Cassandra: Keyspaces

Keyspaces are the top level building blocks to organize the data in Cassandra. Whereas Cluster is a container for keyspaces.

If you are from RDBMS (Relational Database Management Systems) background, you can map keyspace to a database in RDBMS world. Like a database in RDBMS, a keyspace has a name and attributes (replication_factor, class etc.,) associated with it.

In RDBMS, a. database is collection of tables, whereas in Cassandra,  keyspace is a collection of column families. You can map column families to tables in RDBMS.

A keyspace contain

A keyspace can contain

a.   Tables

b.   Views

c.    User-defined Types

d.   Functions

e.   Aggregates

Apart from acting as a container for data structures like tables, indexes, a keyspace can hold some extra important attributes.

For example, while creating a keyspace, you can specify the replication startegy, this attribute deals with 'how data replications are made in cassandra cluster'.

Example

CREATE keyspace cassandratutorial WITH REPLICATION =

{

         'class' : 'SimpleStrategy',

         'replication_factor' : 3

};

Above statement creates a keyspace 'cassandraTutorial' and set the 'replication_factor' to the value 3. That means, data is replicated in 3 different nodes.

Why do we maintain replications of data?

To achieve high availability, Cassandra maintain replications of the data. For example, if a node in the Cassandra cluster fails, then the data is given from other node.

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