Reactive programming with Neo4j

Generate a Spring Boot project using Spring Initializr, and make sure the following options are chosen.

Project type: Maven
Java version : 11
Spring Boot : 2.2.4.RELEASE
Dependencies: Reactive Web, Lombok

Do not add Spring Data Neo4j into the dependencies, we will use the new Spring Data Neo4j Rx instead.

Do not forget to add Lombok to your project dependencies. We'll use Lombok to get the Java codes clean as possible, esp. to generate the getters and setters, hashCode, equals, toString for you at compile time.

Download the generated project skeleton archive and extract the files into your machine.

Open the pom.xml file in the project root folder, add the Spring Data Neo4j RX dependency from the Neo4j team manually.

<dependency>
    <groupId>org.neo4j.springframework.data</groupId>
    <artifactId>spring-data-neo4j-rx-spring-boot-starter</artifactId>
    <version>${spring-data-neo4j-rx.version}</version>
</dependency>

Declare the property spring-data-neo4j-rx.version in properties.

<spring-data-neo4j-rx.version>1.0.0-beta03</spring-data-neo4j-rx.version>

Now let's add some codes to experience the new Reactive features in Spring Data Neo4j Rx.

Create a POJO class for presenting a node in Neo4j graph.

@Node
@Data
@ToString
@Builder
@NoArgsConstructor
@AllArgsConstructor
class Post {

    @Id
    @GeneratedValue
    private Long id;
    private String title;
    private String content;

    @CreatedDate
    private LocalDateTime createdDate;
}

In the above codes, @Node, @Id, @GeneratedValue are located in the package org.neo4j.springframework.data.core.schema which is part of the new Spring Data Neo4j Rx project.

Spring Data Neo4j Rx also supports the annotations from Spring Data Commons project. For example, you can use Id and Persistent from Spring Data Commons instead of the Id and Node annotations from the new Spring Data Neo4j RX.

Spring Data Neo4j Rx supports data auditing as well, so you can use @CreatedDate, @CreatedBy, @LastModifiedDate, @LastModifiedBy as usual, just add @EnableNeo4jAuditing on the application class to enable it. Unfortunately, there is no AuditorAware reactive variant available in SDN RX, thus it can not fill CreatedBy and lastModifiedBy automatically.

Create a Repository class for Post.

interface PostRepository extends ReactiveNeo4jRepository<Post, Long> {
}

Create a Controller to perform CRUD operations.

@RestController()
@RequestMapping(value = "/posts")
@RequiredArgsConstructor
class PostController {

    private final PostRepository posts;

    @GetMapping("")
    public Flux<Post> all() {
        return this.posts.findAll();
    }

    @PostMapping("")
    public Mono<Post> create(@RequestBody Post post) {
        return this.posts.save(post);
    }

    @GetMapping("/{id}")
    public Mono<Post> get(@PathVariable("id") Long id) {
        return Mono.just(id)
                .flatMap(posts::findById)
                .switchIfEmpty(Mono.error(new PostNotFoundException(id)));
    }

    @PutMapping("/{id}")
    public Mono<Post> update(@PathVariable("id") Long id, @RequestBody Post post) {
        return this.posts.findById(id)
                .map(p -> {
                    p.setTitle(post.getTitle());
                    p.setContent(post.getContent());

                    return p;
                })
                .flatMap(this.posts::save);
    }

    @DeleteMapping("/{id}")
    public Mono<Void> delete(@PathVariable("id") Long id) {
        return this.posts.deleteById(id);
    }

}

The above codes are easy to understand, the main difference is here the methods return a Reactor's Flux or Mono type instead.

Similar with WebMVC stack , to handle exception, you can define a @RestControllerAdvice bean and use ExceptionHandler method to handle specifial exceptions.

For example, in the get method, if the Post is not found, throw an exception PostNotFoundException in the reactive flow by Mono.error.

class PostNotFoundException extends RuntimeException {

    PostNotFoundException(Long id) {
        super("Post #" + id + " was not found");
    }
}

When the PostNotFoundException is caught, send a 404 Not Found status code to the client.

@RestControllerAdvice
@Slf4j
class RestExceptionHandler {

    @ExceptionHandler(PostNotFoundException.class)
    ResponseEntity postNotFound(PostNotFoundException ex) {
        log.debug("handling exception::" + ex);
        return ResponseEntity.notFound().build();
    }

}

Almost done. Let's try to initialize some data for demo purpose.

Create a CommandLineRunner bean to insert some data.

