3.2. Relationships

Relationships between nodes are a key part of a graph database. They allow for finding related data. Just like nodes, relationships can have properties.

graphdb-rels-overview.svg

A relationship connects two nodes, and is guaranteed to have valid start and end nodes.

graphdb-rels.svg

As relationships are always directed, they can be viewed as outgoing or incoming relative to a node, which is useful when traversing the graph:

graphdb-rels-dir.svg

Relationships are equally well traversed in either direction. This means that there is no need to add duplicate relationships in the opposite direction (with regard to traversal or performance).

While relationships always have a direction, you can ignore the direction where it is not useful in your application.

Note that a node can have relationships to itself as well:

graphdb-rels-loop.svg

To further enhance graph traversal all relationships have a relationship type. Note that the word type might be misleading here, you could rather think of it as a label. The following example shows a simple social network with two relationship types.

graphdb-rels-twitter.svg

Using relationship direction and type

What How

get who a person follows

outgoing follows relationships, depth one

get the followers of a person

incoming follows relationships, depth one

get who a person blocks

outgoing blocks relationships, depth one

get who a person is blocked by

incoming blocks relationships, depth one


This example is a simple model of a file system, which includes symbolic links:

graphdb-rels-filesys.svg

Depending on what you are looking for, you will use the direction and type of relationships during traversal.

What How

get the full path of a file

incoming file relationships

get all paths for a file

incoming file and symbolic link relationships

get all files in a directory

outgoing file and symbolic link relationships, depth one

get all files in a directory, excluding symbolic links

outgoing file relationships, depth one

get all files in a directory, recursively

outgoing file and symbolic link relationships