Shard key choice
user_id hashed → even distribution. Bad choice: timestamp (hotspots on newest). Choose carefully; changing is painful.
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Shard key choice
user_id hashed → even distribution. Bad choice: timestamp (hotspots on newest). Choose carefully; changing is painful.
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Routing layer
Application code or proxy translates key → shard. Keeps app code simple. Route lookups must be fast (cache the shard map).