@Component
@Slf4j
@RequiredArgsConstructor
class DataInitializer implements CommandLineRunner {

    private final PostRepository posts;

    private final ReactiveNeo4jClient client;

    @Override
    public void run(String[] args) {
        log.info("start data initialization  ...");
        this.posts
                .deleteAll()
                .thenMany(
                        Flux
                                .just("Post one", "Post two")
                                .flatMap(
                                        title -> this.posts.save(Post.builder().title(title).content("content of " + title).build())
                                )
                )
                .log()
                .then()
                .doOnNext(
                        (v) -> client
                                .query("MATCH (p:Post) RETURN p")
                                .fetchAs(Post.class)
                                .mappedBy((t, r) -> (Post) (r.get("p").asObject()))
                                .all()
                                .subscribe(System.out::println)
                )
                .subscribe(
                        null,
                        null,
                        () -> log.info("done initialization...")
                );

    }

}

Benefit from the Spring Boot auto-configuration mechanism, a ReactiveNeo4jClient bean is ready for use. You can use it to interact with the Neo4j low level APIs, such as executing Cypher queries.

To experience the new reactive features, you have to use Neo4j 4.0 which is still under development.

Configure the Neo4j driver properties in application.properties file.

org.neo4j.driver.uri=bolt://localhost:7687
org.neo4j.driver.authentication.username=neo4j
org.neo4j.driver.authentication.password=test

Before running this application, it requires a running Neo4j server. The following code fragment is a docker-compose configuration which allow you run Neo4j server in docker containers.

  neo4j:
    image: neo4j:4
    environment:
      - "NEO4J_AUTH=neo4j/test" # user none as value to disable auth setting
 #     - "NEO4J_ACCEPT_LICENSE_AGREEMENT=yes" # only neede by enterprise version
    ports:
      - 7687:7687 
      - 7474:7474

Run the following command to start a local Neo4j server in Docker container.

docker-compose up neo4j

Start up our application, when it is ready, use curl to test the APIs.

# Get all posts
curl http://localhost:8080/posts
[{"id":0,"title":"Post one","content":"content of Post one","createdDate":null},{"id":1,"title":"Post two","content":"content of Post two","createdDate":null}]


# Get post by id
curl http://localhost:8080/posts/0
{"id":0,"title":"Post one","content":"content of Post one","createdDate":null}

# Get none exsting post
curl -v http://localhost:8080/posts/10
*   Trying ::1...
* TCP_NODELAY set
* Connected to localhost (::1) port 8080 (#0)
> GET /posts/10 HTTP/1.1
> Host: localhost:8080
> User-Agent: curl/7.55.1
> Accept: */*
>
< HTTP/1.1 404 Not Found
< content-length: 0
<
* Connection #0 to host localhost left intact

Like Spring Data Mongo, SDN Rx also provides a @DataNeo4jTest annotation to provide test slice capability.

Add the following dependency into the project pom.xml.

<dependency>
    <groupId>org.neo4j.springframework.data</groupId>
    <artifactId>spring-data-neo4j-rx-spring-boot-test-autoconfigure</artifactId>
    <version>${spring-data-neo4j-rx.version}</version>
    <scope>test</scope>
</dependency>

Create a test class to the SDN Rx features.

@DataNeo4jTest
@Slf4j
public class PostRepositoryTest {

    @Autowired
    private PostRepository posts;

    @BeforeEach
    public void setup() throws IOException {
        log.debug("running setup.....,");
        this.posts.deleteAll()
                .thenMany(testSaveMethod())
                .log()
                .thenMany(testFoundMethod())
                .log()
                .blockLast();// to make the tests work
//                .subscribe(
//                        (data) -> log.info("found post:" + data),
//                        (err) -> log.error("" + err),
//                        () -> log.info("done")
//                );
    }

    private Flux<Post> testSaveMethod() {
        var data = Stream.of("Post one", "Post two")
                .map(title -> Post.builder().title(title).content("The content of " + title).build())
                .collect(Collectors.toList());
        return Flux.fromIterable(data)
                .flatMap(it -> this.posts.save(it));
    }

    private Flux<Post> testFoundMethod() {
        return this.posts
                .findAll(Example.of(Post.builder().title("Post one").build()));
    }

    @AfterEach
    void teardown() {
        //this.posts.deleteAll();
    }

    @Test
    void testAllPosts() {
        posts.findAll().sort(Comparator.comparing(post -> post.getTitle()))
                .as(StepVerifier::create)
                .consumeNextWith(p -> assertEquals("Post one", p.getTitle()))
                .consumeNextWith(p -> assertEquals("Post two", p.getTitle()))
                .verifyComplete();
    }

}

But you have to run a Neo4j server to serve this test, currently SDN Rx does not provide an embedded Neo4j for test scope.

Get the complete codes from my Github.

kongliang/spring-reactive-sample

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