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From Anthony John <>
Subject Re: R and N
Date Fri, 18 Feb 2011 17:28:16 GMT
K - let me state the facts first (As I see know them)
- I do not know the inner workings, so interpret my response with that
caveat. Although, at an architectural level, one should be able to keep
detailed implementation at bay
- Quorum is (N+!)/2 where N is the Replication Factor (RF)
- And consistency is a guarantee if R(ead) + W(rite) > RF (Which Quorum
gives you, but can be achieved via other permutations, depending on whether
Read or Write performance is desired)

No getting to your questions:-
1. If Read at Q is nondeterministic, it would likely have to read the other
(RF-Q) nodes to achieve Quorum on a deterministic value. At which point -
sync'ing all with writes should not be that expensive. But at what point
precisely the read is returned - do not know - you will have to look at the
code. IMO - at this level it should not matter.
2. Should be at the granularity of data divergence
3. Read Repair or Nodetool (which ever comes first)
4. All peer - there is no primary. There might be a connected node - but no
special role/privileges
5. Tries to Q - returns on deterministic read. If not - see (1)
6. Writer supplies timestamp value - can be any value that makes sense
within the scope of data/application.



On Fri, Feb 18, 2011 at 10:28 AM, A J <> wrote:

> 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 ?
> Thanks.
> 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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