Timestamps, as implemented in Riak, Cassandra, et al, are fundamentally unsafe ordering constructs. In order to guarantee consistency you, the user, must ensure locally monotonic and, to some extent, globally monotonic clocks. This is a hard problem, and NTP does not solve it for you. When wall clocks are not properly coupled to the operations in the system, causal constraints can be violated. To ensure safety properties hold all the time, rather than probabilistically, you need logical clocks.
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