DBS-C01 Workload-Specific Database Design Practice Question
A company runs an e-commerce platform on Amazon RDS for PostgreSQL. During a flash sale, the database experiences high write load and read replicas lag significantly. The application uses read replicas for reporting queries. Which design change would most effectively reduce replica lag without compromising write performance?
⚠ Common exam trap
Many exam-takers assume adding more replicas or scaling the primary will solve replication lag, but they fail to recognize that the fundamental replication mechanism in RDS for PostgreSQL (streaming WAL) is the bottleneck, whereas Aurora's shared-storage architecture inherently minimizes 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
✓
Migrate to Amazon Aurora with read replicas.
Amazon Aurora's distributed storage architecture decouples compute from storage, allowing replicas to apply redo logs with minimal overhead compared to RDS for PostgreSQL's physical replication. Aurora's replicas share the same underlying storage volume, so replica lag is significantly reduced even under heavy write loads, while write performance on the primary remains unaffected due to the asynchronous, log-based replication mechanism.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Increase the instance size of the primary database.
Why it's wrong here
Larger instance may handle write load better but does not directly reduce replica lag.
- ✗
Add more read replicas to distribute the reporting load.
Why it's wrong here
Adding replicas does not reduce lag per replica; the underlying replication mechanism remains unchanged.
- ✓
Migrate to Amazon Aurora with read replicas.
Why this is correct
Aurora has faster replication (typically <100ms) and is designed to handle high write loads with minimal replica lag.
- ✗
Convert the RDS instance to a Multi-AZ deployment.
Why it's wrong here
Multi-AZ is for high availability, not for reducing read replica lag.
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