How to Choose a Sharding Key?
A sharding key is the field in a distributed database that determines which shard stores each record, directly impacting performance and scalability.
When to use/Use Cases
Use sharding when a single database cannot handle the scale of data or queries. Common scenarios include social apps, e-commerce platforms, or multi-tenant SaaS systems where data must be distributed evenly.
Example
A social network might shard by user_id, ensuring each user’s posts and friends live on a single shard.
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Why Is It Important
The right shard key ensures balanced load, minimizes cross-shard queries, and supports predictable scalability as the system grows.
Interview Tips
Emphasize high cardinality (many unique values), query alignment (keys often used in lookups), and avoiding hotspots. For interviews, be ready to discuss trade-offs and real-world examples like user-based sharding.
Trade-offs
A hashed key spreads load evenly but may increase scatter-gather queries. A domain-based key (e.g., region) optimizes lookups but risks uneven distribution.
Pitfalls
Avoid low-cardinality keys (like gender), monotonic values (timestamps causing hotspots), or keys unrelated to query patterns, which force expensive scatter-gather operations.
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