C100DBA Sharding Practice Question
A DBA is troubleshooting a sharded cluster where the balancer is not migrating chunks even though there is an imbalance. The DBA runs `sh.getBalancerState()` and it returns `true`. Which of the following is the most likely reason the balancer is not migrating chunks?
⚠ Common exam trap
The trap here is assuming that a `true` balancer state means the balancer is actively migrating chunks at all times, ignoring the effect of the balancing window.
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 balancer window is configured to a time period outside of the current time.
The balancer only runs during the configured balancing window. Even if `sh.getBalancerState()` returns `true`, migrations will not occur outside that window. The DBA should check the balancer window settings and ensure they align with the desired migration times. Other options would either cause broader failures or are not relevant to the balancer's operation.
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 config servers are in a read-only state.
Why it's wrong here
If the config servers were read-only, the entire cluster would be unable to perform metadata updates, and the balancer would be unable to function. However, the scenario states the balancer state is `true`, implying the balancer is enabled. A read-only config server would likely cause broader failures, not just prevent migrations while the balancer appears active.
- ✗
The collection has no indexes on the shard key, so the balancer cannot identify chunks to migrate.
Why it's wrong here
MongoDB automatically creates an index on the shard key when a collection is sharded. The balancer operates on chunk metadata stored in the config servers, not on indexes. Missing indexes would affect query performance, not the balancer's ability to migrate chunks.
- ✓
The balancer window is configured to a time period outside of the current time.
Why this is correct
Even if the balancer is enabled, it only runs during the configured balancing window. If the window is set to a time outside the current time, no migrations will occur. The DBA should check `sh.getBalancerWindow()` to see the active window and adjust it if necessary.
- ✗
The shard key is hashed, and hashed shard keys prevent the balancer from migrating chunks.
Why it's wrong here
Hashed shard keys do not prevent the balancer from migrating chunks. In fact, hashed sharding is designed to distribute data evenly, and the balancer works with hashed chunks just like ranged chunks. The balancer migrates chunks based on chunk counts, regardless of the shard key type.
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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.