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 <aa...@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 <ivytang0...@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
