If you have a lot of money and are willing and able to build a private network with enough redundancy, you can have both C and A in practice, and P will be low enough not to practically affect the service.
See
https://cloud.google.com/blog/products/databases/inside-clou...
Notice who the author is: Eric Brewer
If you really need things to arrive, you'd be sending them on more than one connection. Of course, the difficulty with redundant fiber connections is that really the only way to determine if redundant fiber connections are in the same bundle is with a backhoe.
If you have multiple, let's expand out to 3 instances (not necessary, but helps illustrate some of the issues better). A tells B something 10x. A also tells C something 10x. Even assuming you don't get inconsistent responses (i.e., B has both accepted and rejected the write), what happens if B accepts it, and C rejects it? Likewise, if you have high load, you're going to have race conditions leading to lack of consistency as well (A tells B and C x = 2, meanwhile C tells A and B that x = 3). You are favoring availability > consistency.
Actually I'd say this is something that's correct in theory but not in practice. In theory you can build a network so redundant that you will never experience a partition. In practice you will always have bugs in your redundancy arrangement, operational errors by sysadmins, and so on.
> If you have a lot of money and are willing and able to build a private network with enough redundancy, you can have both C and A in practice, and P will be low enough not to practically affect the service.
You still have to choose whether to drop C, A, or both when P does happen. Five 9s means about an hour of downtime a year; maybe for some businesses that's small enough to be irrelevant, but by no means all.
While it is in theory true that you cannot have a CA system, in practice if you control your H/W and data centers as well, you can build enough redundancy that a P is not more likely than (say) being hit by a meteor. So still not a CA system, but practically CA 99.9..% (add as many 9's as you have money).
Edit: Spelling.
Google has done this. Possibly AT&T also.
> Notice who the author is: Eric Brewer
Note that Professor Brewer is a teacher/academic, and CAP is intended for instructional purposes.
As a DBA, I find CAP totally inadequate for any business purpose.
Try explaining to a business owner, "Yeah, when our network partitioned, we couldn't accept purchases in both partitions." Their response will be, "So all that money you spent on HA was really based on a lie?"
Teacher, academic, and vice president of infrastructure at Google. He wrote about the CAP implications of Spanner here as well:
https://storage.googleapis.com/pub-tools-public-publication-...
If you built a highly redundant network, and it still partitioned, then the actual problem is that you somehow lost 80% of your lines to the internet at the same time. So your response back needs to be that your HA setup kept things running for a while but it would take actual magic to keep going once just about all the servers are unplugged simultaneously. That's not a problem with CAP, that's a problem with explaining the wrong part of the issue.
"All that money spent on HA was what allowed us to accept purchases -at all-"