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Introduction to Sharding: Shard Keys, Application-Level Sharding, Citus and Distributed SQL

Sharding solves write scale and data size, but makes the global ID, cross-shard queries and transactions that are free on one server expensive. In the lab two shards produced the same order_id, and a "best sellers" list merged from each shard's top 10 shared no book with the correct list; the right answer came from partial totals. A bucket-based IShardResolver in .NET, choosing a shard key, co-location in Citus and distributed SQL.