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From Daniel Doubleday <daniel.double...@gmx.net>
Subject Re: Second Cassandra users survey
Date Mon, 07 Nov 2011 19:44:50 GMT
Well - given the example in our case the prefix that determines the endpoints where a token
should be routed to could be something like a user-id

so with 

key = "userid" + "." + "userthingid";

instead of

// this is happening right now
getEndpoints(hash(key))

you would have

getEndpoints("userid")

Since count(users) is much larger than number of nodes in the ring we would still have a balanced
cluster.

I guess what we would need is something like a compound row key

You could almost do something like this with the current code base but I remember that there
are certain assumptions about how keys translate to tokens on the ring make this impossible.


But in essence this would result in another partitioner implementation. 
So you'd have OrderPreserverPartitioner, RandomPartitioner and maybe ShardedPartitioner


On Nov 7, 2011, at 2:26 PM, Peter Lin wrote:

> This feature interests me, so I thought I'd add some comments.
> 
> Having used partition features in existing databases like DB2, Oracle
> and manual partitioning, one of the biggest challenges is keeping the
> partitions balanced. What I've seen with manual partitioning is that
> often the partitions get unbalanced. Usually the developers take a
> best guess and hope it ends up balanced.
> 
> Some of the approaches I've used in the past were zip code, area code,
> state and some kind of hash.
> 
> So my question related deterministic sharding is this, "what rebalance
> feature(s) would be useful or needed once the partitions get
> unbalanced?"
> 
> Without a decent plan for rebalancing, it often ends up being a very
> painful problem to solve in production. Back when I worked mobile
> apps, we saw issues with how OpenWave WAP servers partitioned the
> accounts. The early versions randomly assigned a phone to a server
> when it is provisioned the first time. Once the phone was associated
> to that server, it was stuck on that server. If the load on that
> server was heavier than the others, the only choice was to "scale up"
> the hardware.
> 
> My understanding of Cassandra's current sharding is consistent and
> random. Does the new feature sit some where in-between? Are you
> thinking of a pluggable API so that you can provide your own hash
> algorithm for cassandra to use?
> 
> 
> 
> On Mon, Nov 7, 2011 at 7:54 AM, Daniel Doubleday
> <daniel.doubleday@gmx.net> wrote:
>> Allow for deterministic / manual sharding of rows.
>> 
>> Right now it seems that there is no way to force rows with different row keys will
be stored on the same nodes in the ring.
>> This is our number one reason why we get data inconsistencies when nodes fail.
>> 
>> Sometimes a logical transaction requires writing rows with different row keys. If
we could use something like this:
>> 
>> prefix.uniquekey and let the partitioner use only the prefix the probability that
only part of the transaction would be written could be reduced considerably.
>> 
>> 
>> 
>> On Nov 1, 2011, at 11:59 PM, Jonathan Ellis wrote:
>> 
>>> Hi all,
>>> 
>>> Two years ago I asked for Cassandra use cases and feature requests.
>>> [1]  The results [2] have been extremely useful in setting and
>>> prioritizing goals for Cassandra development.  But with the release of
>>> 1.0 we've accomplished basically everything from our original wish
>>> list. [3]
>>> 
>>> I'd love to hear from modern Cassandra users again, especially if
>>> you're usually a quiet lurker.  What does Cassandra do well?  What are
>>> your pain points?  What's your feature wish list?
>>> 
>>> As before, if you're in stealth mode or don't want to say anything in
>>> public, feel free to reply to me privately and I will keep it off the
>>> record.
>>> 
>>> [1] http://www.mail-archive.com/cassandra-dev@incubator.apache.org/msg01148.html
>>> [2] http://www.mail-archive.com/cassandra-user@incubator.apache.org/msg01446.html
>>> [3] http://www.mail-archive.com/dev@cassandra.apache.org/msg01524.html
>>> 
>>> --
>>> Jonathan Ellis
>>> Project Chair, Apache Cassandra
>>> co-founder of DataStax, the source for professional Cassandra support
>>> http://www.datastax.com
>> 
>> 


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