advancedSystem Design for the Persistence Layer — Read Replicas, Sharding & CQRS
What's the key difference between what read replicas scale versus what sharding scales?
Read replicas scale read throughput only — every write still goes through the single primary. Sharding splits the data itself across multiple independent database instances, so each shard handles both reads AND writes for its own subset of data, genuinely scaling both dimensions, at a real cost in cross-shard query complexity.
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