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From David Boxenhorn <da...@lookin2.com>
Subject Re: Facebook messaging and choice of HBase over Cassandra - what can we learn?
Date Mon, 22 Nov 2010 12:31:46 GMT
Yes, but the value is supposed to be 11, since the write failed.

On Mon, Nov 22, 2010 at 2:27 PM, André Fiedler <fiedler.andre@googlemail.com
> wrote:

> 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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