To setup the project I created a public github account and began setting up the infrastructure for a spring web project using maven as build system. So I added the dependencies for the springframework libraries, put the web.xml for the DispatcherServlet and the applicationContext.xml in the webapp directory.

            TODO setup code?

With this setup I was ready for the first spike: creating a simple MovieController showing a static view. Check. Next was the setup for Spring Data Graph. I looked at the README at github and then checked it with the manual. Quite a lot of maven setup for aspectj but otherwise not so much to add. I added just a few lines to my spring configuration.

            TODO config code?

I spun up jetty to see if there were any obvious issues with the config. Check.

Next were relationships. Direct relationships didn't require any annotation. Unfortunately I had none of those. So I went for the Role relationship between Movie and Actor. It had to be annotated with @RelationshipEntity and the @StartNode and @EndNode had to be marked. So my Role looked like this:

@RelationshipEntity
class Role {
    @EndNode
    Movie movie;
    @StartNode
    Actor actor;
    String role;
}
    

When writing a test for that I tried to create the relationship entity with new, but got an exception saying that this was not allowed. Some weird restriction about having only correctly constructed RelationshipEntities. So I remembered a relateTo method from the list of introduced methods on the NodeEntities. After quickly checking it turned out to be exactly what I needed. I added the method for connecting movies and actors to the actor - seemed more natural.

public Role playedIn(Movie movie, String roleName) {
    Role role = relateTo(movie, Role.class, "ACTS_IN");
    role.setRole(roleName);
    return role;
}
            

What was left - accessing those relationships. I already had the appropriate fields in both classes. Time to annotate them correctly. For the fields providing access to the entities on the other side of the relationship this was straightforward. Providing the target type again (thanks to Java's type erasure) and the relationship type (that I learned from the Neo4j lesson before) there was only the direction left. Which defaults to OUTGOING so only for the movie I had to specify it.

@NodeEntity
class Movie {
    @Indexed
    int id;
    String title;
    int year;
    @RelatedTo(elementClass = Actor.class, type = "ACTS_IN", direction = Direction.INCOMING)
    Set<Actor> cast;
}

@NodeEntity
class Actor {
    @Indexed
    int id;
    String name;
    @RelatedTo(elementClass = Movie.class, type = "ACTS_IN")
    Set<Movie> cast;

    public Role playedIn(Movie movie, String roleName) {
        Role role = relateTo(movie, Role.class, "ACTS_IN");
        role.setRole(roleName);
        return role;
    }
}
            

While reading about those relationship-sets I learned that they are handled by managed collections of spring data graph. So whenever I add something to the set or remove it, it automatically reflects that in the underlying relationships. Neat. But this also meant I mustn't initialize the fields. Something I will certainly forget not to do in the future, so watch out for it.

I didn't forget to add test for those. So I could assure that the collections worked as advertised (and also ran into the intialization problem above).

But I still couldn't access the Role relationships. There was more to read about this. For accessing the relationship in between the nodes there was a separate annotation @RelatedToVia. And I had to declare the field as readonly Iterable<Role>. That should make sure that I never tried to add Roles (which I couldn't create on my own anyway) to this field. Otherwise the annotation attributes were similar to those used for @RelatedTo. So off I went, creating my first real relationship (just kidding).

            @NodeEntity
            class Movie {
                @Indexed
                int id;
                String title;
                int year;
                @RelatedTo(elementClass = Actor.class, type = "ACTS_IN", direction = Direction.INCOMING)
                 Set<Actor> cast;

                @RelatedToVia(elementClass = Role.class, type = "ACTS_IN", direction = Direction.INCOMING)
                Iterable<Roles> roles;
            }
        

After the tests proved that those relationship fields really mirrored the underlying relationships in the graph and instantly reflected additions and removals I was satisfied with my domain so far and went for some coffee and chocolate.

Time to put this on display. But I needed some test data first. So I wrote a small class for populating the database which could be called from my controller. To make it safe to call it several times I added index lookups to check for existing entries. A simple /populate endpoint for the controller that called it would be enough for now.

