Explain Shard Rebalancing Without Downtime.

When to Use

• Scaling systems: When adding or removing shards as data or traffic grows.
• Performance tuning: To remove hotspots and balance load evenly.
• Hardware upgrades: Migrating shards to new nodes without impacting availability.

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Example

An e-commerce platform may move user data between shards during live traffic to prevent one shard from overloading.

Why Is It Important

It ensures continuous availability, maintains consistent performance, and allows systems to scale horizontally without service interruptions.

Interview Tips

Explain approaches like dual-writes, temporary replication, and rebalancing via proxies. Emphasize data consistency, load testing, and rollback planning.

Trade-offs

Pros: Zero downtime, better scalability. Cons: Increased complexity, higher resource usage during migration.

Pitfalls

Failing to update routing tables, mishandling in-flight writes, or skipping validation can cause data inconsistency or temporary outages.