Isn’t this a “no true Scotsman” argument? By this definition any eventually consistent system can never be considered linearizable. Right?
==================== Jordan Zimmerman > On Aug 18, 2019, at 1:47 PM, Karolos Antoniadis <karo...@gmail.com> wrote: > > Hi Jordan, > > When Aphyr tested ZooKeeper, he did not seem to know that it is not > linearizable. See here: https://github.com/jepsen-io/jepsen/issues/399, where > I pointed-out that even with *sync + read*, ZooKeeper might return stale > data. > > ZooKeeper can only be considered linearizable if we assume that specific > timing constraints apply. Naturally, in a real system, we cannot make such > assumptions if we want to be 100% safe. > For instance, if the time(TCP timeout) > (syncLimit * tickTime), ZooKeeper > provides linearizable reads. However, if this does not hold (e..g, skewed > clocks), then ZooKeeper might return stale data. > To conclude, I do not think we can argue that ZooKeeper is linearizable. > > Cheers, > Karolos > > > On Sun, 18 Aug 2019 at 11:34, Jordan Zimmerman <jor...@jordanzimmerman.com> > wrote: > >> ZooKeeper _is_ linearizable. I’m pretty sure the ZAB paper talks about it. >> Aphyr does as well here: https://aphyr.com/posts/291-jepsen-zookeeper >> >> ==================== >> Jordan Zimmerman >> >>> On Aug 18, 2019, at 1:23 PM, Karolos Antoniadis <karo...@gmail.com> >> wrote: >>> >>> Hello everyone, >>> >>> I was wondering on the exact consistency guarantees that ZooKeeper >> provides. >>> It seems that ZooKeeper does not provide strong consistency (i.e., >>> linearizability) since reads could potentially return arbitrarily old >>> values. >>> On the other hand, ZooKeeper provides sequential consistency, since the >>> order of operations of a specific client is respected and all operations >>> appear to take place in some total order ( >>> https://jepsen.io/consistency/models/sequential). >>> However, ZooKeeper provides linearizable writes, and therefore it >> provides >>> something stronger than sequential consistency, but still not as strong >> as >>> linearizability. In other words, ZooKeeper guarantees are somewhere >> between >>> sequential consistency and linearizability. >>> Is there a specific name for the specific consistency guarantees that >>> ZooKeeper provides? >>> What would the ZooKeeper community claim about the consistency guarantees >>> of ZooKeeper? >>> >>> Best Regards, >>> Karolos >>
