Why always [reactor-tcp-nio-2]

[1528110566]

I've created a new project, pom.xml is as follows

<?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">
    <parent>
        <artifactId>spring-boot-starter-parent</artifactId>
        <groupId>org.springframework.boot</groupId>
        <version>2.7.8</version>
    </parent>
    <modelVersion>4.0.0</modelVersion>

    <groupId>org.example</groupId>
    <artifactId>r2dbc-demo</artifactId>
    <version>1.0-SNAPSHOT</version>

    <properties>
        <maven.compiler.source>8</maven.compiler.source>
        <maven.compiler.target>8</maven.compiler.target>
    </properties>
    <dependencies>
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.data</groupId>
            <artifactId>spring-data-r2dbc</artifactId>
        </dependency>
        <dependency>
            <groupId>org.mariadb</groupId>
            <artifactId>r2dbc-mariadb</artifactId>
        </dependency>
        <dependency>
            <groupId>io.r2dbc</groupId>
            <artifactId>r2dbc-spi</artifactId>
        </dependency>
        <dependency>
            <groupId>io.r2dbc</groupId>
            <artifactId>r2dbc-pool</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-webflux</artifactId>
        </dependency>
    </dependencies>
</project>

This controller gets me some data from DB, getTest retruns first record, getTest2 returns all records. There are about 10000 records in DB.

@RestController
public class TestController {
    @Autowired
    private R2dbcEntityTemplate template;

    @GetMapping("/test1")
    public Mono<Test> getTest() {
        return template.select(Test.class).first();
    }

    @GetMapping("/test2")
    public Flux<Test> getTest2() {
        return template.select(Test.class).all();
    }
}

application.yml

spring:
  r2dbc:
    url: r2dbc:mariadb://localhost:3306/r2dbc?userSSL=false
    username: root
    password: 123456
logging:
  level:
    root: trace

Here is my question, I request /test2 first, then /test1, /test1 will be wait until /test2 is complete.
log file shows that only 1 thread is handling my request, which is reactor-tcp-nio-2.
I tried to custom LoopResources, but I got the same result.
This problem has been bothering me for a long time. Could someone help me about it?
😭😭😭

[mp911de]

That's how netty dispatches connections across threads. This is not a problem, it is a consequence of the default EventExecutorChooser.

[1528110566]

Sorry, I don't understand it clearly. Why not reactor-tcp-nio-1 to reactor-tcp-nio-n in circles?
I found this when I debug the code
reactor.netty.resources.ColocatedEventLoopGroup

@Override
public EventLoop next() {
	EventLoop loop = nextInternal();
	return loop != null ? loop : eventLoopGroup.next();
}

nextInternal() returns reactor-tcp-nio-2, and eventLoopGroup.next() won't be called forever.

[1528110566]

Here is my solution. I custom a NioEventLoopGroup instead of using ColocatedEventLoopGroup by default, and put it into runOn() function in SimpleClient.
Things looks normal for now.
But I still don't know why Netty using ColocatedEventLoopGroup by default and why the initialization of NioSocketChannel always under one same single thread(like reactor-tcp-nio-2).

@Bean
public ConnectionFactoryOptionsBuilderCustomizer customizer() {
    return builder -> {
        NioEventLoopGroup eventLoopGroup = new NioEventLoopGroup(6, new ThreadFactory() {
            private final AtomicInteger i = new AtomicInteger(0);
            @Override
            public Thread newThread(Runnable r) {
                Thread thread = new Thread(r);
                thread.setName("r2dbc-tcp-nio-" + i.getAndIncrement());
                return thread;
            }
        });
        LoopResources loopResources = useNative -> eventLoopGroup;
        builder.option(LOOP_RESOURCES, loopResources);
    };
}

[mp911de]

Leaving another reference: r2dbc/r2dbc-pool#190