Quarkus Neo4j

Neo4j is a graph database management system developed by Neo4j, Inc. Neo4j is a native graph database focused not only on the data itself, but especially on the relations between data. Neo4j stores data as a property graph, which consists of vertices or nodes as we call them, connected with edges or relationships. Both of them can have properties.

Neo4j offers Cypher, a declarative query language much like SQL. Cypher is used to for both querying the graph and creating or updating nodes and relationships. As a declarative language it used to tell the database what to do and not how to do it.

Learn more about Cypher in the Neo4j Cypher manual. Cypher is not only available in Neo4j, but for example coming to Apache Spark. A spec called OpenCypher is available, too.

Clients communicate over the so-called Bolt protocol with the database.

Neo4j - as the most popular graph database according to DB-Engines ranking - provides a variety of drivers for various languages.

The Quarkus Neo4j extension is based on the official Neo4j Java Driver. The extension provides an instance of the driver configured ready for usage in any Quarkus application. You will be able to issue arbitrary Cypher statements over Bolt with this extension. Those statements can be simple CRUD statements as well as complex queries, calling graph algorithms and more.

The driver itself is released under the Apache 2.0 license, while Neo4j itself is available in a GPL3-licensed open-source "community edition", with online backup and high availability extensions licensed under a closed-source commercial license.

Programming model

The driver and thus the Quarkus extension support three different programming models:

  • Blocking database access (much like standard JDBC)

  • Asynchronous programming based on JDK’s completable futures and related infrastructure

  • Reactive programming based on Reactive Streams

The reactive programming model is only available when connected against a 4.0+ version of Neo4j. Reactive programming in Neo4j is fully end-to-end reactive and therefore requires a server that supports backpressure.

In this guide you will learn how to

  • Add the Neo4j extension to your project

  • Configure the driver

  • And how to use the driver to access a Neo4j database

This guide will focus on asynchronous access to Neo4j, as this is ready to use for everyone. At the end of this guide, there will be a reactive version, which needs however a 4.0 database version.

The domain

As with some other guides, the application shall manage fruit entities.

package org.acme.neo4j;

public class Fruit {
    public UUID id;

    public String name;

    public Fruit() {
        // This is needed for the REST-Easy JSON Binding
    }

    public Fruit(String name) {
        this.name = name;
    }

    public Fruit(UUID id, String name) {
        this.id = id;
        this.name = name;
    }
}

Prerequisites

To complete this guide, you need:

  • JDK 11+ installed with JAVA_HOME configured appropriately

  • an IDE

  • Apache Maven 3.8.1+

  • Access to a Neo4j Database

  • Optional Docker for your system

Setup Neo4j

The easiest way to start a Neo4j instance is a locally installed Docker environment.

docker run --publish=7474:7474 --publish=7687:7687 -e 'NEO4J_AUTH=neo4j/secret' neo4j:4.4

This starts a Neo4j instance, that publishes its Bolt port on 7687 and a web interface on http://localhost:7474.

Have a look at the download page for other options to get started with the product itself.

Solution

We recommend that you follow the instructions in the next sections and create the application step by step. However, you can go right to the completed example.

Clone the Git repository: git clone https://github.com/quarkusio/quarkus-quickstarts.git, or download an archive.

The solution is located in the neo4j-quickstart directory. It contains a very simple UI to use the JAX-RS resources created here, too.

Creating the Maven project

First, we need a new project. Create a new project with the following command:

mvn io.quarkus.platform:quarkus-maven-plugin:2.14.2.Final:create \
    -DprojectGroupId=org.acme \
    -DprojectArtifactId=neo4j-quickstart \
    -DclassName="org.acme.datasource.GreetingResource" \
    -Dextensions="neo4j,resteasy-reactive-jackson"
cd neo4j-quickstart

It generates:

  • the Maven structure

  • a landing page accessible on http://localhost:8080

  • example Dockerfile files for both native and jvm modes

  • the application configuration file

  • an org.acme.datasource.GreetingResource resource

  • an associated test

The Neo4j extension has been added already to your pom.xml. In addition, we added resteasy-reactive-jackson, which allows us to expose Fruit instances over HTTP in the JSON format via JAX-RS resources. If you have an already created project, the neo4j extension can be added to an existing Quarkus project with the add-extension command:

