After a short break, let us continue the discussion. Think about a graph database as a document database, with a special type of documents, relations. A simple example is a social network:

There are four documents and three relations in this example. Relations in a graph database are more than just a pointer. A relation can be unidirectional or bidirectional, but more importantly, a relation is typed, I may be associated to you in several ways, you may be a client, family or my alter ego. And the relation itself can carry information. In the case of the relation document in the example above, we simply record the type of the association.

And that is about it, mostly. Once you figured out that graph database can be seen as document databases with a special document type, you are pretty much done.

Except that graph database has one additional quality that make them very useful. They allow you to perform graph operations. The most basic graph operation is traversal. For example, let us say that I want to know who of my friends is in town so I can go and have a drink. That is pretty easy to do, right? But what about indirect friends? Using a graph database, I can define the following query:

new GraphDatabaseQuery
{
   SourceNode = ayende,
   MaxDepth = 3,
   RelationsToFollow = new[]{"As Known As", "Family", "Friend", "Romantic", "Ex"},
   Where = node => node.Location == ayende.Location,
   SearchOrder = SearchOrder.BreadthFirst
}.Execute();

I can execute more complex queries, filtering on the relation properties, considering weights, etc.

Graph databases are commonly used to solve network problems. In fact, most social networking sites use some form of a graph database to do things like “You might know…”.

Because graph databases are intentionally design to make sure that graph traversal is cheap, they also provide other operations that tend to be very expensive without it. For example, Shortest Path between two nodes. That turn out to be frequently useful when you want to do things like: “Who can recommend me to this company’s CTO so they would hire me”.