How to reshard a collection?

0-2 years experience

1. 1We have a sharded MongoDB collection using the field 'userId' as the shard key, but we need to change it to 'email'. How would you go about resharding this collection with minimal downtime?
2. 2If you run the reshardCollection command while the balancer is disabled, what effect does that have on data distribution?

2-5 years experience

1. 1During a recent rollout we attempted to reshard a large orders collection and observed the balancer getting stuck. Walk me through how you would diagnose and resolve the issue.
2. 2Explain the trade‑offs between using the reshardCollection command versus creating a new collection and migrating data manually.

5-8 years experience

1. 1Our platform stores petabytes of time‑series data across many shards. We need to change the shard key to improve query patterns. How would you design a resharding strategy that minimizes impact on read/write latency and avoids hot spots?
2. 2What are the performance and operational considerations when resharding a collection that has TTL indexes and a high write throughput?

8+ years experience

1. 1We have multiple microservices that each own different subsets of a sharded MongoDB cluster. A decision is made to change the global shard key for several core collections. How would you coordinate this migration across teams, ensure data consistency, and plan for rollback?
2. 2Discuss the long‑term maintenance implications of frequently resharding collections in a large organization. What policies or tooling would you put in place to reduce operational risk?