An Overview of the NoSQL World - Large Scale and Big Data: Processing and Management - page 295

in addition to storing it locally. Node D will store the keys that fall in the ranges ( A ,
B ), ( B , C ), and ( C , D ). The list of nodes that is responsible for storing a particular key
is called the preference list. The system is designed so that every node in the system
can determine which nodes should be in this list for any particular key.

9.3 NoSQL OPEN SOURCE PROJECTS
In practice, most NoSQL data management systems that are introduced by the key
players (e.g., Bigtable, Dynamo, PNUTS) are meant for their internal use only and
are thus, not available for public users. Therefore, many open-source projects have
been built to implement the concepts of these systems and make it available for
public users [18,54]. Due to the ease in which they can be downloaded and installed,
these systems have attracted a lot of interest from the research community. There are
not many details that have been published about the implementation of most of these
systems. In general, the NoSQL open-source projects can be broadly classified into
the following categories:

• Key-value stores : These systems use the simplest data model, which is a

collection of objects where each object has a unique key and a set of attri-
bute/value pairs.

• Document stores : These systems have the data models that consists of

objects with a variable number of attributes with a possibility of having
nested objects.

• Extensible record stores : They provide variable-width tables (Column

Families) that can be partitioned vertically and horizontally across multiple
nodes.

Here, we give a brief introduction about some of these projects. For the full list,
we refer the reader to the NoSQL database website.*
Cassandra † is presented as a highly scalable, eventually consistent, distributed,
structured key-value store [44,45]. It was open-sourced by Facebook in 2008. It
is designed by Avinash Lakshman (one of the authors of Amazon's Dynamo) and
Prashant Malik (Facebook engineer). Cassandra brings together the distributed
systems technologies from Dynamo and the data model from Google's Bigtable.
Like Dynamo, Cassandra is eventually consistent. Like Bigtable, Cassandra pro-
vides a column family-based data model richer than typical key/value systems.
In Cassandra's data model, column is the lowest/smallest increment of data. It is
a tuple (triplet) that contains a name, a value, and a timestamp. A column family
is a container for columns, analogous to the table in a relational system. It con-
tains multiple columns, each of which has a name, value, and a timestamp, and
are referenced by row keys. A keyspace is the first dimension of the Cassandra
hash, and is the container for column families. Keyspaces are of roughly the same
granularity as a schema or database (i.e., a logical collection of tables) in RDBMS.

* http://NoSQL-database.org/.
† http://cassandra.apache.org/.