DBS-C01 Workload-Specific Database Design Practice Question
A gaming company uses Amazon DynamoDB for player session data. Each session has a partition key of `game_id` and a sort key of `session_id`. The table has a global secondary index (GSI) on `player_id` for leaderboard queries. Recently, the company noticed that write traffic to the GSI is causing throttling on the base table, even though the base table's write capacity is not fully utilized. What is the MOST likely cause?
⚠ Common exam trap
Test-takers frequently assume throttling on the base table is always caused by the base table's own capacity settings, overlooking that GSIs have independent capacity and can cause back-pressure on the base table when their partition key design leads to hot spots.
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
✓
The GSI is not designed with a high-cardinality partition key, causing write hot spots on the GSI.
A global secondary index (GSI) has its own provisioned read and write capacity, separate from the base table. If the GSI's partition key (player_id) has low cardinality (e.g., only a few distinct player_id values), writes to the base table will concentrate on a small number of GSI partitions, causing throttling on the GSI. This throttling on the GSI then back-pressures the base table, resulting in write throttling on the base table even if its own write capacity is underutilized.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The application is using strongly consistent reads on the GSI, which consumes double the read capacity.
Why it's wrong here
Consistent reads affect read capacity, not write capacity.
- ✓
The GSI is not designed with a high-cardinality partition key, causing write hot spots on the GSI.
Why this is correct
A hot GSI partition can throttle writes, affecting the base table writes.
- ✗
Point-in-time recovery (PITR) is enabled, consuming extra write capacity.
Why it's wrong here
PITR uses separate storage and does not consume write capacity from the table.
- ✗
The table's auto-scaling settings are not configured correctly for the GSI.
Why it's wrong here
Auto-scaling adjusts table capacity, but if a GSI partition is hot, throttling can still occur.
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