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From André Fiedler <fiedler.an...@googlemail.com>
Subject Re: Facebook messaging and choice of HBase over Cassandra - what can we learn?
Date Mon, 22 Nov 2010 12:27:31 GMT
Doesn´t sync Cassandra all nodes if the network is up again? I think this
was one of the reasons, storing a timestamp at every key/value pair?
So i think the response will only temporary be 11. If all nodes have synct
it should be 12? Or isn´t that so?

greetings André

2010/11/22 Samuel Carrière <samuel.carriere@gmail.com>

> >Cassandra can work in a consistent way, see some of this discussion and
> the Consistency section here
> http://wiki.apache.org/cassandra/ArchitectureOverview
> >
> >If you always read and write with CL.Quorum (or the other way discussed)
> you will have consistency. Even if some of the replicas are temporarily
> inconsistent, or off line or whatever. Your reads will >be consistent, i.e.
> every client will get the same value or the read will not work. If you want
> to work at a lower or higher consistency you can.
> >
> >Eventually all replicas of a value will become consistent.
> >
> >There are a number of reasons why cassandra may not be a good fit, and I
> would guess something else would be a problem before the consistency model.
> >
> >Hope that helps.
> >Aaron
>
> Hello,
>
> I like cassandra a lot and I'm sure it can be used in many use cases,
> but I'm not sure we can say that we have strong consistency,
> even if we read and write with CL.Quorum.
>
> Firstly, we can only expect consistency at the column level. Reading
> and writing with CL.Quorum gives you most of the time
> a consistent value for each individual column, but it does not mean if
> gives you a consistent view of your data.
> (Because cassandra gives you no isolation and no transactions, your
> application has to deal with data inconsistencies).
>
> Secondly, I may be wrong, but I'm not sure consistency at the column
> level is guaranteed. Here is an example, with a replication
> factor of 3.
> Imagine that the current value of col1 is 11. Your application tries
> to write "col1 = 12" with CL.Quorum.
> Imagine the write arrives to node 1, but that the new value is not
> transmitted to nodes 2 and 3 because of network failures. So
> the write fails (this is the expected behaviour), but node 1 still has
> the new value (there is no rollback).
>
> Then, imagine that the network is back to normal, and that another
> client asked for the value of col1, with CL.Quorum. Here,
> the value of the response is not guaranteed. If the client asks for
> the value to node 2 and node 3, the response will be 11, but
> if he asks to node 1 and node 2 or 3, the response will be 12.
>
> Am I missing something ?
>
> Samuel
>

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