Neo4j and Cypher: Using MERGE with Schema Indexes/Constraints

I wrote about cypher’s MERGE function a couple of weeks ago, and over the last few days, I’ve been exploring how it works with schema indexes and unique constraints.

An Exciting Time to Be a Developer

There is so much that could be said about the merging of Neo4j and Cypher right now, but it is certainly reasonable to point out that this merger will likely result in many exciting developments in the programming world. 

Programmers virtually always appreciate it when they are given the products and tools they require to get their job done properly, and now is the time for steps like this to be taken. The fact that Neo4J and Cypher have decided to merge means that the upsides of both will soon be apparent. 

You deserve to use all of the best tools to make informed decisions about your next software project, and a great way to make it happen is to use what has been given to you regarding product functionality. This is to say that you can use both the upsides of Neo4J and Cypher to come up with the exact tools you need to make a difference in your sphere of influence.

Could Other Products Soon Merge?

There has been some strong demand for other software development products to consider merging. Coders and programmers want to use their favorite projects in exactly how they were meant to be used, and this means getting them to merge in ways that are useful to the programmers. They just want to be able to squeeze as much use out of each program as they possibly can.

You want to make sure that you can see what is going on with your codes as you are directly applying them to whichever problem you are working on at this time. To be sure, it is not an easy task, but no one ever said it would be easy. The important thing is that you get the work done so that you can start to become more productive in the coding you are doing now. 

A common use case with Neo4j is to model users and events where an event could be a tweet, Facebook post, or Pinterest pin. The model might look like this:

We’d like to ensure that we don’t get duplicate users or events, and MERGE provides the semantics to do this:

MERGE (u:User {id: {userId}})
MERGE (e:Event {id: {eventId}})
MERGE (u)-[:CREATED_EVENT]->(m)
RETURN u, e

We’d like to ensure that we don’t get duplicate users or events and MERGE provides the semantics to do this:

MERGE ensures that a pattern exists in the graph. Either the pattern already exists, or it needs to be created.

import org.neo4j.cypher.javacompat.ExecutionEngine;
import org.neo4j.cypher.javacompat.ExecutionResult;
import org.neo4j.graphdb.GraphDatabaseService;
import org.neo4j.graphdb.factory.GraphDatabaseFactory;
import org.neo4j.helpers.collection.MapUtil;
import org.neo4j.kernel.impl.util.FileUtils;
 
...
 
public class MergeTime
{
    public static void main(String[] args) throws Exception
    {
        String pathToDb = "/tmp/foo";
        FileUtils.deleteRecursively(new File(pathToDb));
 
        GraphDatabaseService db = new GraphDatabaseFactory().newEmbeddedDatabase( pathToDb );
        final ExecutionEngine engine = new ExecutionEngine( db );
 
        ExecutorService executor = Executors.newFixedThreadPool( 50 );
        final Random random = new Random();
 
        final int numberOfUsers = 10;
        final int numberOfEvents = 50;
        int iterations = 100;
        final List<Integer> userIds = generateIds( numberOfUsers );
        final List<Integer> eventIds = generateIds( numberOfEvents );
        List<Future> merges = new ArrayList<>(  );
        for ( int i = 0; i < iterations; i++ )
        {
            Integer userId = userIds.get(random.nextInt(numberOfUsers));
            Integer eventId = eventIds.get(random.nextInt(numberOfEvents));
            merges.add(executor.submit(mergeAway( engine, userId, eventId) ));
        }
 
        for ( Future merge : merges )
        {
            merge.get();
        }
 
        executor.shutdown();
 
        ExecutionResult userResult = engine.execute("MATCH (u:User) RETURN u.id as userId, COUNT(u) AS count ORDER BY userId");
 
        System.out.println(userResult.dumpToString());
 
    }
 
    private static Runnable mergeAway(final ExecutionEngine engine,
                                      final Integer userId, final Integer eventId)
    {
        return new Runnable()
        {
            @Override
            public void run()
            {
                try
                {
                    ExecutionResult result = engine.execute(
                            "MERGE (u:User {id: {userId}})\n" +
                            "MERGE (e:Event {id: {eventId}})\n" +
                            "MERGE (u)-[:CREATED_EVENT]->(m)\n" +
                            "RETURN u, e",
                            MapUtil.map( "userId", userId, "eventId", eventId) );
 
                    // throw away
                    for ( Map<String, Object> row : result ) { }
                }
                catch ( Exception e )
                {
                    e.printStackTrace();
                }
            }
        };
    }
 
    private static List<Integer> generateIds( int amount )
    {
        List<Integer> ids = new ArrayList<>();
        for ( int i = 1; i <= amount; i++ )
        {
            ids.add( i );
        }
        return ids;
    }
}

We create a maximum of 10 users and 50 events and then do 100 iterations of random (user, event) pairs with 50 concurrent threads. Afterward, we execute a query that checks how many users of each id have been created and get the following output:

+----------------+
| userId | count |
+----------------+
| 1      | 6     |
| 2      | 3     |
| 3      | 4     |
| 4      | 8     |
| 5      | 9     |
| 6      | 7     |
| 7      | 5     |
| 8      | 3     |
| 9      | 3     |
| 10     | 2     |
+----------------+
10 rows

Next, I added a schema index on users and events to see if that would make any difference, something Javad Karabi recently asked on the user group.

CREATE INDEX ON :User(id)
CREATE INDEX ON :Event(id)

We wouldn’t expect this to make a difference as schema indexes don’t ensure uniqueness, but I ran it anyway t and got the following output:

+----------------+
| userId | count |
+----------------+
| 1      | 2     |
| 2      | 9     |
| 3      | 7     |
| 4      | 2     |
| 5      | 3     |
| 6      | 7     |
| 7      | 7     |
| 8      | 6     |
| 9      | 5     |
| 10     | 3     |
+----------------+
10 rows

If we want to ensure the uniqueness of users and events, we need to add a unique constraint on the id of both of these labels:

CREATE CONSTRAINT ON (user:User) ASSERT user.id IS UNIQUE
CREATE CONSTRAINT ON (event:Event) ASSERT event.id IS UNIQUE

Now if we run the test, we’ll only end up with one of each user:

+----------------+
| userId | count |
+----------------+
| 1      | 1     |
| 2      | 1     |
| 3      | 1     |
| 4      | 1     |
| 5      | 1     |
| 6      | 1     |
| 7      | 1     |
| 8      | 1     |
| 9      | 1     |
| 10     | 1     |
+----------------+
10 rows

We’d see the same result if we ran a similar query checking for the uniqueness of events.

As far as I can tell, this duplication of nodes that we merge on only happens if you try to create the same node twice concurrently. Once the node has been created, we can use MERGE with a non-unique index, and a duplicate node won’t get created.

All the code from this post is available as a gist if you want to play around with it.