You cannot guarantee consistency and availability simultaneously in the face of network partitions. Once the line of communications is cut or overloaded (slow enough = a partition), you have to pick one or the other. It's basic physics.
If two entities can't communicate, they can't synchronize state, so one (or both) of them have to quit acting like they have a consistent view of the data.
It's not exactly clear from the paper what CA should mean.
I've seen people claim it means "you guarantee both consistency and availability, as long as there are no network partitions (you don't have to handle those because you haven't chosen P)". That's a supportable claim. So you can do that, but it's kind of a useless choice, because as long as there are no network partitions, both CP and AP systems can also guarantee full consistency and availability.
I lay the CAP family out thus:
* CP: on network partition, lose availability
* AP: on network partition, lose consistency
* CA: on network partition, lose both
It seems to be a common thread among distributed systems engineers who claim to have beaten CAP: "network partitions don't matter for whatever reason, so therefore I can always guarantee both availability and consistency and so CAP must be wrong yaaay!!"
> If two entities can't communicate, they can't synchronize state, so one (or both) of them have to quit acting like they have a consistent view of the data.
With the exception of quantum entanglement, of course.
Comments
There is no CA.
You cannot guarantee consistency and availability simultaneously in the face of network partitions. Once the line of communications is cut or overloaded (slow enough = a partition), you have to pick one or the other. It's basic physics.
If two entities can't communicate, they can't synchronize state, so one (or both) of them have to quit acting like they have a consistent view of the data.
It's not exactly clear from the paper what CA should mean.
I've seen people claim it means "you guarantee both consistency and availability, as long as there are no network partitions (you don't have to handle those because you haven't chosen P)". That's a supportable claim. So you can do that, but it's kind of a useless choice, because as long as there are no network partitions, both CP and AP systems can also guarantee full consistency and availability.
I lay the CAP family out thus:
* CP: on network partition, lose availability
* AP: on network partition, lose consistency
* CA: on network partition, lose both
It seems to be a common thread among distributed systems engineers who claim to have beaten CAP: "network partitions don't matter for whatever reason, so therefore I can always guarantee both availability and consistency and so CAP must be wrong yaaay!!"
Sorry, no, nice try.
> If two entities can't communicate, they can't synchronize state, so one (or both) of them have to quit acting like they have a consistent view of the data.
With the exception of quantum entanglement, of course.