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From A J <>
Subject Re: R and N
Date Fri, 18 Feb 2011 16:28:18 GMT
Couple of more related questions:

5. For reads, does Cassandra first read N nodes or just the R nodes it
selects ? I am thinking unless it reads all the N nodes, how will it
know which node has the latest write.

6. Who decides the timestamp that gets inserted into the timestamp
field of every column. I would guess the coordinator node picks up its
system's timestamp.  If that is true, the clocks on all the nodes
should be synchronized, right ? Otherwise conflict resolution cannot
be done correctly.
For a distributed system, this is not always possible. How do folks
get around this issue ?


On Fri, Feb 18, 2011 at 10:23 AM, A J <> wrote:
> Questions about R and N (and W):
> 1. If I set R to Quorum and cassandra identifies a need for read
> repair before returning, would the read repair happen on R nodes (I
> mean subset of R that needs repair) or N nodes before the data is
> delivered to the client ?
> 2. Also does the repair happen at level of row (key) or at level of column ?
> 3. During write, if W is met but N-W is not met for some reason; would
> cassandra try to repair N-W nodes in the background as and when it
> can. Or the N-W are only repaired when a read is issued ?
> 4. What is the significance of the 'primary' replica for writes from
> usage point ? Writes to primary and non-primary replicas all happen
> simultaneously. Ensuring W is decided irrespective of it being primary
> or not. Ensuring R is decided by any of the R nodes out of N.
> I know the tokens are divided per the primary replica. But other than
> that, for read and write operations, do the primary replica play any
> special role ?
> Thanks.

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