./mvnw quarkus:add-extension -Dextensions="neo4j"

Otherwise, you can manually add this to the dependencies section of your pom.xml file:

<dependency>
    <groupId>io.quarkiverse.neo4j</groupId>
    <artifactId>quarkus-neo4j</artifactId>
    <version>2.0.0</version>
</dependency>

Configuring

The Neo4j driver can be configured with standard Quarkus properties:

src/main/resources/application.properties
# Those are the default values and are implicitly assumed
quarkus.neo4j.uri = bolt://localhost:7687
quarkus.neo4j.authentication.username = neo4j
quarkus.neo4j.authentication.password = secret

You’ll recognize the authentication here that you passed on to the docker command above.

Having done that, the driver is ready to use, there are however other configuration options, detailed below.

Dev Services (Configuration Free Databases)

Quarkus supports a feature called Dev Services that allows you to create various datasources without any config. In the case of Neo4j this support applies to the single Neo4j driver instance. Dev Services will bring up a Neo4j container if you didn’t explicit add the default values or configured custom values for any of quarkus.neo4j.uri, quarkus.neo4j.authentication.username or quarkus.neo4j.authentication.password. If Neo4j seems to be reachable via the default properties, Dev Services will also step back.

Otherwise, Quarkus will automatically start a Neo4j container when running tests or dev-mode, and automatically configure the connection.

When running the production version of the application, the Neo4j connection need to be configured as normal, so if you want to include a production database config in your application.properties and continue to use Dev Services we recommend that you use the %prod. profile to define your Neo4j settings.

Configuration property fixed at build time - All other configuration properties are overridable at runtime

Configuration property

Type

Default

If DevServices has been explicitly enabled or disabled. DevServices is generally enabled by default, unless there is an existing configuration present. When DevServices is enabled Quarkus will attempt to automatically configure and start a database when running in Dev or Test mode.

Environment variable: QUARKUS_NEO4J_DEVSERVICES_ENABLED

boolean

The container image name to use, for container based DevServices providers.

Environment variable: QUARKUS_NEO4J_DEVSERVICES_IMAGE_NAME

string

neo4j:4.4

This value can be used to specify the port to which the bolt-port of the container is exposed. It must be a free port, otherwise startup will fail. A random, free port will be used by default. Either way, a messsage will be logged on which port the Neo4j container is reachable over bolt.

Environment variable: QUARKUS_NEO4J_DEVSERVICES_BOLT_PORT

int

This value can be used to specify the port to which the http-port of the container is exposed. It must be a free port, otherwise startup will fail. A random, free port will be used by default. Either way, a messsage will be logged on which port the Neo4j Browser is available.

Environment variable: QUARKUS_NEO4J_DEVSERVICES_HTTP_PORT

int

Additional environment entries that can be added to the container before its start.

Environment variable: QUARKUS_NEO4J_DEVSERVICES_ADDITIONAL_ENV

Map<String,String>

Using the driver

General remarks

The result of a statement consists usually of one or more org.neo4j.driver.Record. Those records contain arbitrary values, supported by the driver. If you return a node of the graph, it will be a org.neo4j.driver.types.Node.

We add the following method to the Fruit, as a convenient way to create them:

public static Fruit from(Node node) {
    return new Fruit(UUID.fromString(node.get("id").asString()), node.get("name").asString());
}

