Project Reactor is a fully non-blocking foundation with back-pressure support included. Although most libraries out there support asynchronous methods thus assist on its usage, there are some cases where a library contains complex blocking methods without an asynchronous implementation. Calling this methods inside a reactor stream would have bad results. Instead we need to make those method to async ones or find if there is a workaround.

Provided you might be short on time and is not possible to contribute a patch to the tool used, or you cannot identify how to reverse engineer the blocking call and implement a non blocking version, then it makes sense to utilise some threads.

First let’s import the dependencies for our project

    <dependencyManagement>
        <dependencies>
            <dependency>
                <groupId>io.projectreactor</groupId>
                <artifactId>reactor-bom</artifactId>
                <version>2020.0.11</version>
                <type>pom</type>
                <scope>import</scope>
            </dependency>
        </dependencies>
    </dependencyManagement>

    <dependencies>
        <dependency>
            <groupId>io.projectreactor</groupId>
            <artifactId>reactor-core</artifactId>
        </dependency>
        <dependency>
            <groupId>io.projectreactor</groupId>
            <artifactId>reactor-test</artifactId>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.junit.jupiter</groupId>
            <artifactId>junit-jupiter-engine</artifactId>
            <version>5.8.1</version>
            <scope>test</scope>
        </dependency>
    </dependencies>

Let’s start with out blocking service

    public String get(String url) throws IOException {
        HttpURLConnection connection = (HttpsURLConnection) new URL(url).openConnection();
        connection.setRequestMethod("GET");
        connection.setDoOutput(true);
        try(InputStream inputStream = connection.getInputStream()) {
            return new String(inputStream.readAllBytes(), StandardCharsets.UTF_8);
        }
    }

We used HttpsURLConnection since we know for sure that it is a blocking call. To do so we need a Scheduler. For the blocking calls we shall use the boundedElastic scheduler. A scheduler can also be created by an existing executor service.

So let’s transform this method to a non-blocking one.

package com.gkatzioura.blocking;

import reactor.core.publisher.Mono;
import reactor.core.scheduler.Schedulers;

public class BlockingAsyncService {

    private final BlockingService blockingService;

    public BlockingAsyncService(BlockingService blockingService) {
        this.blockingService = blockingService;
    }

    private Mono<String> get(String url) {
        return Mono.fromCallable(() -> blockingService.get(url))
                .subscribeOn(Schedulers.boundedElastic());
    }

}

What we can see is a Mono created from the callable method. A scheduler subscribes to this mono and thus will receive the event emitted, which shall be scheduled for execution.

Let’s have a test

package com.gkatzioura.blocking;

import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;

import reactor.core.publisher.Mono;
import reactor.test.StepVerifier;

class BlockingAsyncServiceTest {

    private BlockingAsyncService blockingAsyncService;

    @BeforeEach
    void setUp() {
        blockingAsyncService = new BlockingAsyncService(new BlockingService());
    }

    @Test
    void name() {
        StepVerifier.create(
                            Mono.just("https://www.google.com/")
                                .map(s -> blockingAsyncService.get(s))
                                .flatMap(s -> s)
                    )
                .consumeNextWith(s -> s.startsWith("<!doctype"))
                .verifyComplete();
    }
}

That’s it! Obviously the best thing to do is to find a way to make this blocking call into an async call and try to find a workaround using the async libraries out there. When it’s not feasible we can fallback on using Threads.

Since Reactor has taken over the Java world it was inevitable the a reactive sql library would be there.
In this blog we shall use r2dbc with h2 and reactor.

We shall start with the dependencies needed.

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.5.2</version>
    </parent>

    <groupId>com.gkatzioura</groupId>
    <artifactId>r2dbc-reactor</artifactId>
    <version>1.0-SNAPSHOT</version>

    <properties>
        <maven.compiler.source>11</maven.compiler.source>
        <maven.compiler.target>11</maven.compiler.target>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-data-r2dbc</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.data</groupId>
            <artifactId>spring-data-commons</artifactId>
        </dependency>

        <dependency>
            <groupId>com.h2database</groupId>
            <artifactId>h2</artifactId>
            <scope>runtime</scope>
        </dependency>
        <dependency>
            <groupId>io.r2dbc</groupId>
            <artifactId>r2dbc-h2</artifactId>
            <scope>runtime</scope>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>io.projectreactor</groupId>
            <artifactId>reactor-test</artifactId>
            <scope>test</scope>
        </dependency>
    </dependencies>

</project>

We imported spring data from r2dbc, the h2 r2dbc driver, the h2 binary as well as the test utils.

