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From "Matthias Broecheler (JIRA)" <j...@apache.org>
Subject [jira] [Created] (CASSANDRA-9231) Support Routing Key as part of Partition Key
Date Thu, 23 Apr 2015 18:14:39 GMT
Matthias Broecheler created CASSANDRA-9231:
----------------------------------------------

             Summary: Support Routing Key as part of Partition Key
                 Key: CASSANDRA-9231
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-9231
             Project: Cassandra
          Issue Type: Wish
          Components: Core
            Reporter: Matthias Broecheler


Provide support for sub-dividing the partition key into a routing key and a non-routing key
component. Currently, all columns that make up the partition key of the primary key are also
routing keys, i.e. they determine which nodes store the data. This proposal would give the
data modeler the ability to designate only a subset of the columns that comprise the partition
key to be routing keys. The non-routing key columns of the partition key identify the partition
but are not used to determine where to store the data.

Consider the following example table definition:
CREATE TABLE foo (
  a int,
  b int,
  c int,
  d int,
  PRIMARY KEY  (([a], b), c ) );

(a,b) is the partition key, c is the clustering key, and d is just a column. In addition,
the square brackets identify the routing key as column a. This means that only the value of
column a is used to determine the node for data placement (i.e. only the value of column a
is murmur3 hashed to compute the token). In addition, column b is needed to identify the partition
but does not influence the placement.

This has the benefit that all rows with the same routing key (but potentially different non-routing
key columns of the partition key) are stored on the same node and that knowledge of such co-locality
can be exploited by applications build on top of Cassandra.
Currently, the only way to achieve co-locality is within a partition. However, this approach
has the limitations that: a) there are theoretical and (more importantly) practical limitations
on the size of a partition and b) rows within a partition are ordered and an index is build
to exploit such ordering. For large partitions that overhead is significant if ordering isn't
needed.
In other words, routing keys afford a simple means to achieve scalable node-level co-locality
without ordering while clustering keys afford page-level co-locality with ordering. As such,
they address different co-locality needs giving the data modeler the flexibility to choose
what is needed for their application.



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