principleModerate
External consistency vs linearizability
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linearizabilityexternalconsistency
Problem
In Spanner, TrueTime & The CAP Theorem, Eric Brewer writes:
One subtle thing about Spanner is that it gets serializability from
locks, but it gets external consistency (similar to linearizability) from TrueTime [emphasis added].
What is the definition of external consistency, and how does it differ from linearizability?
One subtle thing about Spanner is that it gets serializability from
locks, but it gets external consistency (similar to linearizability) from TrueTime [emphasis added].
What is the definition of external consistency, and how does it differ from linearizability?
Solution
External consistency doesn't have a fixed meaning. In this context, it has the meaning appearing in the very next sentence in the document:
For any two transactions, $T_1$ and $T_2$ (even if on opposite sides of the globe):
if $T_2$ starts to commit after $T_1$ finishes committing, then the timestamp for $T_2$ is greater than the timestamp for $T_1$.
For any two transactions, $T_1$ and $T_2$ (even if on opposite sides of the globe):
if $T_2$ starts to commit after $T_1$ finishes committing, then the timestamp for $T_2$ is greater than the timestamp for $T_1$.
Context
StackExchange Computer Science Q#71003, answer score: 13
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