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From aaron morton <aa...@thelastpickle.com>
Subject Re: Behavior on inconsistent reads
Date Mon, 14 May 2012 10:56:18 GMT
What sort of corruption are you thinking about ?

Whenever the first CL nodes involved in a read do not agree on the "current" value a  process
is run to resolve their differences. This can result in an a node that is out of sync getting
repaired.

Cheers

-----------------
Aaron Morton
Freelance Developer
@aaronmorton
http://www.thelastpickle.com

On 11/05/2012, at 11:17 PM, Carpenter, Curt wrote:

> I (now) understand that the point of this is to get the most recent copy (at least of
the nodes checked) if all replicas simply haven’t been updated to the latest changes. But
what about dealing with corruption? What if the most recent copy is corrupt? With a Zookeeper-based
transaction system on top, corruption is all I’m worried about.
>  
> From: Dave Brosius [mailto:dbrosius@mebigfatguy.com] 
> Sent: Thursday, May 10, 2012 10:03 PM
> 
> If you read at Consistency of at least quorum, you are guaranteed that at least one of
the nodes has the latest data, and so you get the right data. If you read with less than quorum
it would be possible for all the nodes that respond to have stale data.
> 
> On 05/10/2012 09:46 PM, Carpenter, Curt wrote:
> Hi all, newbie here. Be gentle.
>  
> From http://www.datastax.com/docs/1.0/cluster_architecture/about_client_requests:
> “Thus, the coordinator first contacts the replicas specified by the consistency level.
The coordinator will send these requests to the replicas that are currently responding most
promptly. The nodes contacted will respond with the requested data; if multiple nodes are
contacted, the rows from each replica are compared in memory to see if they are consistent.
If they are not, then the replica that has the most recent data (based on the timestamp) is
used by the coordinator to forward the result back to the client.
> 
> To ensure that all replicas have the most recent version of frequently-read data, the
coordinator also contacts and compares the data from all the remaining replicas that own the
row in the background, and if they are inconsistent, issues writes to the out-of-date replicas
to update the row to reflect the most recently written values. This process is known as read
repair. Read repair can be configured per column family (usingread_repair_chance), and is
enabled by default.
> 
> For example, in a cluster with a replication factor of 3, and a read consistency level
of QUORUM, 2 of the 3 replicas for the given row are contacted to fulfill the read request.
Supposing the contacted replicas had different versions of the row, the replica with the most
recent version would return the requested data. In the background, the third replica is checked
for consistency with the first two, and if needed, the most recent replica issues a write
to the out-of-date replicas.”
> 
>  
> Always returns the most recent? What if the most recent write is corrupt? I thought the
whole point of a quorum was that consistency is verified before the data is returned to the
client. No?
>  
> Thanks,
>  
> Curt


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