C100DBA Sharding Practice Question
A production MongoDB 6.0 sharded cluster has a collection with a ranged shard key on the field `customerId`. Over time, the team notices that nearly all new documents have monotonically increasing `customerId` values, and one shard is receiving a disproportionate share of writes while the other shards remain relatively idle. The balancer is enabled and running. Which of the following is the most likely explanation for this imbalance?
⚠ Common exam trap
The trap here is assuming the balancer can fully compensate for a poor shard key choice, when in fact a monotonic key will always create a write hotspot regardless of balancing.
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 shard key is monotonically increasing, causing all new inserts to target the highest chunk on a single shard.
With a monotonically increasing shard key, new inserts always target the chunk with the highest key range. Because chunks are assigned to shards, that highest chunk lives on one shard at a time, making it a write hotspot. The balancer will eventually migrate chunks, but the stream of new inserts keeps hitting the newest chunk, so the imbalance persists.
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 shard key is monotonically increasing, causing all new inserts to target the highest chunk on a single shard.
Why this is correct
A monotonically increasing shard key like an incrementing `customerId` means new documents always fall into the highest chunk, which resides on one shard until the balancer migrates it. This creates a hot shard for writes, a well-known anti-pattern for ranged shard keys with sequential values.
- ✗
The shard key field is not indexed on the shards, forcing all writes to a single primary.
Why it's wrong here
MongoDB automatically creates an index on the shard key if one does not exist when sharding a collection. The absence of an index would affect query performance and possibly cause errors, but it would not cause writes to concentrate on one shard. The imbalance is due to the key's monotonic nature.
- ✗
The config servers are overloaded and cannot process chunk migration commands.
Why it's wrong here
Config server overload would affect metadata operations and migrations, but the scenario states the balancer is running and the imbalance is consistent with a monotonic shard key, not a config server bottleneck. Config server performance issues typically manifest as broader cluster problems, not a predictable write concentration on one shard.
- ✗
The collection has too few chunks, so the balancer cannot distribute them evenly across shards.
Why it's wrong here
Even with many chunks, a monotonically increasing key still concentrates new writes on the highest chunk. The balancer migrates chunks, but inserts continue to target the newest range. The issue is the key pattern, not the chunk count, and the balancer would eventually spread existing chunks.
About these practice questions
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official MongoDB exam blueprint
This C100DBA practice question is part of Courseiva's free MongoDB certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the C100DBA exam.