DBS-C01 Workload-Specific Database Design Practice Question
A social media startup is designing a database for user activity feeds. Each user follows up to 5,000 other users. The feed must show the latest 100 posts from followed users with latency under 200ms. Reads are 10x writes. Which database design best meets these requirements?
⚠ Common exam trap
Candidates often choose Option C (GSI on user_id and timestamp) thinking it enables efficient querying, but they overlook the need to query across multiple followed users and merge results, which DynamoDB cannot do without application-level sorting and pagination, violating the 200ms latency SLA.
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
✓
Use Amazon DynamoDB with a fan-out on write pattern, storing each user's feed in a separate partition
The fan-out on write pattern with DynamoDB ensures each user's feed is pre-computed and stored in a separate partition, allowing reads to fetch the latest 100 posts with sub-200ms latency by querying a single item collection. This pattern optimizes for the 10:1 read-to-write ratio by shifting work to writes, which are less frequent, and avoids expensive joins or scans at read time.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Use Amazon RDS for PostgreSQL with read replicas and materialized views refreshed every minute
Why it's wrong here
Materialized views would not provide real-time updates and read replicas add replication lag.
- ✗
Use Amazon ElastiCache for Redis as a primary data store with sorted sets per user
Why it's wrong here
Redis is not designed for durable primary storage; data loss risk is high.
- ✗
Use a single DynamoDB table with a global secondary index on user_id and timestamp
Why it's wrong here
Without fan-out, querying posts from 5,000 followed users would require multiple queries and client-side merging, exceeding latency goals.
- ✓
Use Amazon DynamoDB with a fan-out on write pattern, storing each user's feed in a separate partition
Why this is correct
Fan-out on write ensures feeds are pre-computed for fast reads, meeting latency and throughput requirements.
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