On Thu, Jan 3, 2013 at 9:00 AM, <riak-users-requ...@lists.basho.com> wrote:
> From: qaspar <snoope...@gmail.com> > > Hi, > > Can someone confirm this? If it's true, what exactly is the purpose of > offering the if_not_modified flag? > > Kaspar > The consistency guarantees of if_not_modified depend on the default R value configured on the bucket properties. INM request perform an internal from the node coordinating the request get to retrieve the object to get its vclock to compare to the incoming object. The get uses the R value that is the default for the bucket. The default value of R for a bucket is "quorum", although the default for PR is 0. Thus, by default INM strongly guarantees no stale writes, but only if no primary replica nodes have failed or have become partitioned. This also assumes that you are using at least an W of "quorum", and ideally a PR of the same. If you change the default bucket PR to "quorum", then it guarantee this also when primary nodes fail or there is a partition, but it does this at the expense of availability. The INM only guarantees no stale writes, and only does so when R, PR, W, and WR are such to to guarantee strong consistency. It does not guard against concurrent writes. Multiple writers can succeed even when using INM if they write at the same time. If you have allow_mult set to true in the bucket, that means the client must handle sibling resolution. If allow_mult is set to false and last_write_wins is set to true, concurrent writes will succeed, but the value with the latest timestamp will win. If last_write_wins is set to false while allow_multi is also false, Riak will keep siblings internally, but use vector clocks to resolve the siblings, and if that fails, fall back on timestamp, to only return a single value.
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