C100DBA Sharding Practice Question
A company stores IoT sensor data with a high ingestion rate. They use a monotonically increasing timestamp as the shard key. What is the most likely performance bottleneck for this cluster?
⚠ Common exam trap
Examinees mistakenly believe that monotonically increasing shard keys distribute writes evenly because new documents are continuously added to the database.
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
✓
All new inserts will target the shard holding the highest range of values.
Monotonically increasing shard keys like timestamps result in all write operations being directed to a single shard, specifically the one containing the maximum range. This creates a hot shard where the CPU and I/O capacity of one node are exhausted while others remain idle. Understanding write distribution is critical for maintaining high-throughput ingest systems in MongoDB's distributed architecture.
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 balancer will move chunks too frequently, causing network saturation.
Why it's wrong here
While the balancer moves chunks to maintain even distribution across the cluster, it does not solve the immediate problem of a single shard receiving all incoming writes. Excessive balancing can consume bandwidth, but the primary bottleneck here is the inability to parallelize the ingestion process across multiple shards.
- ✓
All new inserts will target the shard holding the highest range of values.
Why this is correct
In ranged sharding, documents with values exceeding the current max chunk range are placed in the right-most chunk. If the shard key is a timestamp, every new record goes to this single shard. This negates the horizontal scaling benefits of sharding by creating a single point of contention.
- ✗
Read operations will fail because the query router cannot locate the data.
Why it's wrong here
The mongos query router uses metadata from config servers to identify which shard contains specific ranges. While write performance suffers, read operations remain functional. The router successfully maps the shard key to the correct shard, but the overall cluster health is degraded due to the uneven write load distribution.
- ✗
The shard key must be unique, and timestamps often collide in high-volume systems.
Why it's wrong here
MongoDB does not require shard keys to be unique unless they are also the primary key. If multiple documents share a timestamp, they simply belong to the same chunk. The bottleneck is physical resource exhaustion on one shard, not a constraint violation or index conflict within the database engine.
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.