DBS-C01 Workload-Specific Database Design Practice Question
A company runs a document management system using Amazon DocumentDB (with MongoDB compatibility). The application stores large documents (up to 5 MB each) and frequently fetches them by document ID. The team notices increased latency during peak hours. They need to reduce read latency. Which action is MOST effective?
⚠ Common exam trap
AWS often tests the misconception that scaling up the primary instance (Option D) is equivalent to scaling out read capacity, but in DocumentDB, read replicas are the correct solution for read-heavy workloads because they provide horizontal read scaling without overloading the primary.
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
✓
Add read replicas to the cluster
Adding read replicas to the DocumentDB cluster is the most effective action because it offloads read traffic from the primary instance, directly reducing read latency during peak hours. DocumentDB supports up to 15 read replicas that are kept in sync via the cluster's replication mechanism, and the application's frequent fetches by document ID are read-heavy operations that benefit from distributing the load across multiple replicas.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Add read replicas to the cluster
Why this is correct
Read replicas offload read traffic and reduce latency.
- ✗
Shard the collection across multiple DocumentDB clusters
Why it's wrong here
Sharding adds complexity; not needed for this simple access pattern.
- ✗
Implement Amazon ElastiCache for Redis in front of DocumentDB
Why it's wrong here
Adds complexity and requires application changes.
- ✗
Increase the instance class of the primary instance
Why it's wrong here
Vertical scaling helps but not as cost-effective as horizontal scaling.
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