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From Ying Tang <ivytang0...@gmail.com>
Subject Re: the process of reading and writing
Date Fri, 03 Sep 2010 06:49:35 GMT
Hi Aaron
Thanks for your reply.

In you text , does the coordinator means the random client that user send
request to ?
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 range , this node will be
this key's coordinator.


On Fri, Sep 3, 2010 at 2:36 PM, Aaron Morton <aaron@thelastpickle.com>wrote:

> AKAIK,
> For read the coordinator sends the request to the number of nodes specified
> in the RF. RR is kicked off on the coordinator node after the read has
> completed. There is no key coordinator, what would you do if it as down ?
> The first node in the list of replication nodes is considered special, but
> not that special. (In a normal read only the first node is asked for the
> data, others nodes are asked for a digest)
>
> write same as read. One hop from the coordinator node to the nodes that
> will do the write. The one hop part is discussed in the paper.
>
> N is the number of copies of the data that will be stored. W is the
> consistency level the client is happy to accept to say that the write has
> succeed, after W have ack'd to the coordinator it will ack to the client.
> But it's more complicated that that, search the archives for a big
> discussion on Handed Hint Off
>
> If you client always operates such that R+W>N you have consistency. 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 returns to as soon as the first
> node does. It will then look at the result from the other nodes and kick off
> the Read Repair is needed. But this is after your read request has
> completed.
>
> Aaron
>
>
>
> On 03 Sep, 2010,at 03:19 PM, Ying Tang <ivytang0812@gmail.com> wrote:
>
> Recently , i read the paper about Cassandra again .
> And now i have some concepts about  the reading and writing .
>
> We all know Cassandra uses NWR ,
> When read :
> the request ---> a random node in Cassandra .This node acts as a proxy ,and
> it routes the request.
> Here ,
> 1. the proxy node route this request to this key's coordinator , the
> coordinator then routes request to other N-1 nodes   OR   the proxy routes
> the read request to N nodes ?
> 2. If it is the former situation , the read repair occurs on the  key's
> coordinator ?
>    If  it is the latter , the  read repair occurs on the proxy node ?
>
> When write :
> the request ---> a random node in Cassandra .This node acts as a proxy ,and
> it routes the request.
> Here ,
> 3. the proxy node route this request to this key's coordinator , the
> coordinator then routes request to other N-1 nodes   OR   the proxy routes
> the request to N nodes ?
>
>
> 4. The N isn't the data's copy numbers , it's just a  range . In this  N
> range , there must be W copies .So W is the copy numbers.
> So in this N range , R+W>N can guarantee the data's validity. Right?
>
>
>
>
> --
> Best regards,
>
> Ivy Tang
>
>
>
>


-- 
Best regards,

Ivy Tang

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