DBS-C01 Workload-Specific Database Design Practice Question
A company is building a real-time chat application using Amazon DynamoDB. Each message has a conversation ID, timestamp, sender, and content. The primary access pattern is to retrieve the most recent 50 messages for a given conversation, ordered by timestamp. Which table design minimizes cost and latency?
⚠ Common exam trap
The trap here is that candidates may over-engineer the solution by adding unnecessary components like DAX or GSIs, when DynamoDB's native sort key and query parameters directly solve the access pattern with minimal cost and latency.
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 conversation ID as partition key and timestamp as sort key; query with ScanIndexForward=false and Limit=50.
Using conversation ID as the partition key and timestamp as the sort key allows a single Query operation with ScanIndexForward=false to retrieve items in reverse chronological order, and Limit=50 ensures only the most recent 50 messages are returned. This design minimizes cost by avoiding full table scans or additional indexes, and minimizes latency by leveraging DynamoDB's native sort key ordering without needing external caching or streams.
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 a composite primary key of conversation ID and message ID, and enable DynamoDB Streams to process messages.
Why it's wrong here
Message ID does not preserve order; streams are not needed.
- ✗
Use DynamoDB Accelerator (DAX) to cache the most recent messages.
Why it's wrong here
DAX is unnecessary and adds cost; the query is already efficient.
- ✓
Use conversation ID as partition key and timestamp as sort key; query with ScanIndexForward=false and Limit=50.
Why this is correct
Directly supports the access pattern without additional indexes.
- ✗
Use conversation ID as partition key and a GSI on timestamp.
Why it's wrong here
GSI adds unnecessary cost; the sort key can be used directly.
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