            TODO code

The next thing was to allow users to search for some movies. So I needed some fulltext-search capabilities. As the index provider implementation of Neo4j builds on lucene I was delighted to see that fulltext indexes are supported out of the box.

So I happily annotated the title field of my Movie class with @Index(fulltext=true) and was told with an exception that I have to specify a separate index name for that. So it became @Indexed(fulltext = true, indexName = "search"). The corresponding finder method is called findAllByQuery. So there was my second repository method for searching movies. To restrict the size of the returned set I just added a limit for now that cuts the result after that many entries.

public void List<Movie> searchForMovie(String query, int count) {
    List<Movie> movies=new ArrayList<Movie>(count);
    for (Movie movie : movieFinder.findAllByQuery("title", query)) {
        movies.add(movie);
        if (count-- == 0) break;
    }
    return movies;
}

I then used this result in the controller to render a list of movies driven by a search box. The movie properties and the cast was accessed by the getters in the domain classes.

TODO jsp fragment
            

So I took the User class that I already coded up before and made it a full fledged Spring Data Graph member.

@NodeEntity
class User {
    @Indexed
    String login;
    String name;
    String password;
    @RelatedTo(elementClass=Movie.class, type="RATED")
    Set<Rating> ratings;

    @RelatedTo(elementClass=User.class, type="FRIEND")
    Set<User> friends;

    public Rating rate(Movie movie, int stars, String comment) {
        return relateTo(movie, Rating.class, "RATED").rate(stars, comment);
    }
    public void befriend(User user) {
        this.friends.add(user);
    }
}
class Rating {
    @StartNode User user;
    @EndNode Movie movie;
    int stars;
    String comment;
    public Rating rate(int stars, String comment) {
       this.stars=stars; this.comment = comment;
       return this;
    }
}
            

I also put a ratings field into the movie to be able to show its ratings. And a method to average the stars it got.

class Movie {
    @RelatedToVia(elementClass=Rating.class, type="RATED", direction = Direction.INCOMING)
    Iterable<Rating> ratings;

    public int getStars() {
        int stars, int count;
        for (Rating rating : ratings) {
            stars += rating.getStars(); count++;
        }
        return count == 0 ? 0 : stars / count;
    }
}
            

Fortunately my tests showed my the division by zero error when calculating the stars for a movie without ratings. I also added a few user and ratings to the database population code. And three methods to rate movies, lookup users and add friends to the repository.

TODO code
            

A graph database is a storage engine that is specialized in storing and retrieving vast networks of data. It efficiently stores nodes and relationship and allows high performance traversal of those structures. With property graphs it is possible to add an arbitrary number of properties to nodes and relationships.

The interface org.neo4j.graphdb.GraphDatabaseService provides access to the storage engine. Its features include creating and retrieving Nodes and Relationships, managing indexes, via an IndexManager, database lifecycle callbacks, transation management and more.

The EmbeddedGraphDatabaseService is an implementation of GraphDatabaseService that is used to embed Neo4j in a Java application. This implmentation is used so as to provide the highest and tightest integration. There are other, remote implementations that provide access to Neo4j stores via REST.

Using the API of GraphDatabaseService it is easy to create nodes and relate them to each other. Relationships are named. Both nodes and relationships can have properties. Property values can be primitive Java types and Strings, byte arrays for binary data, or arrays of other Java primitives or Strings. Node creation and modification has to happen within a transaction, while reading from the graph store can be done with or without a transaction.

GraphDatabaseService graphDb = new EmbeddedGraphDatabase( "helloworld" );
Transaction tx = graphDb.beginTx();
try {

	Node firstNode = graphDb.createNode();
	Node secondNode = graphDb.createNode();
	firstNode.setProperty( "message", "Hello, " );
	secondNode.setProperty( "message", "world!" );

	Relationship relationship = firstNode.createRelationshipTo( secondNode, 
		DynamicRelationshipType.of("KNOWS") );
	relationship.setProperty( "message", "brave Neo4j " );
	tx.success();
} finally {
	tx.finish();
}
			

Getting a single node or relationship and examining it is not the main use case of a graph database. Fast graph traversal and application of graph algorithms are. Neo4j provides means via a concise DSL to define TraversalDescriptions that can then be applied to a start node and will produce a stream of nodes and/or relationships as a lazy result using an Iterable.

