Possibility of a CAS API

Martin Bruse zondolfin at gmail.com
Sat Feb 25 02:46:48 EST 2012


Here is the working link:
https://github.com/ProjectDaisy/blodsband/blob/master/lib/blodsband/riak/bucket.rb#L484

The universe will always punish you if you send long emails via smartphone
:/

On Sat, Feb 25, 2012 at 4:21 AM, Armon Dadgar <armon.dadgar at gmail.com>wrote:

> It sounds like the "If-Unmodified-Since" and "If-None-Match" flags could
> do what I
> need, but the docs specify "it is possible for the condition to evaluate
> to true for
> multiple requests if the requests occur at the same time."
>
> From my understanding, the KV vnode's process their requests in a serial
> fashion.
> I'm not sure I fully understand how It could be that the request evaluates
> to true
> for multiple requests, if the PUTs are handled serially.
>
> If it is a matter of the vnodes being interleaved, would it be solvable by
> setting w = r = n?
>
> I'm not convinced that a CAS operation is inevitably subject to data races.
> There are proven techniques for avoiding races at the cost of latency,
> which is acceptable in certain situations.
>
> I will take a look at Zab, thanks for the reference!
>
>  Best Regards,
>
> Armon Dadgar
>
> On Feb 24, 2012, at 6:09 PM, Dietrich Featherston wrote:
>
> If you need CAS semantics, then coordinate that outside of riak. Any
> check-then-act type of operation where atomicity is important is going to
> leave some room for a data race in a system with the distribution semantics
> of riak. Would suggest thinking about the problem in such a way that
> handling of siblings is tolerant of duplicate writes and eventually the
> correct value bubbles up to the readers. That or do the coordination of
> unique indexes in something not dynamo shaped.
>
> I can't say I'm intimately familiar with the work yet, but others have
> prototyped/postulated consistency layers on top of riak (a la zab) that
> might more closely match what you're trying to do. None of this is in a
> released / supported version of riak to my knowledge though.
>
> Thanks,
> D
>
>
> On Fri, Feb 24, 2012 at 4:41 PM, Armon Dadgar <armon.dadgar at gmail.com>wrote:
>
>> As part of a new feature we are working on, we've run into
>> a situation where it would be incredibly convenient to have a
>> check-and-set (CAS) API for Riak KV. In short, we are trying to build
>> a unique index of a bucket, using a second bucket which acts as a
>> reverse index.
>>
>> The CAS API would operate in the same manner as a PUT, except it
>> should take a "last vclock". The new value + last vclock are submitted
>> to the responsible vnodes. The vnodes respond if the last vclock
>> for the key matches the specified last value. If we get "r" nodes
>> responding
>> that the last value matches, then we should commit the write. This method
>> is basically a two-phase commit.
>>
>> It would also be great if no-value sentinel could be specified to indicate
>> the CAS should only succeed if there is not already a key. We need this
>> to make sure uniqueness constraints are not violated.
>>
>> I wanted to gauge the interest from the community in something like this,
>> and see if I could get thoughts from the Basho team on if this could be
>> implemented.
>>
>>  Best Regards,
>>
>> Armon Dadgar
>>
>>
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>
>
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