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From Даниел Симеонов <dsimeo...@gmail.com>
Subject Re: questions about consistency
Date Wed, 21 Apr 2010 11:40:21 GMT
Hi Paul,
   about the last answer I still need some more clarifications, as I
understand it if QUORUM is used, then reads doesn't get old values either?
Or am I wrong?
Thank you very much!
Best regards, Daniel.

2010/4/21 Paul Prescod <prescod@gmail.com>

> I'm not an expert, so take what I say with a grain of salt.
>
> 2010/4/21 Даниел Симеонов <dsimeonov@gmail.com>:
> > Hello,
> >    I am pretty new to Cassandra and I have some questions, they may seem
> > trivial, but still I am pretty new to the subject. First is about the
> lack
> > of a compareAndSet() operation, as I understood it is not supported
> > currently in Cassandra, do you know of use cases which really require
> such
> > operations and how these use cases currently workaround this .
>
> I think your question is paradoxical. If the use case really requires
> the operation then there is no workaround by definition. The existence
> of the workaround implies that the use case really did not require the
> operation.
>
> Anyhow, vector clocks are probably relevant to this question and your next
> one.
>
> > Second topic I'd like to discuss a little bit more is about the read
> repair,
> > as I understand is that it is being done by the timestamps supplied by
> the
> > client application servers. Since computer clocks (which requires
> > synchronization algorithms working regularly) diverge there should be a
> time
> > frame during which the order of the client request written to the
> database
> > is not guaranteed, do you have real world experiences with this? Is this
> > similar to the casual consistency (
> > http://en.wikipedia.org/wiki/Causal_consistency ) .What happens if two
> > application servers try to update the same data and supply one and the
> same
> > timestamp (it could happen although rarely), what if they try to update
> > several columns in batch operation this way, is there a chance that the
> > column value could be intermixed between the two update requests?
>
> All of this is changing with vector clocks in Cassandra 0.7.
>
> https://issues.apache.org/jira/browse/CASSANDRA-580
>
> > I have one last question about the consistency level ALL, do you know of
> > real use cases where it is required (instead of QUORUM) and why (both
> read
> > and write)?
>
> It would be required when your business rules do not allow any client
> to read the old value. For example if it would be illegal to provide
> an obsolete stock value.
>
> > Thank you very much for your help to better understand 'Cassandra'!
> > Best regards, Daniel.
> >
>

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