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From "Dominique De Vito (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-5054) new partitioner for rowkey pairs (P1, P2) targeting hierarchical data
Date Tue, 11 Dec 2012 13:55:22 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-5054?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13528973#comment-13528973
] 

Dominique De Vito commented on CASSANDRA-5054:
----------------------------------------------

Let's analyze the number of data communicated between Cassandra nodes, for RandomPartitioner
and  RandomPartitioner4Pair (the new submitted partitioner).

The test is the following:
* we read/write columns associated with rowkeys like (P1, _) where P1 is fixed/constant and
the second element of the pair is changing.
* N is the number of nodes of the cluster
* L is the number of lines read or written for a given replica, for the test
* all cluster nodes are used as (possible) coordinators.
* RF=3

We evaluate the exchanges' cost between the coordinator and other nodes, to determine the
reponse's cost for each partitioner.


* RandomPartitioner

Hypothesis: each node stores 1/N of all the lines, and the read or write lines (of the tests)
are equally distributed within the cluster.

So, the coordinator stores itself L / N lines among those used for responding to the request.

The number of communicated lines between the coordinator and other nodes is: 2 * L/N + 3 *
(L – L/N)

The number of communications between the coordinator and other nodes is: N-1


* RandomPartitioner4Pair (new partitioner)

As rowkeys requested are like (P1, _), with P1 fixed, all rowkeys are expected to be stored
on 3 nodes only (due to RF=3).

2 cases:
1)	the coordinator is one of the 3 nodes storing the requested lines
2)	the other case

For lines:
1) the number of communicated lines between the coordinator and other nodes is: 2 * L
2) the number of communicated lines between the coordinator and other nodes is: 3 * L

In average: (3 * 2 * L + (N-3) * 3 * L) / N

For communications:
1) the number of communications between the coordinator and other nodes is: 2
2) the number of communications between the coordinator and other nodes is: 3

In average: (3 * 2 + (N-3) * 3) / N


* Conclusion (1/2)

- For the number of communicated lines between the coordinator and other nodes: both partitioners
are roughly quite equal.

RandomPartitioner4Pair: (3 * 2 * L + (N-3) * 3 * L) / N
= 3 L * (N-1) / N
= L * (3 N – 3) / N

RandomPartitioner: 2 * L/N + 3 * (L – L/N)
= 3 * L – L / N
= L * (3 – 1 / N)
= L * (3 N – 1) / N

In fact, RandomPartitioner4Pair has a little advantage over RandomPartitioner : cost(RandomPartitioner4Pair)
< cost(RandomPartitioner). 

Here, all cluster nodes are used as (possible) coordinators. So, the calculated costs above
are average costs.
So, in average, RandomPartitioner4Pair (the submitted new partitioner) is a little better
than RandomPartitioner.

But, if we use Astyanax, it's possible to choose the coordinator among the replica.
And in that case, the cost of using RandomPartitioner4Pair is then much better than RandomPartitioner.

- For the number of communications between the coordinator and other nodes: RandomPartitioner4Pair
has an advantage.


* Conclusion (2/2)

RandomPartitioner4Pair (the submitted new partitioner) is a little better than RandomPartitioner
in average, or much more better than RandomPartitioner when used with Astyanax, when choosing
the coordinator among the replica.



                
> new partitioner for rowkey pairs (P1, P2) targeting hierarchical data
> ---------------------------------------------------------------------
>
>                 Key: CASSANDRA-5054
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-5054
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Contrib, Core
>            Reporter: Dominique De Vito
>            Priority: Trivial
>         Attachments: RandomPartitioner4Pair.java
>
>
> This new partitioner is submitted here both for validation and for proposal to other
Cassandra users (may be this partitioner has its place within Cassandra core, or into some
'contrib' directory).
> This new partitioner is a variant of RandomPartitioner with special token computation
for composite rowkeys of size 2.
> The use case of this partitioner is about rowkeys of hierarchical data, that is, rowkeys
like (directory id, file id).
> This partitioner computes tokens so that rows with same "directory id" have a great chance
to be on the same node: the goal is that, when Cassandra is asked about multiple file ids
for the same directory id, a limited number of nodes should be asked.
> So, in case of a composite rowkey of size 2 like (P1, P2), the partitioner computes the
token as follows: <code>merge(getHighBits(md5(P1)), getLowBits(md5(P2)))</code>.
> In case of a rowkey that is NOT a pair, the partitioner returns the same value than RandomPartitioner.
> This partitioner is expected to be used with Cassandra 1.1 or above.
> Cassandra stores in sstables the pair (token, rowkey). Since v1.1, during a compaction
phase, Cassandra does not sort anymore on token only, but 
> sort on the pair (token, rowkey). So, if a custom partitioner produces token collisions
(that is, multiple rowkeys with the same token), it won't be a pb with v1.1 because, in order
to retrieve a row, Cassandra is going to compare the full pair (token, rowkey).
> As the percent of row collisions for this partitioner is unknown, it's expected to be
used with Cassandra 1.1 or above.

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