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Directory-Based Sharding - Building Block - Database Scaling | System Design Interview Prep | Repovive

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System Design Interview Prep22 sections · 691 units23h total

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Directory-Based Sharding

Lookup table approach

Directory-based sharding uses a lookup service to map keys to shards:

Lookup Table:
user_123 → Shard A
user_456 → Shard B
user_789 → Shard A

Advantages:

Maximum flexibility in data placement
Can move individual records between shards
Supports custom placement logic

Disadvantages:

Lookup service becomes a bottleneck
Additional latency for every query
Lookup table must be highly available

Use directory sharding when you need fine-grained control or when data access patterns are unpredictable.