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the best way i've heard it phrased is 'given the presence of a network partition, you must choose whether to maintain consistency or availability'. this does not mean that any network partition will make data unavailable if you choose C. it only means that some network partitions will make some data unavailable to some machines. picking A does not guarantee that all data will be available to all machines in the presence of a partition either. given a CP system with 3 way replication requiring a quorum to make progress, i would argue that the set of partitions in which data becomes unavailable yet would still be available had AP been chosen is very small and not worth worrying about. in systems designed to be up 100% of the time where partitions are the exception rather than the norm CP is almost always the right choice. in systems designed for network partitons, like replication to mobile devices or laptops or whatever, AP is almost always the right choice. the problem with trying to apply the CAP theorem to the real world is that the CAP theorem's definition of availability is not the same as most people's definition of availability in practice.



> in systems designed for network partitons, like replication to mobile devices or laptops or whatever, AP is almost always the right choice.

Although it is pretty much the only choice for general purpose file sync, it's still not a good choice. It is difficult to train non-technical staff to deal with inconsistencies on resync, and they don't want to have to deal with it. I've had success with Synctus precisely because it guarantees consistency (it is CP, and any one node keeps the A for a given file). Of course, this only works for mostly-on systems.

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I take this to mean that if the nodes are disconnected from each other, Synctus disallows all access to certain files on node A; i.e. the files that node B currently owns.

Do I understand correctly?

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Yes, that's right. Although currently it provides read-only access if a replica (not necessarily known to be the latest) is available, so I suppose that's not quite fully C in the read-only case. In the future, this might be a configurable option.

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yes that's certainly true. if there isn't a sensible option for a merge policy and non technical users have to resolve it manually you're fighting a losing battle in an AP system.

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