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Date: Fri, 18 Feb 2011 11:28:16 -0600
Message-ID: <AANLkTimyuBF384dYc+gA3p09Ztj+0W-aDJvzvSQGXfAi@mail.gmail.com>
Subject: Re: R and N
From: Anthony John <chirayithaj@gmail.com>
To: user@cassandra.apache.org
Cc: A J <s5alye@gmail.com>
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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.

HTH,

-JA

On Fri, Feb 18, 2011 at 10:28 AM, A J <s5alye@gmail.com> 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 <s5alye@gmail.com> 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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K - let me state the facts first (As I see know them)<div>- I do not know t=
he inner workings, so interpret my response with that caveat. Although, at =
an architectural level, one should be able to keep detailed implementation =
at bay</div>
<div>- Quorum is (N+!)/2 where N is the Replication Factor (RF)</div><div>-=
 And consistency is a guarantee if R(ead) + W(rite) &gt; RF (Which Quorum g=
ives you, but can be achieved via other permutations, depending on whether =
Read or Write performance is desired)</div>
<div><br></div><div>No getting to your questions:-=A0<br><div>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&#39;ing =
all with writes should not be that expensive. But at what point precisely t=
he read is returned - do not know - you will have to look at the code. IMO =
- at this level it should not matter.</div>
<div>2. Should be at the granularity of data divergence</div><div>3. Read R=
epair or Nodetool (which ever comes first)</div><div>4. All peer - there is=
 no primary. There might be a connected node - but no special role/privileg=
es</div>
<div>5. Tries to Q - returns on deterministic read. If not - see (1)</div><=
div>6. Writer supplies timestamp value - can be any value that makes sense =
within the scope of data/application.</div><div><br></div><div>HTH,</div>
<div><br></div><div>-JA</div><div><br><div class=3D"gmail_quote">On Fri, Fe=
b 18, 2011 at 10:28 AM, A J <span dir=3D"ltr">&lt;<a href=3D"mailto:s5alye@=
gmail.com" target=3D"_blank">s5alye@gmail.com</a>&gt;</span> wrote:<br><blo=
ckquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #c=
cc solid;padding-left:1ex">

Couple of more related questions:<br>
<br>
5. For reads, does Cassandra first read N nodes or just the R nodes it<br>
selects ? I am thinking unless it reads all the N nodes, how will it<br>
know which node has the latest write.<br>
<br>
6. Who decides the timestamp that gets inserted into the timestamp<br>
field of every column. I would guess the coordinator node picks up its<br>
system&#39;s timestamp. =A0If that is true, the clocks on all the nodes<br>
should be synchronized, right ? Otherwise conflict resolution cannot<br>
be done correctly.<br>
For a distributed system, this is not always possible. How do folks<br>
get around this issue ?<br>
<br>
Thanks.<br>
<div><div></div><div><br>
<br>
<br>
On Fri, Feb 18, 2011 at 10:23 AM, A J &lt;<a href=3D"mailto:s5alye@gmail.co=
m" target=3D"_blank">s5alye@gmail.com</a>&gt; wrote:<br>
&gt; Questions about R and N (and W):<br>
&gt; 1. If I set R to Quorum and cassandra identifies a need for read<br>
&gt; repair before returning, would the read repair happen on R nodes (I<br=
>
&gt; mean subset of R that needs repair) or N nodes before the data is<br>
&gt; delivered to the client ?<br>
&gt; 2. Also does the repair happen at level of row (key) or at level of co=
lumn ?<br>
&gt;<br>
&gt; 3. During write, if W is met but N-W is not met for some reason; would=
<br>
&gt; cassandra try to repair N-W nodes in the background as and when it<br>
&gt; can. Or the N-W are only repaired when a read is issued ?<br>
&gt;<br>
&gt; 4. What is the significance of the &#39;primary&#39; replica for write=
s from<br>
&gt; usage point ? Writes to primary and non-primary replicas all happen<br=
>
&gt; simultaneously. Ensuring W is decided irrespective of it being primary=
<br>
&gt; or not. Ensuring R is decided by any of the R nodes out of N.<br>
&gt; I know the tokens are divided per the primary replica. But other than<=
br>
&gt; that, for read and write operations, do the primary replica play any<b=
r>
&gt; special role ?<br>
&gt;<br>
&gt; Thanks.<br>
&gt;<br>
</div></div></blockquote></div><br></div>
</div>

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