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From Narendra Sharma <>
Subject Re: How to find what node a key is on
Date Thu, 24 Mar 2011 00:30:42 GMT
The logic to find the node is not complicated. You compute the MD5 hash of
the key. Create sorted list of tokens assigned to the nodes in the ring.
 Find the first token greater than the hash. This is the first node. Next in
the list is the replica, which depends on the RF. Now this is simple because
this assumes SimpleStrategy for replica placement. For other strategies
finding replicas will be more involved.

Cassandra is a distributed databases. Each node is aware of the state of the
cluster and token distribution. Moving the logic into client is possible but
the benefits are way less compared to pain. At the same time doing it for a
large cluster would be more painful.

I would discourage you from going that route.


On Wed, Mar 23, 2011 at 5:16 PM, Sameer Farooqui <>wrote:

> No problems with read performance, just curious about what kind of overhead
> was being added b/c we're doing read tests.
> If it's easy to figure out where the row is stored, I'd be interested in
> trying it. If not, don't worry about it.
> - Sameer
> On Wed, Mar 23, 2011 at 4:31 PM, aaron morton <>wrote:
>> Each row is stored on RF nodes, and your read will be sent to CL number of
>> nodes. Messages only take a single hop from the coordinator to each node the
>> read is performed on, so the networking overhead varies with the number of
>> nodes involved in the request.  There are man factors other than networking
>> that influence the speed of a read request.
>> There are features available to determine which nodes holds replicas for a
>> particular key. AFAIK they are not intended for use by clients.
>> Are you currently having problems with read performance ?
>> Hope that helps.
>> Aaron
>> On 24 Mar 2011, at 11:53, Sameer Farooqui wrote:
>> Does anybody know if it's possible to find out what node a specific
>> key/row lives on?
>> We have a 30 node cluster and I'm curious how much faster it'll be to read
>> data directly from the node that stores the data.
>> We're using random partitioner, by the way.
>> *Sameer Farooqui
>> *Accenture Technology Labs

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