[Info-vax] [OT] Data/system integrity, was: Re: HBVS, was: Re: WASD 11.0 announced.

[lawrencedo99 at gmail.com]

On Friday, July 1, 2016 at 1:10:05 AM UTC+12, Kerry Main wrote:

> ... the CAP "theorem" is outdated ...

On the contrary. It’s never been disproved. All that’s happened is that the hard requirements for one or the other have been loosened somewhat as a compromise.

Was there some kind of suggestion further up this thread that somehow VMS clustering was a way around CAP? It wasn’t. Remember the concept of cluster quorums? Consider the following sequence:

* You have a process running on node N of your cluster.
* The cluster becomes partitioned.
* Node N is in a partition that is unable to achieve quorum.

What happens to the process? To guarantee consistency, it has to stop processing until that node can rejoin the quorum. Which means you no longer have availability for that process. Or you continue processing and risk inconsistency.

So you see, in the presence of partitioning, you can either keep consistency or availability, but not both.

CAP theorem QED.