Cassandra: The trouble with timestamps Cassandra uses timestamps to determine the most recent update...

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Cassandra: The trouble with timestamps
Cassandra uses timestamps to determine the most recent update to a column. The timestamp is provided by the client application. The latest timestamp always wins when requesting data, so if multiple client sessions update the same columns in a row concurrently, the most recent update is the one that readers see.
wall clocks
Logical clocks: Lamport clocks, Vector clocks
NTP, clock drift, leap seconds
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Cassandra breaks a row up into columns that can be updated independently.
timestamps are only used to pick a “winning” update within a single column or collection element. (A timestamp tie will also result in a deterministic, commutative result.) Lightweight transactions are available when linearizability is 
important.
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Some folks have asked whether Cassandra or Riak in last-write-wins mode are monotonically consistent, or whether they can guarantee read-your-writes, and so on. This is a fascinating question, and leads to all sorts of interesting properties about clocks and causality.

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