C100DBA Sharding Practice Question
An engineering team must shard a high-volume collection tracking global financial transactions. The chosen shard key is based on a timestamp field containing the exact millisecond of each transaction. Why will this specific shard key choice severely degrade cluster write performance over time?
⚠ Common exam trap
Candidates often believe that high-precision timestamps naturally distribute data well, forgetting that monotonically increasing values send 100% of writes to a single shard chunk.
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
✓
Monotonically increasing timestamp values direct all concurrent write operations exclusively to a single chunk located on the current maximum shard, creating a severe insertion bottleneck.
Timestamp shard keys cause write amplification on a single shard because monotonically increasing values always route insertions to the highest chunk in the cluster. This violates key sharding principles by preventing horizontal write scalability. Recognizing insert bottlenecks helps DBAs design balanced, high-throughput architectures using hashed keys or compound prefixes to distribute incoming write operations effectively across multiple cluster shards.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Monotonically increasing timestamp values direct all concurrent write operations exclusively to a single chunk located on the current maximum shard, creating a severe insertion bottleneck.
Why this is correct
Monotonically increasing timestamp values direct all concurrent write operations exclusively to a single chunk located on the current maximum shard, creating a severe insertion bottleneck. Because the cluster cannot distribute active inserts across multiple shards simultaneously, the designated shard experiences severe resource exhaustion while all other cluster shards remain entirely underutilized during peak ingestion periods.
- ✗
The MongoDB balancer will immediately purge all historical chunks older than twenty-four hours to conserve disk space, resulting in accidental data loss for compliance records.
Why it's wrong here
The MongoDB balancer will immediately purge all historical chunks older than twenty-four hours to conserve disk space, resulting in accidental data loss for compliance records. The automated balancer process exclusively migrates existing chunks across shards based on data size disparities and never deletes stored documents or drops historical ranges independently.
- ✗
Queries filtering by date ranges will fail because the query router cannot project timestamps across distributed shards without a secondary hashed index on the object identifier.
Why it's wrong here
Queries filtering by date ranges will fail because the query router cannot project timestamps across distributed shards without a secondary hashed index on the object identifier. Mongos query routers seamlessly scatter-gather date range predicates across all relevant shards regardless of whether the underlying shard key is hashed or explicitly ranged.
- ✗
MongoDB will automatically convert the millisecond integers into floating-point numbers, causing precision mismatch errors during chunk migration coordination phases.
Why it's wrong here
MongoDB will automatically convert the millisecond integers into floating-point numbers, causing precision mismatch errors during chunk migration coordination phases. MongoDB fully supports high-precision numeric types and BSON dates natively without forcing hazardous implicit type conversions that could trigger data corruption or chunk migration failures.
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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.