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From Kyle Gibson <kyle.gib...@frozenonline.com>
Subject Re: Replicating to all nodes
Date Thu, 14 Jul 2011 00:32:07 GMT
Thanks for the reply Peter.

The goal is to configure a cluster in which reads and writes can
complete successfully even if only 1 node is online. For this to work,
each node would need the entire dataset. Your example of a 3 node ring
with RF=3 would satisfy this requirement. However, if two nodes are
offline, CL.QUORUM would not work, I would need to use CL.ONE. If all
3 nodes are online, CL.ONE is undershooting, I would want to use
CL.QUORUM (or maybe CL.ALL). Or does CL.ONE actually function this
way, somewhat?

A complication occurs when you want to add another node. Now there's a
4 node ring, but only 3 replicas, so each node isn't guaranteed to
have all of the data, so the cluster can't completely function when
N-1 nodes are offline. So this is why I would like to have the RF
scale relative to the size of the cluster. Am I mistaken?

Thanks!

On Wed, Jul 13, 2011 at 6:41 PM, Peter Schuller
<peter.schuller@infidyne.com> wrote:
>> Read and write operations should succeed even if only 1 node is online.
>>
>> When a read is performed, it is performed against all active nodes.
>
> Using QUORUM is the closest thing you get for reads without modifying
> Cassandra. You can't make it wait for all nodes that happen to be up.
>
>> When a write is performed, it is performed against all active nodes,
>> inactive/offline nodes are updated when they come back online.
>
> Writes always go to all nodes that are up, but if you want to wait for
> them before returning "OK" to the client than no - except CL.ALL
> (which means you don't survive one being down) and CL.QUORUM (which
> means you don't wait for all if all are up).
>
>> I don't believe it does. Currently the replication factor is hard
>> coded based on key space, not a function of the number of nodes in the
>> cluster. You could say, if N = 7, configure replication factor = 7,
>> but then if only 6 nodes are online, writes would fail. Is this
>> correct?
>
> No. Reads/write fail according to the consistency level. The RF +
> consistency level tells how many nodes must be up and successfully
> service the request in order for the operation to succeed. RF just
> tells you the number of total nodes int he replicate set for a key;
> whether an operation fails is up to the consistency level.
>
> I would ask: Why are you trying to do this? It really seems you're
> trying to do the "wrong" thing. Why would you ever want to replicate
> to all? If you want 3 copies in total, then do RF=3 and keep a 3 node
> ring. If you need more capacity, you add nodes and retain RF. If you
> need more redundancy, you have to increase RF. Those are two very
> different axis along which to scale. I cannot think of any reason why
> you would want to tie RF to the total number of nodes.
>
> What is the goal you're trying to achieve?
>
> --
> / Peter Schuller (@scode on twitter)
>

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