Data Model
==========

Graph
-----

`Neo4j`_  uses a `Graph`_ data model.

.. image:: ../../images/Neo4j-DataModel.svg

* A Graph contains `Nodes`_ and `Relationships`_.
* `Nodes`_ have `Properties`_.
* `Relationships`_ organize the graph.
* `Relationships`_ have `Properties`_.
* `Relationships`_ are directed links between nodes.
* `Properties`_ are key-value pairs where the key is a string.
* The values of `Properties`_ in `Neo4j`_ can have one of several `Types`_
* Queries are done by `Traversing`_ the Graph navigating from one Node to another along a `Path`_.
* Indexes look-up `Nodes`_ or `Relationships`_.

Comparison
----------

* A graph database `transforms a RDBMS`_

 * If one takes the stack of rows stored in RDBMS tables, separate the rows as individual nodes and keeps the relationships, the data becomes visible as a graph.

* A graph database `creates relationsips in Column-Family`_

 * A Column-Family database uses *families* to group rows.
 * When stored in a graph the families can become hierarchical. 

* A graph database `navigates a document store`_

 * A document store can be seen as a graph, by asking *what is the relationship between a field of the document and the document ?*

* A graph database `elaborates a Key-Value store`_

 * When the values have Keys themselves, the underlying structure is the one of a graph.

Internals
---------

You don't need to read this section if you are only interested in using Neo4j in an application.

But if you are curious about how Neo4j does the internal magic, you may find this interesting.

The `internal storage`_ of `Neo4j`_ uses a `double-linked-list`_ data structure.

The content of these data structures is stored the following collection of files:

* /var/lib/neo4j/data/graph.db/neostore
* /var/lib/neo4j/data/graph.db/neostore.id
* /var/lib/neo4j/data/graph.db/neostore.nodestore.db
* /var/lib/neo4j/data/graph.db/neostore.nodestore.db.id
* /var/lib/neo4j/data/graph.db/neostore.propertystore.db
* /var/lib/neo4j/data/graph.db/neostore.propertystore.db.arrays
* /var/lib/neo4j/data/graph.db/neostore.propertystore.db.arrays.id
* /var/lib/neo4j/data/graph.db/neostore.propertystore.db.id
* /var/lib/neo4j/data/graph.db/neostore.propertystore.db.index
* /var/lib/neo4j/data/graph.db/neostore.propertystore.db.index.id
* /var/lib/neo4j/data/graph.db/neostore.propertystore.db.index.keys
* /var/lib/neo4j/data/graph.db/neostore.propertystore.db.index.keys.id
* /var/lib/neo4j/data/graph.db/neostore.propertystore.db.strings
* /var/lib/neo4j/data/graph.db/neostore.propertystore.db.strings.id
* /var/lib/neo4j/data/graph.db/neostore.relationshipstore.db
* /var/lib/neo4j/data/graph.db/neostore.relationshipstore.db.id
* /var/lib/neo4j/data/graph.db/neostore.relationshiptypestore.db
* /var/lib/neo4j/data/graph.db/neostore.relationshiptypestore.db.id
* /var/lib/neo4j/data/graph.db/neostore.relationshiptypestore.db.names
* /var/lib/neo4j/data/graph.db/neostore.relationshiptypestore.db.names.id

The filenames are self-descriptive of their content.

.. _Graph:  http://docs.neo4j.org/chunked/stable/what-is-a-graphdb.html
.. _Neo4j: http://neo4j.org/
.. _navigates a document store: http://docs.neo4j.org/chunked/stable/tutorial-comparing-models.html#_a_graph_database_navigates_a_document_store
.. _elaborates a Key-Value store: http://docs.neo4j.org/chunked/stable/tutorial-comparing-models.html#_a_graph_database_elaborates_a_key_value_store
.. _creates relationsips in Column-Family: http://docs.neo4j.org/chunked/stable/tutorial-comparing-models.html#_a_graph_database_relates_column_family
.. _transforms a RDBMS: http://docs.neo4j.org/chunked/stable/tutorial-comparing-models.html#_a_graph_database_transforms_a_rdbms
.. _Nodes: http://docs.neo4j.org/chunked/stable/graphdb-neo4j-nodes.html
.. _Relationships: http://docs.neo4j.org/chunked/stable/graphdb-neo4j-relationships.html
.. _Properties: http://docs.neo4j.org/chunked/stable/graphdb-neo4j-properties.html
.. _Path: http://docs.neo4j.org/chunked/stable/graphdb-neo4j-paths.html
.. _Traversing: http://docs.neo4j.org/chunked/stable/graphdb-neo4j-traversal.html
.. _Types: http://docs.neo4j.org/chunked/stable/graphdb-neo4j-properties.html
.. _internal storage: http://digitalstain.blogspot.com/2011/11/rooting-out-redundancy-new-neo4j.html
.. _double-linked-list: http://en.wikipedia.org/wiki/Doubly_linked_list