Consistent hashing
Ring of virtual nodes. Data mapped by hash of key. Replicas on next N nodes clockwise. Cassandra adopted.
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Sloppy quorum + hinted handoff
If preferred replica down, write to next available with hint. When original returns, replay hint. Availability > strict quorum.
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Vector clocks + client resolution
Concurrent writes tracked. Client sees all versions, resolves. Application-level semantics.