remember the mnemonic “SAP”: Switch to async, Add resources, Parallel apply.
PCDE Monitor and optimize database performance Practice Question
This PCDE practice question tests your understanding of monitor and optimize database performance. 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.
You are managing a Cloud SQL for MySQL instance that is experiencing high replication lag. The instance uses semi-synchronous replication. Which THREE actions could reduce the replication lag?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Change replication mode from semi-synchronous to asynchronous.
Option A is correct because switching from semi-synchronous to asynchronous replication removes the requirement for the primary to wait for at least one replica to acknowledge receipt of each transaction. In semi-synchronous mode, the primary waits for acknowledgment, which can introduce latency and contribute to replication lag under high write loads. Asynchronous replication allows the primary to commit transactions immediately without waiting, reducing the time the primary spends waiting for replica acknowledgment and thus lowering replication lag.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 replication mode from semi-synchronous to asynchronous.
Why this is correct
Async replication removes the acknowledgement wait, reducing lag.
Related concept
Read the scenario before looking for a memorised answer.
✗
Reduce the binlog retention period on the primary.
Why it's wrong here
Binlog retention affects storage, not replication speed.
✓
Configure the replica to use parallel replication (slave_parallel_workers > 0).
Why this is correct
Parallel replication allows the replica to apply multiple transactions concurrently.
Related concept
Read the scenario before looking for a memorised answer.
✓
Increase the machine type of the replica.
Why this is correct
A more powerful replica can apply changes faster.
Related concept
Read the scenario before looking for a memorised answer.
✗
Enable binary log compression on the primary.
Why it's wrong here
Compression reduces storage but not replication latency.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that reducing log retention or compressing logs directly reduces replication lag, when in fact these actions affect storage or network transfer, not the apply speed or acknowledgment delay that cause lag.
Detailed technical explanation
How to think about this question
In semi-synchronous replication, the primary waits for acknowledgment from at least one replica (by default, after the replica has written the event to its relay log but before it has applied it). This wait can become a bottleneck if the replica is slow to acknowledge, especially under high concurrency. Parallel replication (slave_parallel_workers > 0) allows the replica to apply transactions from different databases concurrently, reducing the apply backlog. Increasing the replica's machine type provides more CPU and memory resources to handle the apply workload faster, directly addressing the lag caused by slow apply on the replica.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. 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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Monitor and optimize database performance — This question tests Monitor and optimize database performance — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Change replication mode from semi-synchronous to asynchronous. — Option A is correct because switching from semi-synchronous to asynchronous replication removes the requirement for the primary to wait for at least one replica to acknowledge receipt of each transaction. In semi-synchronous mode, the primary waits for acknowledgment, which can introduce latency and contribute to replication lag under high write loads. Asynchronous replication allows the primary to commit transactions immediately without waiting, reducing the time the primary spends waiting for replica acknowledgment and thus lowering replication lag.
What should I do if I get this PCDE question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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