← SOA-C02·Amazon Web Services

Question 459 of 1,546

Cost and Performance Optimization →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is to upgrade the instance class of the read replicas to a larger type with more CPU and memory. This is correct because replica lag in Amazon RDS for PostgreSQL is primarily driven by the read replica’s ability to replay Write-Ahead Log (WAL) data as fast as the primary generates it; a larger instance class directly increases the WAL replay rate by providing more processing power and memory to handle the write-heavy replay workload. On the AWS Certified SysOps Administrator Associate SOA-C02 exam, this scenario tests your understanding that scaling read replicas independently is a cost-effective RDS replica lag reduction strategy—upgrading the primary instance would be wasteful since the bottleneck is on the replica side. A common trap is assuming you must upgrade the primary or add more replicas, but the key is matching the replica’s compute capacity to the peak write rate. Memory tip: “Replica lag is a replay race—give the follower the bigger engine, not the leader.”

SOA-C02 Cost and Performance Optimization Practice Question

This SOA-C02 practice question tests your understanding of cost and performance optimization. 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 runs a read-heavy database workload on Amazon RDS for PostgreSQL with a primary instance and two read replicas. The SysOps administrator observes that the read replicas frequently experience high replica lag during peak hours, causing stale reads for the application. The administrator needs to reduce replica lag while optimizing costs. The workload is predictable, with spikes during business hours and low traffic at night. Which combination of actions should the administrator take?

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.

Question 1hardmultiple choice

Read the full NAT/PAT explanation →

A
Convert the read replicas to Multi-AZ instances to improve the replication process and reduce lag.
Why wrong: Multi-AZ for read replicas provides standby replicas for failover, but it does not reduce replica lag. In fact, running Multi-AZ adds overhead because the primary in the replica's AZ must also replicate to the standby.
B
Upgrade the instance class of the read replicas to a larger type with more CPU and memory to handle the increased WAL replay rate.
Replica lag occurs when the replica cannot keep up with the rate of changes from the primary. Increasing the replica's instance size gives it more resources to apply WAL data faster, reducing lag. This directly addresses the performance bottleneck.
C
Add additional read replicas to distribute the read load and reduce the lag on each individual replica.
Why wrong: Adding more replicas does not reduce the lag on existing replicas; it may increase the load on the primary because it must stream the WAL to more replicas, potentially worsening the lag. The solution is to make each replica more powerful.
D
Upgrade the primary DB instance to a larger class with increased IOPS to reduce the amount of data that needs to be replicated.
Why wrong: While upgrading the primary could improve overall throughput, the replica lag is specifically caused by the replicas' inability to apply changes quickly. The replicas are the bottleneck. Upgrading the primary is less cost-effective and may not solve the lag issue.

Full breakdown with real-world context →

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

✓

Upgrade the instance class of the read replicas to a larger type with more CPU and memory to handle the increased WAL replay rate.

Option B is correct because upgrading the read replica instance class provides more CPU and memory, which directly increases the WAL replay rate. In RDS for PostgreSQL, replica lag is primarily caused by the replica's inability to apply WAL changes as fast as the primary generates them. A larger instance class alleviates this bottleneck without incurring the cost of upgrading the primary instance.

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.

✗
Convert the read replicas to Multi-AZ instances to improve the replication process and reduce lag.
Why it's wrong here
Multi-AZ for read replicas provides standby replicas for failover, but it does not reduce replica lag. In fact, running Multi-AZ adds overhead because the primary in the replica's AZ must also replicate to the standby.
✓
Upgrade the instance class of the read replicas to a larger type with more CPU and memory to handle the increased WAL replay rate.
Why this is correct
Replica lag occurs when the replica cannot keep up with the rate of changes from the primary. Increasing the replica's instance size gives it more resources to apply WAL data faster, reducing lag. This directly addresses the performance bottleneck.
Clue confirmation
The clue word "primary" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Add additional read replicas to distribute the read load and reduce the lag on each individual replica.
Why it's wrong here
Adding more replicas does not reduce the lag on existing replicas; it may increase the load on the primary because it must stream the WAL to more replicas, potentially worsening the lag. The solution is to make each replica more powerful.
✗
Upgrade the primary DB instance to a larger class with increased IOPS to reduce the amount of data that needs to be replicated.
Why it's wrong here
While upgrading the primary could improve overall throughput, the replica lag is specifically caused by the replicas' inability to apply changes quickly. The replicas are the bottleneck. Upgrading the primary is less cost-effective and may not solve the lag issue.

Common exam traps

Common exam trap: answer the scenario, not the keyword

The trap here is that candidates often confuse replica lag with primary performance, leading them to upgrade the primary (Option D) or add more replicas (Option C), when the real bottleneck is the replica's WAL replay capacity.

Detailed technical explanation

How to think about this question

PostgreSQL read replicas use asynchronous streaming replication, where the primary sends WAL segments to replicas, which then replay them. Replica lag occurs when the replay rate (apply rate) falls behind the receive rate, often due to insufficient CPU or I/O on the replica. In a read-heavy workload with predictable spikes, using a larger replica instance class (e.g., moving from db.r5.large to db.r5.xlarge) directly improves the apply rate, while smaller replicas can be used during low-traffic periods via scheduled scaling to optimize costs.

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 startup's cloud architect reviews their monthly bill and notices costs are higher than expected for a long-running batch job. Switching from on-demand instances to Reserved Instances — or using Spot/Preemptible VMs — can reduce compute costs by up to 72 %. Questions like this test whether you understand the tradeoffs between commitment, flexibility, and cost across cloud pricing models.

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.

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FAQ

Questions learners often ask

What does this SOA-C02 question test?

Cost and Performance Optimization — This question tests Cost and Performance Optimization — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: Upgrade the instance class of the read replicas to a larger type with more CPU and memory to handle the increased WAL replay rate. — Option B is correct because upgrading the read replica instance class provides more CPU and memory, which directly increases the WAL replay rate. In RDS for PostgreSQL, replica lag is primarily caused by the replica's inability to apply WAL changes as fast as the primary generates them. A larger instance class alleviates this bottleneck without incurring the cost of upgrading the primary instance.

What should I do if I get this SOA-C02 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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