>Does this mean that Spanner is a CA system
as defined by CAP? The short answer is “no” technically, but “yes” in effect and its users can and do
assume CA.
>The purist answer is “no” because partitions can happen and in fact have happened at Google, and during
(some) partitions, Spanner chooses C and forfeits A. It is technically a CP system. We explore the impact
of partitions below.
...
>Conclusion
>---------
>Spanner reasonably claims to be an “effectively CA” system despite operating over a wide area, as it is
always consistent and achieves greater than 5 9s availability.
>The purist answer is “no” because partitions can happen and in fact have happened at Google, and during (some) partitions, Spanner chooses C and forfeits A. It is technically a CP system. We explore the impact of partitions below.
...
>Conclusion
>---------
>Spanner reasonably claims to be an “effectively CA” system despite operating over a wide area, as it is always consistent and achieves greater than 5 9s availability.
From the paper