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From Tyler Hobbs <>
Subject Re: Key to Token clarification
Date Sat, 06 Nov 2010 18:43:37 GMT
Minor correction: instead of replicating at the 'previous (replicas-1)
nodes' it's replicated to the next (RF - 1) nodes clockwise, assuming you're
using SimpleStrategy.  NetworkTopologyStrategy and similar skip nodes in the
ring to make sure you meet your cross-datacenter requirements.

Which node a key goes to depends on the partitioner.  If you're using
RandomPartitioner, then you take an MD5 hash of the key, and that gives you
a location on the ring.  The order preserving partitioners use the actual
key instead of a hash.

- Tyler

On Fri, Nov 5, 2010 at 5:54 PM, Christian Decker <
> wrote:

> Hi all,
> I'm trying to figure out some minor understanding problems. As I see it for
> each Keyspace each node takes care about a certain tokenRange (describe_ring
> gives me the assignment), these TokenRanges have a list of nodes that hold
> replicas, a start token and an end token. The tokens are of type byte[] and
> can be interpreted as BigInteger. Tokens are usually taken care of at the
> node with the next higher token, and the previous (replicas-1)-nodes. Should
> a token be larger than the largest token of a node it'll wrap around to the
> smallest. So if I have a single node it'll tell me that it has TokenRange X
> to X (X being the same token in both cases), that means that it basically
> takes care of the entire possible token range, right?
> So far it's all DHT as usual, the point I'm having problems is the mapping
> of the byte[] keys of the Column Family system to the token system used to
> identify the responsible nodes for a given key. You guessed it I want to be
> able to, given a key, find the node that is storing the value that goes with
> it.
> Also how reliable are the informations I'm getting from describe_ring are.
> What actions trigger a token assignment change and what affects which nodes
> change the nodes holding the replicas (new node join aside)?
> Regards,
> Chris

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