Add a FruitResource skeleton like this and @Inject a org.neo4j.driver.Driver instance and a ThreadContext instance:

src/main/java/org/acme/neo4j/FruitResource.java
package org.acme.neo4j;

import java.net.URI;
import java.util.List;
import java.util.Map;
import java.util.concurrent.CompletionException;
import java.util.concurrent.CompletionStage;

import javax.inject.Inject;
import javax.ws.rs.Consumes;
import javax.ws.rs.DELETE;
import javax.ws.rs.GET;
import javax.ws.rs.POST;
import javax.ws.rs.Path;
import javax.ws.rs.Produces;
import javax.ws.rs.core.MediaType;
import javax.ws.rs.core.Response;
import javax.ws.rs.core.Response.ResponseBuilder;
import javax.ws.rs.core.Response.Status;

import org.eclipse.microprofile.context.ThreadContext;
import org.neo4j.driver.Driver;
import org.neo4j.driver.async.AsyncSession;
import org.neo4j.driver.async.ResultCursor;
import org.neo4j.driver.exceptions.NoSuchRecordException;

@Path("/fruits")
@Consumes(MediaType.APPLICATION_JSON)
@Produces(MediaType.APPLICATION_JSON)
public class FruitResource {

    @Inject
    Driver driver;

    @Inject
    ThreadContext threadContext; (1)
}
1 The ThreadContext is related to context propagation with completion stage. A completion stage, unlike Mutiny, does not have a hook to automatically "capture and restore" the context. So, we need to use this construct in later steps when using the connections asynchronous api.

Reading nodes

Add the following method to the fruit resource:

FruitResource#get
@GET
public CompletionStage<Response> get() {
    AsyncSession session = driver.asyncSession(); (1)
    CompletionStage<List<Fruit>> cs = session
            .executeReadAsync(tx -> tx
                    .runAsync("MATCH (f:Fruit) RETURN f ORDER BY f.name") (2)
                    .thenCompose(cursor -> cursor (3)
                            .listAsync(record -> Fruit.from(record.get("f").asNode()))));
    return threadContext.withContextCapture(cs) (4)
            .thenCompose(fruits -> (5)
            session.closeAsync().thenApply(signal -> fruits))
            .thenApply(Response::ok) (6)
            .thenApply(ResponseBuilder::build);
}
1 Open a new, asynchronous session with Neo4j
2 Execute a query. This is a Cypher statement.
3 Retrieve a cursor, list the results and create Fruits. This must happen inside the transactional function, not outside.
4 Wrap the completion stage so that the current context is captured, and restored it before calling a continuation method of the completion stage. With that, the context is restored and available in any callback.
5 Close the session after processing
6 Create a JAX-RS response

Now start Quarkus in dev mode with:

./mvnw compile quarkus:dev

and retrieve the endpoint like this

curl localhost:8080/fruits

There are not any fruits, so let’s create some.

Creating nodes

The POST method looks similar. It uses transaction functions of the driver:

FruitResource#create
@POST
public CompletionStage<Response> create(Fruit fruit) {
    AsyncSession session = driver.asyncSession();
    CompletionStage<Fruit> cs = session
            .executeWriteAsync(tx -> tx
                    .runAsync(
                            "CREATE (f:Fruit {id: randomUUID(), name: $name}) RETURN f",
                            Map.of("name", fruit.name))
                    .thenCompose(ResultCursor::singleAsync)
                    .thenApply(record -> Fruit.from(record.get("f").asNode())));
    return threadContext.withContextCapture(cs)
            .thenCompose(persistedFruit -> session
                    .closeAsync().thenApply(signal -> persistedFruit))
            .thenApply(persistedFruit -> Response
                    .created(URI.create("/fruits/" + persistedFruit.id))
                    .build());
}

As you can see, we are now using a Cypher statement with named parameters (The $name of the fruit). The node is returned, a Fruit entity created and then mapped to a 201 created response.

A curl request against this path may look like this:

curl -v -X "POST" "http://localhost:8080/fruits" \
     -H 'Content-Type: application/json; charset=utf-8' \
     -d $'{
  "name": "Banana"
}'

The response contains an URI that shall return single nodes.

