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Subject: Re: Minimum Cluster size to accommodate a single node failure
Date: Wed, 18 Jun 2014 16:40:18 +1000
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Yes your thinking is correct.

This article from TLP sums it all up beautifully =
http://thelastpickle.com/blog/2011/06/13/Down-For-Me.html=20

Ben Bromhead
Instaclustr | www.instaclustr.com | @instaclustr | +61 415 936 359

On 18 Jun 2014, at 4:18 pm, Prabath Abeysekara =
<prabathabeysekara@gmail.com> wrote:

> Sorry, the title of this thread has to be "Minimum cluster size to =
survive a single node failure".
>=20
>=20
> On Wed, Jun 18, 2014 at 11:38 AM, Prabath Abeysekara =
<prabathabeysekara@gmail.com> wrote:
> Hi Everyone,
>=20
> First of all, apologies if the $subject was discussed previously in =
this list before. I've already gone through quite a few email trails on =
this but still couldn't find a convincing answer which really made me =
raise this question again here in this list.
>=20
> If my understanding is correct, a 3 node Cassandra cluster would =
survive a single node failure while the Replication Factor is set to 3 =
with consistency levels are assigned QUORUM for read/write operations. =
For example, let's consider the following configuration.
>=20
> * Number of nodes in the cluster : 3
> * Replication Factor : 3
> * Read/Write consistencies : QUORUM (this evaluates to 2 when RF is =
set to 3)
>=20
> Here's how I expect it to work.
>=20
> Whenever a read operation takes place, the Cassandra cluster =
coordinator node that receives the read request would try to read from =
at least two replicas before responding to the client. With Read =
consistency being 2 (+ all raws being available in all three nodes), we =
should be able to survive a single node failure in this particular =
instance for read operations. Similarly, for write requests, even in the =
middle of a single node failure, the writes should be allowed as the =
Write consistency is set to 2?=20
>=20
> Can someone please confirm whether what's mentioned above is correct?=20=

> (Please note that I'm trying to figure out the minimum node numbers =
and I indeed am aware of the fact that there are other factors also to =
be considered in order to come up with the most optimal numbers for a =
given cluster requirement).
>=20
>=20
> Cheers,
> Prabath
> --=20
> Prabath
>=20
>=20
>=20
> --=20
> Prabath


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<html><head><meta http-equiv="Content-Type" content="text/html charset=us-ascii"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div>Yes your thinking is correct.</div><div><br></div><div>This article from TLP sums it all up beautifully&nbsp;<a href="http://thelastpickle.com/blog/2011/06/13/Down-For-Me.html">http://thelastpickle.com/blog/2011/06/13/Down-For-Me.html</a>&nbsp;</div><div><br><div>
<div><div><div>Ben Bromhead</div><div></div></div><div>Instaclustr |&nbsp;<a href="https://www.instaclustr.com/">www.instaclustr.com</a>&nbsp;|&nbsp;<a href="http://twitter.com/instaclustr">@instaclustr</a>&nbsp;| +61 415 936 359</div></div>

</div>
<br><div><div>On 18 Jun 2014, at 4:18 pm, Prabath Abeysekara &lt;<a href="mailto:prabathabeysekara@gmail.com">prabathabeysekara@gmail.com</a>&gt; wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><div dir="ltr">Sorry, the title of this thread has to be "<b>Minimum cluster size to survive a single node failure</b>".<div class="gmail_extra"><br><br><div class="gmail_quote">On Wed, Jun 18, 2014 at 11:38 AM, Prabath Abeysekara <span dir="ltr">&lt;<a href="mailto:prabathabeysekara@gmail.com" target="_blank">prabathabeysekara@gmail.com</a>&gt;</span> wrote:<br>


<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr">Hi Everyone,<div><br></div><div>First of all, apologies if the $subject was discussed previously in this list before. I've already gone through quite a few email trails on this but still couldn't find a convincing answer which really made me raise this question again here in this list.</div>


<div><br></div><div><div style="font-family:arial,sans-serif;font-size:13px">If my understanding is correct, a <b>3 node Cassandra cluster</b> would survive a single node failure while the Replication Factor is set to 3 with consistency levels are assigned QUORUM for read/write operations. For example, let's consider the following configuration.</div>


<div style="font-family:arial,sans-serif;font-size:13px"><br></div><div style="font-family:arial,sans-serif;font-size:13px">* Number of nodes in the cluster : 3</div><div style="font-family:arial,sans-serif;font-size:13px">


* Replication Factor : 3</div><div style="font-family:arial,sans-serif;font-size:13px">* Read/Write consistencies : QUORUM (this evaluates to 2 when RF is set to 3)</div><div style="font-family:arial,sans-serif;font-size:13px">


<br></div><div style="font-family:arial,sans-serif;font-size:13px">Here's how I expect it to work.</div><div style="font-family:arial,sans-serif;font-size:13px"><br></div><div style="font-family:arial,sans-serif;font-size:13px">


Whenever a read operation takes place, the Cassandra cluster coordinator node that receives the read request would try to read from at least two replicas before responding to the client. With Read consistency being 2 (+ all raws being available in all three nodes), we should be able to survive a single node failure in this particular instance for read operations. Similarly, for write requests, even in the middle of a single node failure, the writes should be allowed as the Write consistency is set to 2?&nbsp;</div>


<div style="font-family:arial,sans-serif;font-size:13px"><br></div><div style="font-family:arial,sans-serif;font-size:13px">Can someone please confirm whether what's mentioned above is correct?&nbsp;</div><div style="font-family:arial,sans-serif;font-size:13px">


(Please note that I'm trying to figure out the&nbsp;<b>minimum</b>&nbsp;node numbers and I indeed am aware of the fact that there are other factors also to be considered in order to come up with the most optimal numbers for a given cluster requirement).</div>


<div><br></div><div><br></div><div>Cheers,</div><div>Prabath</div><span><font color="#888888">-- <br>Prabath<br>
</font></span></div></div>
</blockquote></div><br><br clear="all"><div><br></div>-- <br>Prabath<br>
</div></div>
</blockquote></div><br></div></body></html>
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