TraversalDescription traversalDescription = Traversal.description()
          .depthFirst()
          .relationships( KNOWS )
          .relationships( LIKES, Direction.INCOMING )
          .prune( Traversal.pruneAfterDepth( 5 ) );
for ( Path position : traversalDescription.traverse( myStartNode )) {
    System.out.println( "Path from start node to current position is " + position );
}

The best way for retrieving start nodes for traversals is using Neo4j's index facilities. The GraphDatabaseService provides access to the IndexManager which in turn retrieves named indexes for nodes and relationships. Both can be indexed with property names and values. Retrieval is done by query methods on Index to return an IndexHits iterator.

IndexManager indexManager = graphDb.index();
Index<Node> nodeIndex = indexManager.forNodes("a-node-index");
nodeIndex.add(node, "property","value");
for (Node foundNode = nodeIndex.get("property","value")) {
    assert node.getProperty("property").equals("value");
}

Note: Spring Data Graph provides auto-indexing via the @Indexed annotation, while this still is a manual process when using the Neo4j API.

Behind the scenes Spring Data Graph leverages AspectJ aspects to modify the behavior of simple POJO entities to be able to be backed by a graph store. Each entity is backed by a node that holds its properties and relationships to other entities. AspectJ is used to intercept field access and to reroute it to the backing state (either its properties or relationships). For relationship entities the fields are similarly mapped to properties. There are two specially annotated fields for the start and the end node of the relationship.

The aspect introduces some internal fields and some public methods to the entities for accessing the backing state via getPersistentState() and creating relationships with relateTo and retrieving relationship entities viagetRelationshipTo. It also introduces finder methods like find(Class<? extends NodeEntity>, TraversalDescription) and equals and hashCode delegation.

Spring Data Graph internally uses an abstraction called EntityState that the field access and instantiation advices of the aspect delegate to, keeping the aspect code very small and focused to the pointcuts and delegation code. The EntityState then uses a number of FieldAccessor factories to create a FieldAccessor instance per field that does the specific handling needed for the concrete field.

Entities are declared using the @NodeEntity annotation. Relationship entities use the @RelationshipEntity annotation.

@NodeEntity
public class Movie {
	String title;
}

@RelationshipEntity
public class Role {
	@StartNode
	private Actor actor;
	@EndNode
	private Movie movie;
}

@NodeEntity
public class Movie {
	private Actor topActor;
}
@NodeEntity
public class Person {
	@RelatedTo(type = "topActor", direction = Direction.INCOMING)
	private Movie wasTopActorIn;
}
@NodeEntity
public class Actor {
	@RelatedTo(type = "ACTS_IN", elementClass = Movie.class)
	private Set<Movie> movies;
}

@NodeEntity
public class Actor {
	@RelatedToVia(type = "ACTS_IN", elementClass = Role.class)
	private Iterable<Role> roles;
}

The Neo4j graph database can use different index providers for exact lookups and fulltext searches. Lucene is used as a index provider implementation. There is support for distinct indexes for nodes and relationships which can be configured to be of fulltext or exact types.

Using the standard Neo4j API, Nodes and Relationships and their indexed field-value combinations have to be added manually to the appropriate index. When using Spring Data Graph, this task is simplified by eased by applying an @Indexed annotation on entity fields. This will result in updates to the index on every change. Numerical fields are indexed numerically so that they are available for range queries. All other fields are indexed with their string representation. The @Indexed annotation can also set the index-name to be used. If @Indexed annotates the entity class, the index-name for the whole entity is preset to that value. Not providing index names defaults them to "node" and "relationship" respectively.