Read single nodes

This time, we ask for a read-only transaction. We also add some exception handling, in case the resource is called with an invalid id:

FruitResource#getSingle
@GET
@Path("/{id}")
public CompletionStage<Response> getSingle(String id) {
    AsyncSession session = driver.asyncSession();
    return threadContext.withContextCapture(session
            .executeReadAsync(tx -> tx
                    .runAsync("MATCH (f:Fruit) WHERE f.id = $id RETURN f", Map.of("id", id))
                    .thenCompose(ResultCursor::singleAsync))
            .handle((record, exception) -> {
                if (exception != null) {
                    Throwable source = exception;
                    if (exception instanceof CompletionException) {
                        source = exception.getCause();
                    }
                    Status status = Status.INTERNAL_SERVER_ERROR;
                    if (source instanceof NoSuchRecordException) {
                        status = Status.NOT_FOUND;
                    }
                    return Response.status(status).build();
                } else {
                    return Response.ok(Fruit.from(record.get("f").asNode())).build();
                }
            }))
            .thenCompose(response -> session.closeAsync().thenApply(signal -> response));
}

A request may look like this:

curl localhost:8080/fruits/42
In case Neo4j has been setup as a cluster, the transaction mode is used to decide whether a request is routed to a leader or a follower instance. Write transactions must be handled by a leader, whereas read-only transactions can be handled by followers.

Deleting nodes

Finally, we want to get rid of fruits again and we add the DELETE method:

FruitResource#delete
@DELETE
@Path("{id}")
public CompletionStage<Response> delete(String id) {
    AsyncSession session = driver.asyncSession();
    return threadContext.withContextCapture(session
            .executeWriteAsync(tx -> tx
                    .runAsync("MATCH (f:Fruit) WHERE f.id = $id DELETE f", Map.of("id", id))
                    .thenCompose(ResultCursor::consumeAsync) (1)
            ))
            .thenCompose(response -> session.closeAsync())
            .thenApply(signal -> Response.noContent().build());
}
1 There is no result for us, only a summary of the query executed.

A request may look like this

curl -X DELETE localhost:8080/fruits/42

And that’s already the most simple CRUD application with one type of nodes. Feel free to add relationships to the model. One idea would be to model recipes that contain fruits. The Cypher manual linked in the introduction will help you with modelling your queries.

Next steps

Packaging

Packaging your application is as simple as ./mvnw clean package. It can be run with java -jar target/quarkus-app/quarkus-run.jar.

With GraalVM installed, you can also create a native executable binary: ./mvnw clean package -Dnative. Depending on your system, that will take some time.

Connection Health Check

If you are using the quarkus-smallrye-health extension, quarkus-neo4j will automatically add a readiness health check to validate the connection to Neo4j.

So when you access the /q/health/ready endpoint of your application you will have information about the connection validation status.

This behavior can be disabled by setting the quarkus.neo4j.health.enabled property to false in your application.properties.

Driver metrics

If you are using a metrics extension and specify the config property quarkus.neo4j.pool.metrics.enabled=true, the Neo4j extension will expose several metrics related to the Neo4j connection pool.

Explore Cypher and the Graph

There are tons of options to model your domain within a Graph. The Neo4j docs, the sandboxes and more are a good starting point.

Going reactive

If you have access to Neo4j 4.0, you can go fully reactive.

To make life a bit easier, we will use Mutiny for this.

Mutiny

The following example uses Mutiny reactive types. If you are not familiar with Mutiny, check Mutiny - an intuitive reactive programming library.

The reactive fruit resources streams the name of all fruits:

ReactiveFruitResource.java
package io.quarkus.it.neo4j;

import static org.reactivestreams.FlowAdapters.toFlowPublisher;
import static org.reactivestreams.FlowAdapters.toPublisher;

import java.util.Map;

import javax.inject.Inject;
import javax.ws.rs.Consumes;
import javax.ws.rs.GET;
import javax.ws.rs.POST;
import javax.ws.rs.Path;
import javax.ws.rs.Produces;
import javax.ws.rs.core.MediaType;

import org.jboss.resteasy.reactive.ResponseStatus;
import org.neo4j.driver.Driver;
import org.neo4j.driver.reactive.ReactiveSession;
import org.reactivestreams.Publisher;

import io.smallrye.mutiny.Multi;
import io.smallrye.mutiny.Uni;

