In you text , = does the coordinator means the random client that user send request to ?=A0=
Do you mean no matter how many W is assigned to , the data will = copy on N node ? Just the client will think this write action is successful= when W nodes are be written ?

Ps. The key coordinator doesn't mean a single node = that is responsible for all nodes's key range . The key coordinator is = the primary node that is responsible for a key range . If a key is in its r= ange , this node will be this key's coordinator.

On Fri, Sep 3, 2010 at 2= :36 PM, Aaron Morton wrote:
AKAIK,=A0
For read the coordinator sends the request to= the number of nodes specified in the RF. RR is kicked off on the=A0coordin= ator=A0node after the read has completed. There is no key=A0coordinator, wh= at would you do if it as down ? The first node in the list of replication n= odes is considered special, but not that special. (In a normal read only th= e first node is asked for the data, others nodes are asked for a digest)

write same as read. One hop from the coordinator node t= o the nodes that will do the write. The one hop part is discussed in the pa= per.

N is the number of copies of the data that wi= ll be stored. W is the consistency level the client is happy to accept to s= ay that the write has succeed, after W have ack'd to the=A0coordinator= =A0it will ack to the client. But it's more complicated that that, sear= ch the archives for a big discussion on Handed Hint Off=A0

If you client always operates such that R+W>N you ha= ve=A0consistency. If you drop the R down to 1 you may read data that is not= consitent with the other nodes in the ring, because the coordinator return= s to as soon as the first node does. It will then look at the result from t= he other nodes and kick off the Read Repair is needed. But this is after yo= ur read request has completed.=A0

Aaron

=A0=A0

On 03 Sep, 2010,at 0= 3:19 PM, Ying Tang <ivytang0812@gmail.com> wrote:

Recently , i read the paper a= bout Cassandra again .=A0
And now i have some concepts about =A0the rea= ding and writing .=A0

We all know Cassandra uses N= WR ,
the request ---> a random node in Cassandra .= This node acts as a proxy ,and it routes the request.
Here ,=A0
1.=A0the proxy node route this request to this key= 's coordinator , the coordinator then routes request to other N-1 nodes= =A0 OR =A0 the proxy routes the read request to N nodes ?
2. If = it is the former situation , the read repair occurs on the=A0=A0key's c= oordinator ?=A0
=A0=A0 If =A0it is the latter , the=A0=A0read repair occurs on the pro= xy node ?

When write :
the request = ---> a random node in Cassandra .This node acts as a proxy ,and it route= s the request.
Here ,=A0
3.=A0the proxy node route this request to this key= 's coordinator , the coordinator then routes request to other N-1 nodes= =A0 OR =A0 the proxy routes the request to N nodes ?

<= div>
4. The N isn't the data's copy numbers , it's ju= st a =A0range . In this =A0N range , there must be W copies .So W is the co= py numbers.
So in this N range , R+W>N can=A0guarantee the dat= a's=A0validity. Right?

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Best regards,

Ivy Tang

=

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Best regards,

Ivy Tang

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