C100DBA Sharding Practice Question
When designing a compound shard key in MongoDB, what is the architectural significance of placing a high-cardinality field as the prefix compared to placing a low-cardinality field first?
⚠ Common exam trap
Candidates often mistakenly prioritize field names or data types over cardinality, failing to realize that low-cardinality prefixes lead to monolithic, unsplittable chunks that degrade cluster performance.
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
✓
A high-cardinality prefix ensures fine-grained chunk boundaries and even data distribution, whereas a low-cardinality prefix causes massive chunks that cannot be split effectively.
The shard key prefix dictates how MongoDB organizes and partitions chunk ranges across the cluster. A high-cardinality prefix ensures fine-grained chunk boundaries and prevents massive, unmanageable chunks from forming around repetitive values, which often happens when low-cardinality fields like boolean flags or status codes occupy the leading position.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
A high-cardinality prefix ensures fine-grained chunk boundaries and even data distribution, whereas a low-cardinality prefix causes massive chunks that cannot be split effectively.
Why this is correct
A high-cardinality prefix ensures fine-grained chunk boundaries and even data distribution, whereas a low-cardinality prefix causes massive chunks that cannot be split effectively. Low-cardinality prefixes limit the number of distinct values, creating oversized chunks for each unique value that the balancer cannot split further, leading to severe distribution hotspots.
- ✗
A high-cardinality prefix allows the query router to bypass authentication checks on secondary shards, improving overall cluster query response times.
Why it's wrong here
Authentication and authorisation checks apply uniformly across all shards regardless of shard key composition, so prefix cardinality cannot bypass them. It tempts because high-cardinality prefixes genuinely improve query targeting by letting mongos route to fewer shards, a real performance benefit that has nothing to do with security checks.
- ✗
A low-cardinality prefix enables the WiredTiger storage engine to cache entire indexes in RAM, eliminating disk I/O bottlenecks entirely.
Why it's wrong here
A low-cardinality prefix enables the WiredTiger storage engine to cache entire indexes in RAM, eliminating disk I/O bottlenecks entirely. Index caching behavior depends on RAM availability and index size relative to cache limits, not on the cardinality of the leading shard key field.
- ✗
A low-cardinality prefix automatically triggers hashed shard evaluation, bypassing the need for explicit hashing functions during collection creation.
Why it's wrong here
MongoDB never infers hashed sharding from field cardinality; hashed shard keys require an explicit hashed index and shard key specification. It tempts because low-cardinality prefixes do cause hotspotting and uneven chunk distribution, the problem hashed sharding exists to solve, so administrators reasonably associate the two.
About these practice questions
This C100DBA question is part of Courseiva's 222-question bank — original exam-style content with full explanations and wrong-answer analysis, never real exam questions or exam dumps. Learn why practice questions differ from exam dumps →
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.