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Comment by snewman | Hacker News Reader

To be precise, I would argue this is actually a CA system.

"C" because there are consistency guarantees, which are upheld even in the face of failures / partition.

"A" because the system will continue making progress even after a node failure. What I called a "hiccup" can be made arbitrarily short, in principle at least. The system can work around failed nodes, it does not need to wait for them to be repaired.

Not "P", because if the network falls apart sufficiently, the system will generally not be able to progress.

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> Not "P", because if the network falls apart sufficiently, the system will generally not be able to progress.

Not be able to progress means it is not available.

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.

> 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.

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.

You don't get to sacrifice the P. http://codahale.com/you-cant-sacrifice-partition-tolerance/

That blog is quite wrong about partition tolerance. First, the definition is just bizarre:

"Handling a crashed machine counts as partition-tolerance. (Update: I was wrong about this part."

He then goes on to give Stonebraker crap about claiming that "failures" never happen, simply because he doesn't understand the difference between failures and partitions.

Look, the point of "CAP" is this: if you assume nothing about the network, then you can not guarantee CA in the presence of ARBITRARY network partitions. It doesn't say that you can't provide CA under some or even many network partition scenarios. So, the question you should be asking is "what kinds of network partitions happen in practice?". Stonebraker's point was that network partitions are such rare and wholly catastrophic events that worrying about them pulls focus away from much more practical concerns. Hyperdex' point on partition tolerance (admittedly not clearly spelled out) is much more subtle. They offer tolerance of a specific class of partitions. To simply say they are "AP" or "CP" ignores the very important fact that they do in fact tolerate partitions and maintain the CA. This whole "CAP" pick any two is a gross over-simplification that obscures very real distinctions like this.

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