Query access to the index happens with the Node- and RelationshipFinders that are created via an instance of org.springframework.data.graph.neo4j.finder.FinderFactory. The methods findByPropertyValue and findAllByPropertyValue work on the exact indexes and return the first or all matches. To do range queries, use findAllByRange (please note that currently both values are inclusive).

@NodeEntity
class Person {
    @Indexed(indexName = "people")
    String name;

    // automatically indexed numerically
    @Indexed
    int age;

}

@NodeEntity
@Indexed(indexName="groups")
class Group {
    @Indexed
    String name;

    @RelatedTo(elementClass = Person.class, type = "people" )
    Set<Person> people;
}

NodeFinder<Person> finder = finderFactory.createNodeEntityFinder(Person.class);

// exact finder
Person mark = finder.findByProperyValue("people","name","mark");

// numeric range queries
for (Person middleAgedDeveloper : finder.findAllByRange(null, "age", 20, 40)) {
    Developer developer=middleAgedDeveloper.projectTo(Developer.class);
}

Neo4jTemplate also offers index support, providing auto-indexing for fields at creation time of nodes and relationships. There is an autoIndex method that can also add indexes for a set of fields in one go.

For querying the index, the template offers query-methods that take either the exact match parameters or a query object / query expression and push the results wrapped uniformly as Paths to the supplied PathMapper to be converted or collected.

Spring Data Graph also comes with a type bound Repository-like Finder implementation that provides methods for locating nodes and relationships:

The Finder instances are created via a FinderFactory to be bound to a concrete node or relationship entity class. The FinderFactory is created in the Spring context and can be injected.

NodeFinder<Person> finder = finderFactory.createNodeEntityFinder(Person.class);
Person dave=finder.findById(123);
int people = finder.count();
Person mark = finder.findByPropertyValue("name", "mark");
Iterable<Person> devs = finder.findAllByProperyValue("occupation","developer");
Iterable<Person> davesFriends = finder.findAllByTraversal(dave,
    Traversal.description().pruneAfterDepth(1)
    .relationships(KNOWS).filter(returnAllButStartNode()));

Neo4j is a transactional datastore which only allows modifications within transaction boundaries and fullfills the ACID properties. Reading from the store is also possible outside of transactions.

Spring Data Graph integrates with transaction managers configured using Spring. The simplest scenario of just running the graph database uses a SpringTransactionManager provided by the Neo4j kernel to be used with Spring's JtaTransactionManager. Note: The explicit XML configuration given below is encoded in the Neo4jConfiguration configuration bean that uses Spring's @Configuration functioanlity. This simplifies the configuration. An example is shown further below.

<bean id="transactionManager" class="org.springframework.transaction.jta.JtaTransactionManager">
<property name="transactionManager">
    <bean class="org.neo4j.kernel.impl.transaction.SpringTransactionManager">
        <constructor-arg ref="graphDatabaseService"/>
    </bean>
</property>
<property name="userTransaction">
    <bean class="org.neo4j.kernel.impl.transaction.UserTransactionImpl">
        <constructor-arg ref="graphDatabaseService"/>
    </bean>
</property>
</bean>

<tx:annotation-driven mode="aspectj" transaction-manager="transactionManager"/>

For scenarios running multiple transactional resources there are two options. First of all you can have Neo4j participate in the externally set up transaction manager using the new SpringProvider by enabling the configuration parameter for your graph database. Either via the spring config or the configuration file (neo4j.properties).

<context:annotation-config />
<context:spring-configured/>

<bean id="transactionManager" class="org.springframework.transaction.jta.JtaTransactionManager">
<property name="transactionManager">
    <bean id="jotm" class="org.springframework.data.graph.neo4j.transaction.JotmFactoryBean"/>
</property>
</bean>

<bean class="org.neo4j.kernel.EmbeddedGraphDatabase" destroy-method="shutdown">
<constructor-arg value="target/test-db"/>
<constructor-arg>
    <map>
        <entry key="tx_manager_impl" value="spring-jta"/>
    </map>
</constructor-arg>
</bean>

<tx:annotation-driven mode="aspectj" transaction-manager="transactionManager"/>
                

You can configure a stock XA transaction manager to be used with Neo4j and the other resources (e.g. Atomikos, JOTM, App-Server-TM). For a bit less secure but fast 1 phase commit best effort, use the implementation coming with Spring Data Graph (ChainedTransactionManager). It takes a list of transaction-managers as constructor params and will handle them in order for transaction start and commit (or rollback) in the reverse order.

