External consistency vs linearizability In Spanner, TrueTime &amp; 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?
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 $T_1$.