Supposing that this is our schema.
This schema is a postgresql schema.

create table order_request (
	id uuid NOT NULL constraint or_id_pk primary key,
	created_by varchar,
	created timestamp default now()              not null,
	updated timestamp default now()              not null
);

We shall add it later to test/resources/schema.sql for testing purposes.

Also let’s add a new model

package com.gkatzioura.r2dbc.model;

import java.time.LocalDateTime;
import java.util.UUID;

import org.springframework.data.annotation.Id;
import org.springframework.data.domain.Persistable;
import org.springframework.data.relational.core.mapping.Table;

@Table("order_request")
public class OrderRequest implements Persistable<UUID> {

    @Id
    private UUID id;
    private String createdBy;
    private LocalDateTime created;
    private LocalDateTime updated;

    public void setId(UUID id) {
        this.id = id;
    }

    public String getCreatedBy() {
        return createdBy;
    }

    public void setCreatedBy(String createdBy) {
        this.createdBy = createdBy;
    }

    public LocalDateTime getCreated() {
        return created;
    }

    public void setCreated(LocalDateTime created) {
        this.created = created;
    }

    public LocalDateTime getUpdated() {
        return updated;
    }

    public void setUpdated(LocalDateTime updated) {
        this.updated = updated;
    }

    @Override
    public UUID getId() {
        return id;
    }

    @Override
    public boolean isNew() {
        return created == null;
    }

}

Pay attention to isNew method. This way the repository can identify if the object should be persisted or updated.

Now onwards to our Repository

package com.gkatzioura.r2dbc.repository;

import java.util.UUID;
import org.springframework.data.repository.reactive.ReactiveCrudRepository;
import com.gkatzioura.r2dbc.model.OrderRequest;

public interface OrderRepository extends ReactiveCrudRepository<OrderRequest, UUID> {
}

Let’s put some tests.

As mentioned the schema above will reside in test/resources/schema.sql

We shall add some configuration for the test h2 db. We need to make sure that h2 will pickup the postgresql interface.

package com.gkatzioura.r2dbc;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.core.io.ClassPathResource;
import org.springframework.data.r2dbc.config.AbstractR2dbcConfiguration;
import org.springframework.data.r2dbc.repository.config.EnableR2dbcRepositories;
import org.springframework.r2dbc.connection.init.CompositeDatabasePopulator;
import org.springframework.r2dbc.connection.init.ConnectionFactoryInitializer;
import org.springframework.r2dbc.connection.init.ResourceDatabasePopulator;

import io.r2dbc.h2.H2ConnectionFactory;
import io.r2dbc.spi.ConnectionFactory;

@Configuration
@EnableR2dbcRepositories
public class H2ConnectionConfiguration extends AbstractR2dbcConfiguration  {

    @Override
    public ConnectionFactory connectionFactory() {
        return new H2ConnectionFactory(
                io.r2dbc.h2.H2ConnectionConfiguration.builder()
                                                     .url("mem:testdb;MODE=PostgreSQL;DB_CLOSE_DELAY=-1;")
                                                     .build()
        );
    }

    @Bean
    public ConnectionFactoryInitializer initializer() {
        var initializer = new ConnectionFactoryInitializer();
        initializer.setConnectionFactory(connectionFactory());

        var databasePopulator = new CompositeDatabasePopulator();
        databasePopulator.addPopulators(new ResourceDatabasePopulator(new ClassPathResource("schema.sql")));
        initializer.setDatabasePopulator(databasePopulator);
        return initializer;
    }

}

With this configuration we create a H2 database simulating a Postgresql DB, we create the schemas as well as enable the creation of the R2DBC repositories.

Also let’s add a test.

package com.gkatzioura.r2dbc.repository;

import java.util.UUID;
import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.annotation.Import;
import org.springframework.test.context.junit.jupiter.SpringExtension;
import com.gkatzioura.r2dbc.H2ConnectionConfiguration;
import com.gkatzioura.r2dbc.model.OrderRequest;
import reactor.test.StepVerifier;

@ExtendWith({SpringExtension.class})
@Import({H2ConnectionConfiguration.class})
class OrderRepositoryTest {

    @Autowired
    private OrderRepository orderRepository;

    @Test
    void testSave() {
        UUID id = UUID.randomUUID();
        OrderRequest orderRequest = new OrderRequest();
        orderRequest.setId(id);
        orderRequest.setCreatedBy("test-user");

        var persisted = orderRepository.save(orderRequest)
                                       .map(a -> orderRepository.findById(a.getId()))
                                       .flatMap(a -> a.map(b -> b.getId()));

        StepVerifier.create(persisted).expectNext(id).verifyComplete();
    }
}

That’s it, you can find the code on github.