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Date: Thu, 15 Apr 2010 18:16:26 +0300
Message-ID: <h2xe4b4609b1004150816idea74600nb94843b5b3680179@mail.gmail.com>
Subject: RackAware and replication strategy
From: Ran Tavory <rantav@gmail.com>
To: user@cassandra.apache.org
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I'm reading this on this page
http://wiki.apache.org/cassandra/ArchitectureInternals :

 AbstractReplicationStrategy controls what nodes get secondary, tertiary,
> etc. replicas of each key range. Primary replica is always determined by the
> token ring (in TokenMetadata) but you can do a lot of variation with the
> others. RackUnaware just puts replicas on the next N-1 nodes in the ring.
> RackAware puts the first non-primary replica in the next node in the ring in
> ANOTHER data center than the primary; then the remaining replicas in the
> same as the primary.


So I just want to make sure I got this right and that documentation is up to
date.
I have two data centers and rack-aware.

When replication factor is 2: is it always the case that the primary replica
goes to one DC and the second replica to the second DC?
When replication factor is 3: First replica in DC1, second in DC2 and third
in DC1
When replication factor is 4: First replica in DC1, second in DC2, third in
DC1, fourth in DC1 etc

If I have 4 hosts in each DC, which replication factors make sense?
N=1 - When I don't care about losing data, cool
N=2 - When I want to make sure each DC has a copy; useful for local fast
access and allows recovery if only one host down.
N=3 - If I want to make sure each DC has a copy plus recovery can be made
faster in certain cases, and more resilient to two hosts down.
N=4 - Like N=3 but even more resilient. etc

Say I want to have two replicas in each DC, can this be done?

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<div dir=3D"ltr"><div class=3D"gmail_quote"><div dir=3D"ltr"><div>I&#39;m r=
eading this on this page=C2=A0<a href=3D"http://wiki.apache.org/cassandra/A=
rchitectureInternals" target=3D"_blank">http://wiki.apache.org/cassandra/Ar=
chitectureInternals</a>=C2=A0:</div>
<div><br></div>
<blockquote class=3D"gmail_quote" style=3D"margin-top:0px;margin-right:0px;=
margin-bottom:0px;margin-left:0.8ex;border-left-width:1px;border-left-color=
:rgb(204, 204, 204);border-left-style:solid;padding-left:1ex">
AbstractReplicationStrategy controls what nodes get secondary, tertiary, et=
c. replicas of each key range. Primary replica is always determined by the =
token ring (in TokenMetadata) but you can do a lot of variation with the ot=
hers. RackUnaware just puts replicas on the next N-1 nodes in the ring. Rac=
kAware puts the first non-primary replica in the next node in the ring in A=
NOTHER data center than the primary; then the remaining replicas in the sam=
e as the primary.</blockquote>

<div><br></div><div>So I just want to make sure I got this right and that d=
ocumentation is up to date.</div><div>I have two data centers and rack-awar=
e.</div><div><br></div><div>When replication factor is 2: is it always the =
case that the primary replica goes to one DC and the second replica to the =
second DC?</div>

<div>When replication factor is 3: First replica in DC1, second in DC2 and =
third in DC1</div><div>When replication factor is 4:=C2=A0First replica in =
DC1, second in DC2, third in DC1, fourth in DC1 etc</div><div><br></div><di=
v>

If I have 4 hosts in each DC, which replication factors make sense?</div><d=
iv>N=3D1 - When I don&#39;t care about losing data, cool</div><div>N=3D2 - =
When I want to make sure each DC has a copy; useful for local fast access a=
nd allows recovery if only one host down.</div>

<div>N=3D3 -=C2=A0If I want to make sure each DC has a copy plus recovery c=
an be made faster in certain cases, and more resilient to two hosts down.</=
div><div><div>N=3D4 - Like N=3D3 but even more resilient. etc</div></div><d=
iv><br>

</div><div>Say I want to have two replicas in each DC, can this be done?</d=
iv></div>
</div><br></div>

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