<bean id="transactionManager"
        class="org.springframework.data.graph.neo4j.transaction.ChainedTransactionManager" >
    <constructor-arg>
        <list>
        <bean class="org.springframework.orm.jpa.JpaTransactionManager" id="jpaTransactionManager">
            <property name="entityManagerFactory" ref="entityManagerFactory"/>
        </bean>
        <bean
            class="org.springframework.transaction.jta.JtaTransactionManager">
            <property name="transactionManager">
                <bean class="org.neo4j.kernel.impl.transaction.SpringTransactionManager">
                    <constructor-arg ref="graphDatabaseService" />
                </bean>
            </property>
            <property name="userTransaction">
                <bean  class="org.neo4j.kernel.impl.transaction.UserTransactionImpl">
                    <constructor-arg ref="graphDatabaseService" />
                </bean>
            </property>
        </bean>
        </list>
    </constructor-arg>
</bean>

By default newly created node entities are in a detached state. When persist() is called on the entity it is attached to the graph store and its properties and relationships are persisted as well. Changing an attached entity inside a transaction will write through the changes to the datastore. Whenever an entity is changed outside of a transaction it will be considered detached. The changed data is stored in the entity itself and not written back to the datastore.

All entities that are returned by library functions are initially in an attached state. Changing them outside of a transaction detaches them. For writing the changes back it is necessary to persist() them again.

Persisting an entity not only persists that single entity but will traverse its existing and new relationships and persist the cluster of detached entities that it is part of. The borders of this cluster are formed by attached entities. The persist operation creates its own, implicit transaction. When it is called withina external transaction it participates otherwise it is an atomic operation.

Please keep in mind that the session handling behaviour is still heavily developed. The defaults and also other aspects of the behaviour are likely to change in subsequent releases. At the moment there is no support for the creation of relationships outside of transactions and also more complex operations like creating whole subgraphs outside of transactions is not supported.

@NodeEntity
class Person {
    String name;
}
Person p = new Person().persist();

There are several ways to represent the Java type hierarchy of the data model in the graph. In general for all node and relationship entities type information is needed to perform certain repository operations. That's why the hierarchy up to java.lang.Object of all these classes will be persisted in the graph. Implementations of NodeTypeStrategy take care of persisting this information on entity instance creation. They also provide the repository methods that use this type information to perform their operations like findAll, count etc.

The current implementation uses nodes to represent the Java type hierarchy which are connected via SUBCLASS_OF relationships to their superclass nodes and via INSTANCE_OF relationships to the concrete node entity instance node.

An alternative approach could use indexing operations to perform the same functionality. Or one could skip the NodeTypeStrategy altogether if no strict checks on type conformity are needed, which would allow for a much more flexible data model.

The node and relationship aspects introduce (via ITD - inter type declaration) several methods to the entities that make common tasks easier. Unfortunately these methods are not generified yet, so the results have to be casted to the correct return type.

As the underlying data model of a graph database doesn't imply and enforce strict type constraints like a relational model does, it offers much more flexibility on how to model your domain classes and which of those to use in different contexts.

For instance an order can be used in these contexts: customer, procurement, logistics, billing, fulfillment and many more. Each of those contexts requires its distinct set of attributes and operations. As Java doesn't support mixins one would put the sum of all of those into the entity class and thereby making it very big, brittle and hard to understand. Being able to take a basic order and project it to a different (not related in the inheritance hierarchy or even an interface) order type that is valid in the current context and only offers the attributes and methods needed here would be very benefitial.

