
Inside Cloud Spanner and the CAP Theorem - wwilson
https://cloudplatform.googleblog.com/2017/02/inside-Cloud-Spanner-and-the-CAP-Theorem.html
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tzs
This is marked as a dupe, but is it? The "past" link does not find an earlier
submission of it.

Is it being mixed up with the earlier submission to the announcement of Cloud
Spanner? That article announced it and gave an overview. In that overview it
was mentioned that Cloud Spanner does not violate the CAP Theorem even though
its feature set might make it appear to.

The present submission is to a different article on the same site that is
about the CAP Theorem and how Cloud Spanner achieves its features without
violating CAP.

~~~
zzzcpan
True, this is not dupe, different blog post, this one is technical.

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zzzcpan
"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. [...] during some partitions, Spanner chooses C and forfeits A. It
is technically a CP system. [...] delivers availability that is so high that
most users don't worry about its outages"

This is quite confusing, CAP tradeoffs are about what can be provided
simultaneously, not the system's availability over time, it's a different kind
of availability. I think the tradeoffs in CAP are better explained as
__waiting__ to achieve consensus. If you wait for consensus, you can get
global consistency, but you would have to wait for it somewhere on some level
no matter what every time. Waiting is not good for latency though. So, if you
don't wait for consensus, you can pretty much guarantee good latency, but
would have to rely on CRDTs for consistency. You cannot really assume CP as
CA. You can misunderstand it of course.

~~~
VanillaCafe
Note: the blog post was written by Eric Brewer who invented the CAP theorem.

~~~
zzzcpan
Sure, but it is also a PR post where certain things are worded in a certain
way on purpose.

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sciurus
Should this discussion be merged with
[https://news.ycombinator.com/item?id=13644959](https://news.ycombinator.com/item?id=13644959)
?

~~~
sctb
Thanks, we've moved comments there.

