C100DBA Sharding Practice Question
A MongoDB DBA is managing a sharded cluster where the collection 'orders' is sharded on the field 'orderDate' using ranged sharding. The DBA needs to add a new shard to the cluster to handle increasing data volume. After adding the shard, the balancer begins migrating chunks. During this migration, the DBA runs a query that includes 'orderDate' and notices increased latency. What is the most likely reason for the increased latency?
⚠ Common exam trap
The trap here is attributing latency to the new shard's lack of indexes or config server overload, when the primary cause is resource contention from chunk migration.
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 migration process consumes resources on the shards and may cause temporary performance degradation.
Adding a shard triggers the balancer to migrate chunks to distribute data. This migration process involves copying data and updating metadata, which consumes resources on the source and destination shards. As a result, concurrent queries may experience increased latency until migrations finish. The other options describe unrelated issues or misconceptions about indexing and hot spots.
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 overloaded with metadata updates, causing mongos to delay query routing.
Why it's wrong here
Config servers handle metadata updates during chunk migrations, but they are designed to handle this load. While there may be some overhead, it is not the primary cause of query latency. The main impact is on the shards themselves due to data movement.
- ✗
The query must now be routed to the new shard, which has not yet been indexed, causing a full collection scan.
Why it's wrong here
When a new shard is added, it starts empty and receives chunks via migration. Indexes on the collection are created on the destination shard as part of the migration process. The query is routed based on the shard key and existing chunks; it does not inherently cause a full collection scan on the new shard.
- ✗
The shard key 'orderDate' is monotonically increasing, so all new data is going to the new shard, causing a hot spot.
Why it's wrong here
A monotonically increasing shard key can cause hot spots, but that is a separate issue from adding a new shard. When adding a shard, the balancer distributes existing chunks. The hot spot issue would exist regardless of adding a shard. The increased latency during migration is due to resource contention from the migration itself.
- ✓
The balancer migration process consumes resources on the shards and may cause temporary performance degradation.
Why this is correct
Chunk migrations involve copying data from the source shard to the destination shard and updating metadata. This process consumes CPU, memory, and I/O on both shards, which can increase latency for concurrent queries. The impact is temporary and typically resolves once migrations complete.
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.