@Path("reactivefruits")
@Consumes(MediaType.APPLICATION_JSON)
public class ReactiveFruitResource {

    @Inject
    Driver driver;

    static Uni<Void> sessionFinalizer(ReactiveSession session) { (1)
        return Uni.createFrom().publisher(toPublisher(session.close()));
    }

    @GET
    @Produces(MediaType.SERVER_SENT_EVENTS)
    public Publisher<String> get() {
        // Create a stream from a resource we can close in a finalizer...
        return Multi.createFrom().resource(driver::reactiveSession, (2)
                session -> toPublisher(session.executeRead(tx -> {
                    var result = tx.run("MATCH (f:Fruit) RETURN f.name as name ORDER BY f.name");
                    return toFlowPublisher(
                            Multi.createFrom().publisher(toPublisher(result)).flatMap(v -> toPublisher(v.records())));
                })))
                .withFinalizer(ReactiveFruitResource::sessionFinalizer) (3)
                .map(record -> record.get("name").asString());
    }
}
1 A finalizer that will close a Neo4j driver session in a non-blocking fashion
2 The Multi.createFrom().resource() is used to defer the creation of session until the publisher is subscribed to
3 When the publisher is done, the finalizer will be called

driver.rxSession() returns a reactive session. It exposes its API based on Reactive Streams, most prominently, as org.reactivestreams.Publisher. Those can be used directly, but we found it easier and more expressive to wrap them in reactive types such as the one provided by Mutiny. Typically, in the previous code, the session is closed when the stream completes, fails or the subscriber cancels.

If you want to return a Mutiny! Uni object, you need to be very careful before you convert a Multi into a Uni: The conversion works in such a way, that the first item is emitted and then a cancellation signal is sent to the publisher, that will propagate upto the drivers' session, indicating a cancellation of the transaction, thus doing a rollback.

In most cases you are better off returning a Multi or just a generic Publisher. If you need a Uni, you can still realize this with an emitter:

ReactiveFruitResource.java#create
@POST
@Produces(MediaType.TEXT_PLAIN)
@ResponseStatus(201)
public Uni<String> create(Fruit fruit) {

    return Uni.createFrom().emitter(e -> Multi.createFrom().resource(driver::reactiveSession, (1)
            session -> toPublisher(session.executeWrite(tx -> {
                var result = tx.run(
                        "CREATE (f:Fruit {id: randomUUID(), name: $name}) RETURN f",
                        Map.of("name", fruit.name));
                return toFlowPublisher(
                        Multi.createFrom().publisher(toPublisher(result)).flatMap(v -> toPublisher(v.records())));
            })))
            .withFinalizer(ReactiveFruitResource::sessionFinalizer)
            .map(record -> Fruit.from(record.get("f").asNode()))
            .subscribe().with( (2)
                    persistedFruit -> e.complete("/fruits/" + persistedFruit.id)));
}
1 Here we use Uni.createFrom().emitter() to retrieve an UniEmitter that we use in 2.
2 Notice how we subscribe to a Multi setup in a similar fashion as in the prior example. The subscription will emit the one and only item via the emitter, without a cancellation event.

Configuration Reference

Each of the neo4j and bolt URI schemes permit variants that contain extra encryption and trust information. The +s variants enable encryption with a full certificate check, and the +ssc variants enable encryption, but with no certificate check. This latter variant is designed specifically for use with self-signed certificates. The variants are basically shortcuts over explicit configuration. If you use one of them, Quarkus won’t pass quarkus.neo4j.encrypted and related to the driver creation as the driver prohibits this.
The only check applied when Quarkus detects a secure url (either of +s or +ssc) is to ensure availability of SSL in native image and will throw ConfigurationException if it isn’t available.

Configuration property fixed at build time - All other configuration properties are overridable at runtime

Configuration property

Type

Default

Whether a health check is published in case the smallrye-health extension is present.

Environment variable: QUARKUS_NEO4J_HEALTH_ENABLED

boolean

true

If DevServices has been explicitly enabled or disabled. DevServices is generally enabled by default, unless there is an existing configuration present. When DevServices is enabled Quarkus will attempt to automatically configure and start a database when running in Dev or Test mode.

