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Comment by vikiomega9 | Hacker News Reader

But isn't network reliability a necessary condition for distributed consensus?

No. A query takes place and the network is either working or not at that instant, and the system reacts accordingly. CP means it might not be available, but if it is then you'll get the latest data. AP means you'll always get an answer, but it might not be the latest data.

You can make your network reliable in the sense that better infrastructure lowers failures, but nothing is 100% so CAP is about what happens in the inevitable failure scenario. Whether you have 0 failures or 1 per hour doesn't affect CAP at all and has nothing to do with magically making something CA.

My comment is that you cannot have CP without a reliable network, an unreliable network is equivalent to a partition event. This might be a naive assertion, but how else do you have a partition event? I'm confused about what you were trying to say. The post talks about being effectively CA which is what I understood about spanner when I came across the system.

The network is either working or not at any given instant. During that instant, CAP determines how the system responds.

Reliability is the graph of many instants over some period of time, so 50% reliability over a day means it's not working roughly half of the instants measured for 24 hours.

So it doesn't whether your network is 1%, 99%, or 100% reliable because CAP is saying that when a partition exists, this is how the system works. If the network is 100% reliable, then sure, partitions never happen and you can be CA, but since that is not possible and partitions will always happen eventually, you have to be either CP or AP - and Spanner chooses CP, which is the only accurate description.

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