Now came the unknown - how to put these domain objects into the graph. First we read up about graph databases, especially Neo4j. The Neo4j datamodel consists of nodes and relationships, both of which can have properties. Relationships are first class citizens in Neo4j, meaning we can link together nodes into semantically rich networks - we really liked that. Then we found we could index nodes and relationships by {name, value} pairs to quickly get hold of them as starting points for further processing. We also found we could imperatively traverse of relationships using the core API, and in a declarative way using a query-like Traversal Description.

We also learned that Neo4j was fully transactional and completely upholds ACID guarantees for out data. This is unusual for NoSQL databases, but easier for us to get our head around than non-transactional eventual consistency. It also makes us feel safe, though it also means that we had to manage transactions. Keep that in mind.

Initially we used the core Neo4j API to get a feeling for that. And also to see, how (probably) the domain might look when it's saved in the graph store. After adding the Maven dependency, it was ready to go.

<dependency>
    <groupId>org.neo4j</groupId>
    <artifactId>neo4j</artifactId>
    <version>1.3.M05</version>
</dependency>
        

enum RelationshipTypes implements RelationshipType { ACTS_IN };

GraphDatabaseService gds = new EmbeddedGraphDatabase("/path/to/store");
Node forrest=gds.createNode();
forrest.setProperty("title","Forrest Gump");
forrest.setProperty("year",1994);
gds.index().forNodes("movies").add(forrest,"id",1);

Node tom=gds.createNode();
tom.setProperty("Tom Hanks");

Relationship role=tom.createRelationshipTo(forrest,ACTS_IN);
role.setProperty("role","Forrest Gump");

Node movie=gds.index().forNodes("movies").get("id",1).getSingle();
print(movie.getProperty("title"));
for (Relationship role : movie.getRelationships(ACTS_IN,INCOMING)) {
    Node actor=role.getOtherNode(movie);
    print(actor.getProperty("name") +" as " + role.getProperty("role"));
}