Databases Reference
In-Depth Information
FIGURE 4.23
Key-value pair.
FIGURE 4.24
Cassandra key-value pair (column).
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Low cost of ownership
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Minimal administration
Data model
The Cassandra data model is based on a key-value model, where we have a key that uniquely identi-
fies a value, and this value can be structured or completely unstructured or can also be a collection
of other key-value elements. This is very similar to pointers and linked lists in the world of program-
ming.
Figure 4.23
shows the basic key-value structure.
A key-value pair can represent a simple storage for Person →→Name type of data but cannot
scale much. An alteration to the basic model is done to create a name and value in the key-value pair,
and this would provide a structure to create multiple values and associate a key to the name-value
pair. This creates a tablelike structure described in
Figure 4.24
.
In the updated structure of the key-value notation, we can store Person → Name → John Doe, add
another column called Person → Age → 30, and create multiple storage structures. This defines the
most basic structure in the Cassandra data model called column.
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Column.
A column is an ordered list of values stored as a name-value pair. It is composed of a
column name, a column value, and a third element called timestamp. The timestamp is used to
manage conflict resolution on the server, when there is a conflicting list of values or columns to
be managed. The client sets the timestamp that is stored along with the data, and this is an explicit
operation.
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The column can hold any type of data in this model, varying from characters to Globally Unique
Identifiers (GUID) to blobs. Columns can be grouped into a row called a rowkey. A simple
column by itself limits the values you can represent; to add more flexibility, a group of columns
belonging to a key can be stored together, called a column family. A column family can be loosely
compared to a table in the database comparison.
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Column family.
A column family is a logical and physical grouping of a set of columns that can
be represented by a single key. The flexibility of a column family is the names of columns can
vary from a row to another and the number of columns can vary over a period of time.
Figure 4.25
shows a sample column family.
There is no limitation with creating different column structures in a column family, except the
maintenance of the same is dependent on the application that is creating the different structures.
Conceptually, it is similar to overloading in the object-oriented programming language.