Spring Data Graph offers initial support for projecting node and relationship entities to different target types. All instances of this projected entity share the same backing node or relationship, so data changes are reflected immediately.

This could for instance also be used to handle nodes of a traversal with a unified (simpler) type (e.g. for reporting or auditing) and only project them to a concrete, more functional target type when the business logic requires it.

// not related to Person at all
@NodeEntity
class Trainee {
    String name;
    @RelatedTo(elementClass=Training.class);
    Set<Training> trainings;
}

for (Person person : finder.findAllByProperyValue("occupation","developer")) {
    Developer developer = person.projectTo(Developer.class);
    if (developer.isJavaDeveloper()) {
        trainInSpringData(developer.projectTo(Trainee.class));
    }
}

The Neo4jTemplate offers the convenient API of Spring templates for the Neo4j graph database. There are methods for creating nodes and relationships that automatically set provided properties and optionally index certain fields. Other methods (index, autoindex) will index them.

For the querying operations Neo4jTemplate unifies the result with the Path abstraction that comes from Neo4j. Much like a resultset a path contains nodes() and relationships() starting at a startNode() and ending with aendNode(), the lastRelationship() is also available separately. The Path abstraction also wraps results that contain just nodes or relationships. Using implementations of PathMapper<T> and PathMapper.WithoutResult (comparable with RowMapper and RowCallbackHandler) the paths can be converted to Java objects.

Query methods either take a field / value combination to look for exact matches in the index or a lucene query object or string to handle more complex queries.

Traversal methods are the bread and butter of graph operations. As such, they are fully supported in the Neo4jTemplate. The traverseNext method traverses to the direct neighbours of the start node filtering the relationships according to its parameters.

The traverse method covers the full fledged traversal operation that takes a powerful TraversalDescription (most probably built from the Traversal.description() DSL) and runs it from the start node. Each path that is returned via the traversal is passed to the PathMapper to be processed accordingly.

The Neo4jTemplate provides configurable implicit transactions for all its methods. By default it creates a transaction for each call (which is a no-op if there is already a transaction running). If you call the constructor with the useExplicitTransactions parameter set to true, it won't create any transactions so you have to provide them using @Transactional or the TransactionTemplate.

Neo4jOperations neo = new Neo4jTemplate(grapDatabase);
Node michael = neo.createNode(_("name","Michael"),"name");
Node mark = neo.createNode(_("name","Mark"));
Node thomas = neo.createNode(_("name","Thomas"));
neo.createRelationship(mark,thomas, WORKS_WITH, _("project","spring-data"));
neo.index("devs",thomas, "name","Thomas");
neo.autoIndex("devs",mark, "name");
assert "Mark".equals(neo.query("devs","name","Mark",new NodeNamePathMapper()));

Spring Data Graph supports property based validation support. So whenever a property is changed, it is checked against the annotated constraints (.e.g @Min, @Max, @Size, etc). Validation errors throw a ValidationException. For evaluating the constraints the validation support that comes with Spring is used. To use it a validator has to be registered with the GraphDatabaseContext, if there is none, no validation will be performed (any registered Validator or (Local)ValidatorFactoryBean will be used).

@NodeEntity
class Person {
    @Size(min = 3, max = 20)
    String name;

    @Min(0)
    @Max(100)
    int age;
}

As stated in the requirements chapter, Spring Data Graph projects are easiest to build with Apache Maven. The main dependencies are Spring Data Graph itself, Spring Data Commons, some parts of the Spring Framework and of course the Neo4j graph database.