Environment variable: QUARKUS_NEO4J_DEVSERVICES_ENABLED

boolean

The container image name to use, for container based DevServices providers.

Environment variable: QUARKUS_NEO4J_DEVSERVICES_IMAGE_NAME

string

neo4j:4.4

This value can be used to specify the port to which the bolt-port of the container is exposed. It must be a free port, otherwise startup will fail. A random, free port will be used by default. Either way, a messsage will be logged on which port the Neo4j container is reachable over bolt.

Environment variable: QUARKUS_NEO4J_DEVSERVICES_BOLT_PORT

int

This value can be used to specify the port to which the http-port of the container is exposed. It must be a free port, otherwise startup will fail. A random, free port will be used by default. Either way, a messsage will be logged on which port the Neo4j Browser is available.

Environment variable: QUARKUS_NEO4J_DEVSERVICES_HTTP_PORT

int

The uri this driver should connect to. The driver supports bolt, bolt+routing or neo4j as schemes.

Environment variable: QUARKUS_NEO4J_URI

string

bolt://localhost:7687

If the driver should use encrypted traffic.

Environment variable: QUARKUS_NEO4J_ENCRYPTED

boolean

false

Additional environment entries that can be added to the container before its start.

Environment variable: QUARKUS_NEO4J_DEVSERVICES_ADDITIONAL_ENV

Map<String,String>

Authentication

Type

Default

The login of the user connecting to the database.

Environment variable: QUARKUS_NEO4J_AUTHENTICATION_USERNAME

string

neo4j

The password of the user connecting to the database.

Environment variable: QUARKUS_NEO4J_AUTHENTICATION_PASSWORD

string

neo4j

Set this to true to disable authentication.

Environment variable: QUARKUS_NEO4J_AUTHENTICATION_DISABLED

boolean

false

Configure trust settings for encrypted traffic

Type

Default

Configures which trust strategy to apply when using encrypted traffic.

Environment variable: QUARKUS_NEO4J_TRUST_SETTINGS_STRATEGY

trust-all-certificates, trust-custom-ca-signed-certificates, trust-system-ca-signed-certificates

trust-system-ca-signed-certificates

The file of the certificate to use.

Environment variable: QUARKUS_NEO4J_TRUST_SETTINGS_CERT_FILE

path

If hostname verification is used.

Environment variable: QUARKUS_NEO4J_TRUST_SETTINGS_HOSTNAME_VERIFICATION_ENABLED

boolean

false

Connection pool

Type

Default

Flag, if metrics are enabled.

Environment variable: QUARKUS_NEO4J_POOL_METRICS_ENABLED

boolean

false

Flag, if leaked sessions logging is enabled.

Environment variable: QUARKUS_NEO4J_POOL_LOG_LEAKED_SESSIONS

boolean

false

The maximum amount of connections in the connection pool towards a single database.

Environment variable: QUARKUS_NEO4J_POOL_MAX_CONNECTION_POOL_SIZE

int

100

Pooled connections that have been idle in the pool for longer than this timeout will be tested before they are used again. The value 0 means connections will always be tested for validity and negative values mean connections will never be tested.

Environment variable: QUARKUS_NEO4J_POOL_IDLE_TIME_BEFORE_CONNECTION_TEST

Duration

-0.001S

Pooled connections older than this threshold will be closed and removed from the pool.

Environment variable: QUARKUS_NEO4J_POOL_MAX_CONNECTION_LIFETIME

Duration

1H

Acquisition of new connections will be attempted for at most configured timeout.

Environment variable: QUARKUS_NEO4J_POOL_CONNECTION_ACQUISITION_TIMEOUT

Duration

1M

About the Duration format

The format for durations uses the standard java.time.Duration format. You can learn more about it in the Duration#parse() javadoc.

You can also provide duration values starting with a number. In this case, if the value consists only of a number, the converter treats the value as seconds. Otherwise, PT is implicitly prepended to the value to obtain a standard java.time.Duration format.