Apache Cassandra Learning Step by Step (2): Core Concepts

====15 Feb 2012, by Bright Zheng (IT进行时)====

3. Core Concepts

3.1.  Keyspace

3.1.1.  Intro

A keyspace is the first dimension of the Cassandra hash, and is the container for the ColumnFamilies. Keyspaces are of roughly the same granularity as a schema or database (i.e. a logical collection of tables) in the RDBMS world. They are the configuration and management point for column families, and is also the structure on which batch inserts are applied. In most cases you will have one Keyspace for an application.

3.1.2. CLI

[default@unknown] drop keyspace Tutorial; 876ee520-571f-11e1-0000-242d50cf1ffd Waiting for schema agreement... ... schemas agree across the cluster [default@unknown] create keyspace Tutorial ...  with strategy_options = [{replication_factor:1}] ...  and placement_strategy = 'org.apache.cassandra.locator.SimpleStrategy'; WARNING: [{}] strategy_options syntax is deprecated, please use {} 8daea060-571f-11e1-0000-242d50cf1ffd Waiting for schema agreement... ... schemas agree across the cluster [default@unknown] use Tutorial; Authenticated to keyspace: Tutorial [default@Tutorial]

3.2. Column Family

3.2.1. Intro

A column family is a container for columns, something like the TABLE in a relational system.

Model representation:

ColumnFamily
key	list
binary	1 .. * Columns

Data representation:

ColumnFamily
key	Columns
1	name	value	timestamp
	"firstname"	"Ronald"	1270073054
	"lastname"	"Mathies"	1270073054
	"birthday"	"01/01/1978"	1270073054
2	name	value	timestamp
	"firstname"	"John"	1270084021
	"lastname"	"Steward"	1270084021
	"birthday"	"01/01/1982"	1270084021

3.2.2. CLI

[default@Tutorial] drop column family StateCity; StateCity not found in current keyspace. [default@Tutorial] create column family StateCity ...         with comparator = LongType ...         and default_validation_class = 'UTF8Type' ...         and key_validation_class = 'UTF8Type'; d3cfa8f0-571f-11e1-0000-242d50cf1ffd Waiting for schema agreement... ... schemas agree across the cluster [default@Tutorial]

Where:

1. Comparator is used to validate and compare/sort Column names in the CF. It has following supported types:

Type	Description
BytesType	Simple non-validating byte comparison (Default)
AsciiType	Similar to BytesType, but validates that input is US-ASCII
UTF8Type	UTF-8 encoded string comparison
LongType	Compares values as 64 bit longs
LexicalUUIDType	128 bit UUID compared by byte value
TimeUUIDType	Timestamp compared 128 bit version 1 UUID

Note:

1. The above types are changeable subject to different versions.
2. It is also valid to specify the fully-qualified class name to a customized class that extends org.apache.Cassandra.db.marshal.AbstractType.

3.3. Column

3.3.1. Intro

A Column consists of a name, value and a timestamp.

Model representation:

Column
name	Binary
value	Binary
timestamp	i64

Data representation:

Column
name	value	timestamp
"firstname"	"Ronald"	1270073054

3.3.2. CLI

N/A

3.4. SuperColumn

3.4.1. Intro

A SuperColumn is very similar to a ColumnFamily, it consists of a key and a list of Columns.

Model representation:

SuperColumn
key	list
binary	1 .. * Columns

Data representation:

SuperColumn
key	Columns
1	name	value	timestamp
	"firstname"	"Ronald"	1270073054
	"lastname"	"Mathies"	1270073054
	"birthday"	"01/01/1978"	1270073054
2	name	value	timestamp
	"firstname"	"John"	1270084021
	"lastname"	"Steward"	1270084021
	"birthday"	"01/01/1982"	1270084021

The only difference to ColumnFamily is the usage. A SuperColumn is used within a ColumnFamily. So it adds an extra layer in your data structure, instead of having only a row which consists of a key and a list of columns. We can now have a row which consists of a key and a list of super columns which by itself has keys and per key a list of columns.

Once the ColumnFamily uses SuperColumn, the column type must be “Super”. By default the column type is “Standard” which means common Column.

3.4.2. CLI (TODO)

To be added here!

3.5. Others?

Please refer to http://wiki.apache.org/cassandra/API for more.