<repository>
  <id>spring-maven-milestone</id>
  <name>Springframework Maven Repository</name>
  <url>http://maven.springframework.org/milestone</url>
</repository>
				

<dependency>
  <groupId>org.springframework.data</groupId>
  <artifactId>spring-data-neo4j</artifactId>
  <version>1.0.0.M4</version>
</dependency> 

<dependency>
  <groupId>org.aspectj</groupId>
  <artifactId>aspectjrt</artifactId>
  <version>1.6.11.M2</version>
</dependency>
						

<plugin>
  <groupId>org.codehaus.mojo</groupId>
  <artifactId>aspectj-maven-plugin</artifactId>
  <version>1.0</version>
  <dependencies>
    <!-- NB: You must use Maven 2.0.9 or above or these are ignored (see MNG-2972) -->
    <dependency>
      <groupId>org.aspectj</groupId>
      <artifactId>aspectjrt</artifactId>
      <version>1.6.11.M2</version>
    </dependency>
    <dependency>
      <groupId>org.aspectj</groupId>
      <artifactId>aspectjtools</artifactId>
      <version>1.6.11.M2</version>
    </dependency>
  </dependencies>
  <executions>
    <execution>
      <goals>
        <goal>compile</goal>
        <goal>test-compile</goal>
      </goals>
    </execution>
  </executions>
  <configuration>
    <outxml>true</outxml>
    <aspectLibraries>
      <aspectLibrary>
        <groupId>org.springframework</groupId>
        <artifactId>spring-aspects</artifactId>
      </aspectLibrary>
      <aspectLibrary>
        <groupId>org.springframework.data</groupId>
        <artifactId>spring-datastore-neo4j</artifactId>
      </aspectLibrary>
    </aspectLibraries>
    <source>1.6</source>
    <target>1.6</target>
  </configuration>
</plugin>
						

The concrete configuration for Spring Data Graph is quite verbose as there is no autowiring involved. It sets up the following parts.

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<beans xmlns="http://www.springframework.org/schema/beans"
  xmlns:context="http://www.springframework.org/schema/context"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:datagraph="http://www.springframework.org/schema/data/graph"
  xsi:schemaLocation="
    http://www.springframework.org/schema/beans 
    http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
    http://www.springframework.org/schema/context 
    http://www.springframework.org/schema/context/spring-context-3.0.xsd
    http://www.springframework.org/schema/data/graph 
    http://www.springframework.org/schema/data/graph/datagraph-1.0.xsd
    ">

    <context:annotation-config/>
    <datagraph:config storeDirectory="target/config-test"/>

</beans>
                    

    <context:annotation-config/>

    <bean id="graphDatabaseService" class="org.neo4j.kernel.EmbeddedGraphDatabase" 
      destroy-method="shutdown">
         <constructor-arg index="0" value="target/config-test" />
    </bean>

    <datagraph:config graphDatabaseService="graphDatabaseService"/>
                    

  <context:annotation-config/>

    <datagraph:config storeDirectory="target/config-test"
      entityManagerFactory="entityManagerFactory"/>

    <bean class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean"
      id="entityManagerFactory">
        <property name="dataSource" ref="dataSource"/>
        <property name="persistenceXmlLocation" value="classpath:META-INF/persistence.xml"/>
    </bean>
                    

    <beans>
    ...

        <tx:annotation-driven mode="aspectj" transaction-manager="transactionManager"/>
        <bean class="org.springframework.data.graph.neo4j.config.Neo4jConfiguration"/>

        <bean class="org.springframework.context.annotation.ConfigurationClassPostProcessor"/>

        <bean id="graphDatabaseService" class="org.neo4j.kernel.EmbeddedGraphDatabase"
              destroy-method="shutdown" scope="singleton">
            <constructor-arg index="0" value="target/config-test"/>
        </bean>
    ...
    </beans>
                

Partial graph persistence is achieved by restricting the Spring Data Graph aspects to explicitly annotated parts of the entity. Those fields have to be made transient so that JPA ignores them and won't try to persist those attributes.

A backing node in the graph store is created when the entity has been assigned a JPA id. Only then will the connection between the two stores be kept. Until the entity has been persisted, its state is just kept inside the POJO and flushed to the backing graph store afterwards.

The connection between the two entities is kept via a FOREIGN_ID field in the node that contains the JPA id (currently only single value ids are supported). The entity class can be resolved via the NodeTypeStrategy that preserves the Java type hierarchy within the graph. With the id and class, you can then retrieve the appropriate JPA entity for a given node.

