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copied!<p>There are multiple possible meanings for the word "consistency". See, e.g., <a href="https://stackoverflow.com/questions/4813282/why-is-c-in-cap-theorem-not-same-as-c-in-acid">Why is C in CAP theorem not same as C in ACID?</a> .</p> <p>Plus, some level of debate is also possible as to the meaning of the C in 'ACID' : while it is typically defined in a sense that relates to database integrity ("no transaction shall get to see a database state that violates a declared constraint - modulo the inconsistencies that that transaction has created itself of course"), one commenter said he interpreted it as referring to "the database state as seen (or perhaps better, as effectively used) by any transaction does not change while that transaction is in progress. Paraphrased : transactions are ACID-compliant if they are executing in at least repeatable read mode.</p> <p>If you take the CAP-C to mean "all nodes see the same data at the same time", then availability is necessarily hampered because while the system is busy distributing the data to the various nodes, it cannot allow any transaction access to (the elder versions of) that data. (Unless of course access to elder versions is precisely what is needed, such as when a transaction is running under MVCC.)</p> <p>If you take the CAP-C to mean something along the lines of "no transaction can get to see an inconsistent database state", then essentially the same applies, except that it is now the user's update process that should be locking out access for all other transactions.</p> <p>If you impose a rule to the effect that "whenever a transaction has accessed a particular node N to read from some resource R (assuming R could theoretically be accessed on more than one node), then whenever that transaction accesses R again, it should do so on the same node N.", then I can imagine this will increase your guarantee of "consistency", but you pay in availability, because if node N falls out, then precisely because of the rule imposed, your transaction cannot access R anymore even if it could be done on other nodes.</p> <p>At any rate, I think that if an institution such as Berkeley comes up with a proof of some theorem, then you're on the safe side if you consider vociferous claims such as the one you mention, as marketing lies.</p>

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