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From the technical blog post > 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

by sudhirj 10y ago

From the technical blog post > 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. However, no system provides 100% availability, so the pragmatic question is whether or not Spanner delivers availability that is so high that most users don't worry about its outages. For example, given there are many sources of outages for an application, if Spanner is an insignificant contributor to its downtime, then users are correct to not worry about it. Basically, the underlying system is CP, but A is so high (because of the custom fiber, paxos etc) that they're rounding it off to 100% and calling it CAP.

zzzcpan 10y ago
Except that A in CAP has nothing to do with overall system's availability over time and using it as such is just confusing.
- sudhirj 10y ago
  But it makes sense in this case. The system guarantees CP. But as customer it looks like you're getting CA as well, because A is so high. If you drink the kool-aid, you get C & A & P. The kool-aid isn't too bad, though if they can measurably guarantee A > 99.999999%, I'm happy to round off to 100% and call it CAP.
  - thesandlord 10y ago
    The availability is "only" 99.999%, which IMO is still really high! (I work for Google Cloud)
    - lallysingh 10y ago
      https://uptime.is/99.999 https://uptime.is/99.999 puts that at: Daily: 0.9s Weekly: 6.0s Monthly: 26.3s Yearly: 5m 15.6s
    - the_cap_theorem 10y ago
      Where does it say that? Does google cloud spanner guarantee three nines uptime? On the spanner page it says: >> This feature is not covered by any SLA ;) ;)
      - thesandlord 10y ago
        Once Cloud Spanner leaves beta, it will be covered by an SLA. GCP Alpha/Beta products don't have SLAs for the most part.
takeda 10y ago
So essentially a marketing buzz. "It's always available because we spent a lot of money to make it available" CAP is about how the system deals with partitions not whether it has partitions or not.
- parasubvert 10y ago
  The person who wrote this "marketing buzz" is Eric Brewer, who was the originator of the CAP theorem in the first place. I think he knows what it was about :)
  - the_cap_theorem 10y ago
    See my other comment in this thread on how they still used misleading language in their marketing, regardless of who the author is. Don't fall for these PR tricks. Hiring thought leaders/influencers is one that should be easy to spot. Best to judge claims on their own merit and not by the person that speaks them....
    - parasubvert 10y ago
      What am I supposed to think about someone who creates a throwaway account called "the_cap_theorem"? You look like a competitor that's freaking out and throwing FUD. I don't think Eric's words were misleading. People will judge Cloud Spanner on its own merit, but given the reputation Eric has both academically and professionally, I think Cloud Spanner is deserving of attention, and is not a "PR trick".
      - takeda 10y ago
        I don't care who Eric is, it is still misleading. He knows he is providing misinformation and he tries to cover his a__ by adding that technically it is a CP system, when a partition happens. And that's what matters. Saying it is CA because Google threw some money to reduce chances of a partition happening is very misleading. By that definition you could take any RDBMS and claim that is CA because with enough money you can make sure hardware won't fail. It defeats the purpose what CAP is being used for, which is telling us how the system will behave when partition does happen. It defeats the purpose what Aphyr was doing with his Jepsen tests which observe how given storage solution behave when there's a network partition. The claim that Eric Brewer wrote regarding Spanner reduces makes him lose him credibility to me at least. It almost feels like he was hired by Google, just so he could claim that.

by sudhirj 10y ago

zzzcpan 10y ago

Except that A in CAP has nothing to do with overall system's availability over time and using it as such is just confusing.

sudhirj 10y ago
But it makes sense in this case. The system guarantees CP. But as customer it looks like you're getting CA as well, because A is so high. If you drink the kool-aid, you get C & A & P. The kool-aid isn't too bad, though if they can measurably guarantee A > 99.999999%, I'm happy to round off to 100% and call it CAP.
- thesandlord 10y ago
  The availability is "only" 99.999%, which IMO is still really high! (I work for Google Cloud)
  - lallysingh 10y ago
    https://uptime.is/99.999 https://uptime.is/99.999 puts that at: Daily: 0.9s Weekly: 6.0s Monthly: 26.3s Yearly: 5m 15.6s
  - the_cap_theorem 10y ago
    Where does it say that? Does google cloud spanner guarantee three nines uptime? On the spanner page it says: >> This feature is not covered by any SLA ;) ;)
    - thesandlord 10y ago
      Once Cloud Spanner leaves beta, it will be covered by an SLA. GCP Alpha/Beta products don't have SLAs for the most part.

takeda 10y ago

So essentially a marketing buzz. "It's always available because we spent a lot of money to make it available" CAP is about how the system deals with partitions not whether it has partitions or not.

parasubvert 10y ago
The person who wrote this "marketing buzz" is Eric Brewer, who was the originator of the CAP theorem in the first place. I think he knows what it was about :)
- the_cap_theorem 10y ago
  See my other comment in this thread on how they still used misleading language in their marketing, regardless of who the author is. Don't fall for these PR tricks. Hiring thought leaders/influencers is one that should be easy to spot. Best to judge claims on their own merit and not by the person that speaks them....
  - parasubvert 10y ago
    What am I supposed to think about someone who creates a throwaway account called "the_cap_theorem"? You look like a competitor that's freaking out and throwing FUD. I don't think Eric's words were misleading. People will judge Cloud Spanner on its own merit, but given the reputation Eric has both academically and professionally, I think Cloud Spanner is deserving of attention, and is not a "PR trick".
    - takeda 10y ago
      I don't care who Eric is, it is still misleading. He knows he is providing misinformation and he tries to cover his a__ by adding that technically it is a CP system, when a partition happens. And that's what matters. Saying it is CA because Google threw some money to reduce chances of a partition happening is very misleading. By that definition you could take any RDBMS and claim that is CA because with enough money you can make sure hardware won't fail. It defeats the purpose what CAP is being used for, which is telling us how the system will behave when partition does happen. It defeats the purpose what Aphyr was doing with his Jepsen tests which observe how given storage solution behave when there's a network partition. The claim that Eric Brewer wrote regarding Spanner reduces makes him lose him credibility to me at least. It almost feels like he was hired by Google, just so he could claim that.