The other direction is handled by indexing the Node with the FOREIGN_ID index which contains a concatenation of the fully qualified class name of the JPA entity and the id. So it is possible on instantiation of a JPA id via the entity manager (or some other means like creating the POJO and setting its id manually) to find the matching node using the index facilities and reconnect them.

Using those mechanisms and the Spring Data Graph aspects a single POJO can contain fields that are handled by JPA and other fields (which might be relationships as well) that are handled by Spring Data Graph.

The following example is taken from the Spring Data Graph examples, it is contained in the myrestaurant-social project.

	@Entity
	@Table(name = "user_account")
	@NodeEntity(partial = true)
	public class UserAccount {
	    private String userName;
	    private String firstName;
	    private String lastName;

	    @GraphProperty
	    @Transient
	    String nickname;

	    @RelatedTo(type = "friends", elementClass = UserAccount.class)
	    @Transient
	    Set<UserAccount> friends;

	    @RelatedToVia(type = "recommends", elementClass = Recommendation.class)
	    @Transient
	    Iterable<Recommendation> recommendations;

	    @Temporal(TemporalType.TIMESTAMP)
	    @DateTimeFormat(style = "S-")
	    private Date birthDate;

		@ManyToMany(cascade = CascadeType.ALL)
	    private Set<Restaurant> favorites;

		@Id
	    @GeneratedValue(strategy = GenerationType.AUTO)
	    @Column(name = "id")
	    private Long id;

	    @Transactional
	    public void knows(UserAccount friend) {
	        relateTo(friend, DynamicRelationshipType.withName("friends"));
	    }

		@Transactional
	    public Recommendation rate(Restaurant restaurant, int stars, String comment) {
	        Recommendation recommendation = (Recommendation) relateTo(restaurant, 
				Recommendation.class, "recommends");
	        recommendation.rate(stars, comment);
	        return recommendation;
	    }
	    public Iterable<Recommendation> getRecommendations() {
	        return recommendations;
	    }
	}

Configuring cross-store persistence is done similarly to the default Spring Data Graph operations. As soon as you refer to an entityManagerFactory in the xml-namespace it is set up for cross-store persistence.

<beans xmlns="http://www.springframework.org/schema/beans"
	xmlns:context="http://www.springframework.org/schema/context"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:datagraph="http://www.springframework.org/schema/data/graph"
	xsi:schemaLocation="
		http://www.springframework.org/schema/beans 
		http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
		http://www.springframework.org/schema/context 
		http://www.springframework.org/schema/context/spring-context-3.0.xsd
		http://www.springframework.org/schema/data/graph 
		http://www.springframework.org/schema/data/graph/datagraph-1.0.xsd
		">

	<context:annotation-config/>

    <datagraph:config storeDirectory="target/config-test" 
      entityManagerFactory="entityManagerFactory"/>

    <bean class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean"
      id="entityManagerFactory">
        <property name="dataSource" ref="dataSource"/>
        <property name="persistenceXmlLocation" value="classpath:META-INF/persistence.xml"/>
    </bean>
</beans>

Spring Data Graph comes with a number of samples. The source code of the samples is found on GitHub. The different sample projects are introduced below.

The Hello Worlds sample application is a simple console application with unit tests, that creates some Worlds (entities / nodes) and Rocket Routes (relationships) in a Galaxy (graph) and then reads them back and prints them out.

The unit tests demonstrate some other features of Spring Data Graph. The sample comes with a minimal configuration for Maven and spring to get up and running quickly.

Executing the application creates the following graph in the Graph Database:

A web application that imports datasets from the Internet Movie Database (IMDB) into the graph database. It allows listings of movies with their actors and actors with their roles in different movies. It also uses graph traversal operations to calculate the Kevin Bacon number (distance to an actor that has acted with Kevin Bacon). This sample application shows the basic usage of Spring Data Graph in a more complex setting with several annotated entities and relationships as well as usage of indices and graph traversal.

See the readme file for instruction on how to compile and run the application.

An excerpt of the data stored in the Graph Database after executing the application: