- A
Reduce the value of `max_wal_size`.
Why wrong: Reducing max_wal_size can cause more frequent checkpoints, potentially increasing I/O and lag.
- B
Increase the replica's machine size.
More CPU and memory on the replica help it apply WAL more quickly, reducing lag.
- C
Place the replica in the same zone as the primary.
Same zone placement minimizes network latency for WAL transfer, reducing lag.
- D
Use a higher number of read replicas.
Why wrong: Adding more replicas does not reduce lag for an existing replica; it may even increase load on the primary.
- E
Enable synchronous replication.
Why wrong: Synchronous replication ensures no lag but impacts primary write performance; it does not reduce lag on the replica.
Question 534 of 1,000
Cloud SQL PostgreSQL Replication Lag Reduction
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.
A Cloud SQL for PostgreSQL instance is experiencing high replication lag between primary and read replica. Which TWO actions would reduce the lag?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
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.
Quick Answer
The answer is placing the replica in the same zone as the primary and increasing the replica’s machine size. Replication lag in Cloud SQL for PostgreSQL occurs when the read replica cannot apply Write-Ahead Log (WAL) segments as fast as the primary generates them. By colocating the replica in the same zone, you drastically reduce network latency between the instances, while a larger machine size provides more CPU and memory to accelerate WAL replay. On the Google Professional Cloud Database Engineer exam, this question tests your understanding of read replica scaling and the specific mechanics of PostgreSQL streaming replication. A common trap is assuming that adding more replicas or increasing the primary’s size will help—those actually increase overhead or do not address the replica’s bottleneck. For a quick memory tip, think “Same Zone, Bigger Clone” to recall that both location and compute power directly attack lag.
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 replica's machine size.
Increasing the replica's machine size (Option B) reduces replication lag by providing more CPU and memory resources to apply WAL changes faster. Placing the replica in the same zone as the primary (Option C) minimizes network latency between the instances, which directly reduces the time for WAL data to travel from primary to replica, thus lowering 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.
- ✗
Reduce the value of `max_wal_size`.
Why it's wrong here
Reducing max_wal_size can cause more frequent checkpoints, potentially increasing I/O and lag.
- ✓
Increase the replica's machine size.
Why this is correct
More CPU and memory on the replica help it apply WAL more quickly, reducing lag.
Clue confirmation
The clue word "primary" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✓
Place the replica in the same zone as the primary.
Why this is correct
Same zone placement minimizes network latency for WAL transfer, reducing lag.
Clue confirmation
The clue word "primary" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Use a higher number of read replicas.
Why it's wrong here
Adding more replicas does not reduce lag for an existing replica; it may even increase load on the primary.
- ✗
Enable synchronous replication.
Why it's wrong here
Synchronous replication ensures no lag but impacts primary write performance; it does not reduce lag on the replica.
Common exam traps
Common exam trap: answer the scenario, not the keyword
A common misconception is that reducing `max_wal_size` or adding more replicas will reduce replication lag. In Cloud SQL for PostgreSQL, reducing `max_wal_size` can cause more frequent WAL segment switches, potentially increasing I/O and lag. Adding more replicas does not speed up apply on the existing replica; it may even increase load on the primary. The correct approaches are to increase replica resources or reduce network latency.
Detailed technical explanation
How to think about this question
Replication lag in Cloud SQL for PostgreSQL is primarily caused by the replica's inability to apply WAL segments as fast as the primary generates them. Increasing replica machine size improves apply throughput via more vCPUs and memory for the recovery process. Placing the replica in the same zone reduces network round-trip time (RTT) for WAL streaming, which is critical because PostgreSQL streaming replication uses a single TCP connection and any latency directly impacts the lag.
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 healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this PCDE question test?
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: Increase the replica's machine size. — Increasing the replica's machine size (Option B) reduces replication lag by providing more CPU and memory resources to apply WAL changes faster. Placing the replica in the same zone as the primary (Option C) minimizes network latency between the instances, which directly reduces the time for WAL data to travel from primary to replica, thus lowering 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.
Are there clue words in this question I should notice?
Yes — watch for: "primary". Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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