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From Dr. Martin Grabmüller <Martin.Grabmuel...@eleven.de>
Subject RE: UnavailableException with 3 nodes and RF=2
Date Tue, 14 Sep 2010 08:53:49 GMT
When you write with QUORUM, RF/2+1 of the nodes cassandra *wants to write*
to have to be up.  In your case, RF/2+1 = 2, that means, the two nodes responsible
for the write have to be up, not any two nodes.  Each write which tries to the node 
with token 78502309573904554351249603414557542595  and another node
will fail.
 
QUORUM consistency only gives you more availability when you have a RF of 3 or higher.
 
Martin

________________________________

	From: Chris Jansen [mailto:chris.jansen@cognitomobile.com] 
	Sent: Tuesday, September 14, 2010 10:44 AM
	To: user@cassandra.apache.org
	Subject: UnavailableException with 3 nodes and RF=2
	
	

	Hi All,

	 

	I’m a newbie to Cassandra so I could have a configuration issue here, I am using the latest
stable release 0.6.0.

	 

	I have created a cluster of 3 nodes, a keyspace with RF=2 and a rack unaware replication
strategy. When I write with CL=QUORUM with all 3 nodes commit the data fine, but when I write
with the same CL with one of the nodes down I see an UnavailableException thrown. Surely if
one of the nodes in the cluster is down another should acknowledge the writes and maintain
the quorum, or is there something that I have misunderstood? From what I understand, in this
case with a RF=2 for the quorum writes to succeed I need two nodes to acknowledge the write
(RF/2+1), which I have.

	 

	Here is how the cluster looks when quorum writes succeed:

	 

	192.168.245.2 Up         477.33 KB     78502309573904554351249603414557542595     |<--|

	192.168.245.4 Up         426.74 KB     139625953069891725539207365034742863768    |   |

	192.168.245.1 Up         496.67 KB     163572901304139170217093255272499595459    |-->|

	 

	This is how it looks with one node down and quorum writes fail (I am writing to 192.168.245.1):

	 

	192.168.245.2 Down       423.58 KB     78502309573904554351249603414557542595     |<--|

	192.168.245.4 Up         426.74 KB     139625953069891725539207365034742863768    |   |

	192.168.245.1 Up         496.67 KB     163572901304139170217093255272499595459    |-->|

	 

	Here is the exception that is thrown:

	 

	Cannot write: 9e48b039-7687-4b14-9b40-0096b15fd7b0 RETRYING

	UnavailableException()

	                at org.apache.cassandra.thrift.Cassandra$insert_result.read(Cassandra.java:12303)

	                at org.apache.cassandra.thrift.Cassandra$Client.recv_insert(Cassandra.java:675)

	                at org.apache.cassandra.thrift.Cassandra$Client.insert(Cassandra.java:648)

	                at cassandraclient.Main.writeReadDelete(Main.java:101)

	                at cassandraclient.Main.run(Main.java:188)

	                at java.lang.Thread.run(Thread.java:619)

	 

	If I switch CL=ONE the writes succeed, but I don’t know if the data is being replicated.

	 

	Any help would be greatly appreciated, thanks.

	 

	Chris Jansen




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