Spring Boot 2.1.1 - Eureka Discovery & Spring Boot Admin Server

Question

With Spring Boot v1.5.x, it was possible to have a single service double as a Spring Boot Admin Server and an Eureka Discovery Server. With Spring Boot 2.x, it appears that Spring Boot Admin Server uses the Spring reactive API's (webflux, Netty Server, etc) while Netflix Eureka Discovery Server still uses Tomcat. I would like to know if is possible to either use Netty for Eureka Server or somehow use both Netty and Tomcat in one service. Sample code below

import de.codecentric.boot.admin.server.config.EnableAdminServer;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.cloud.netflix.eureka.server.EnableEurekaServer;

@EnableAdminServer
@EnableEurekaServer
@SpringBootApplication
public class EurekaSpringBootAdminApplication {

    public static void main(String[] args) {
        SpringApplication.run(EurekaSpringBootAdminApplication.class, args);
    }
}

Sample pom below.

<?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>

<groupId>com.foo.bar</groupId>
<artifactId>eureka-springadmin-service</artifactId>
<version>0.0.1-SNAPSHOT</version>
<packaging>jar</packaging>

<name>eureka-springadmin-service</name>
<description>Demo project for Spring Boot</description>

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.1.1.RELEASE</version>
    <relativePath/> <!-- lookup parent from repository -->
</parent>

<properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
    <java.version>1.8</java.version>
    <spring-boot-admin.version>2.1.1</spring-boot-admin.version>
    <spring-cloud.version>Greenwich.M3</spring-cloud.version>
</properties>

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-actuator</artifactId>
    </dependency>
    <dependency>
        <groupId>de.codecentric</groupId>
        <artifactId>spring-boot-admin-starter-server</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-starter-netflix-eureka-server</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-test</artifactId>
        <scope>test</scope>
    </dependency>
</dependencies>

<dependencyManagement>
    <dependencies>
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-dependencies</artifactId>
            <version>${spring-cloud.version}</version>
            <type>pom</type>
            <scope>import</scope>
        </dependency>
        <dependency>
            <groupId>de.codecentric</groupId>
            <artifactId>spring-boot-admin-dependencies</artifactId>
            <version>${spring-boot-admin.version}</version>
            <type>pom</type>
            <scope>import</scope>
        </dependency>
    </dependencies>
</dependencyManagement>

<build>
    <plugins>
        <plugin>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
        </plugin>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-release-plugin</artifactId>
            <version>2.5.3</version>
        </plugin>
    </plugins>
</build>

<repositories>
    <repository>
        <id>spring-milestones</id>
        <name>Spring Milestones</name>
        <url>https://repo.spring.io/milestone</url>
        <snapshots>
            <enabled>false</enabled>
        </snapshots>
    </repository>
</repositories>

UPDATE: So I finally got this to work!! First, lets take a look at the pom.xml below

<properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
    <java.version>1.8</java.version>
    <spring-boot-admin.version>2.1.1</spring-boot-admin.version>
    <spring-cloud.version>Greenwich.M3</spring-cloud.version>
</properties>

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-actuator</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-security</artifactId>
    </dependency>
    <dependency>
        <groupId>de.codecentric</groupId>
        <artifactId>spring-boot-admin-starter-server</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-webflux</artifactId>
        <exclusions>
            <exclusion>
                <artifactId>spring-boot-starter-reactor-netty</artifactId>
                <groupId>org.springframework.boot</groupId>
            </exclusion>
        </exclusions>
    </dependency>
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-starter-netflix-eureka-server</artifactId>
    </dependency>
    <dependency>
        <groupId>io.projectreactor</groupId>
        <artifactId>reactor-core</artifactId>
        <version>3.2.2.RELEASE</version>
        <scope>compile</scope>
    </dependency>
    <dependency>
        <groupId>io.projectreactor.netty</groupId>
        <artifactId>reactor-netty</artifactId>
        <version>0.8.2.RELEASE</version>
        <scope>compile</scope>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-test</artifactId>
        <scope>test</scope>
    </dependency>
    <dependency>
        <groupId>org.springframework.security</groupId>
        <artifactId>spring-security-test</artifactId>
        <scope>test</scope>
    </dependency>
</dependencies>

<dependencyManagement>
    <dependencies>
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-dependencies</artifactId>
            <version>${spring-cloud.version}</version>
            <type>pom</type>
            <scope>import</scope>
        </dependency>
        <dependency>
            <groupId>de.codecentric</groupId>
            <artifactId>spring-boot-admin-dependencies</artifactId>
            <version>${spring-boot-admin.version}</version>
            <type>pom</type>
            <scope>import</scope>
        </dependency>
    </dependencies>
</dependencyManagement>

<build>
    <plugins>
        <plugin>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
        </plugin>
    </plugins>
</build>

<repositories>
    <repository>
        <id>spring-milestones</id>
        <name>Spring Milestones</name>
        <url>https://repo.spring.io/milestone</url>
        <snapshots>
            <enabled>false</enabled>
        </snapshots>
    </repository>
</repositories>

Above is the modified pom.xml. Two changes were made. The first, I added the spring-boot-starter-webflux dependency and then I excluded the spring-boot-starter-reactor-netty dependency from the spring-boot-starter-webflux. Second, I pulled in reactor-core and reactor-netty from io.projectreactor. SBA 2.1.1 relies on the reactive libs, specifically the HttpClient class. This satisfies all the needs of SBA. Eureka Admin Server already has everything is needs to run.

Now lets take a look at the .properties to configure self discovery and registry fetching.

#PROJECT INFORMATION
spring.application.name= Eureka and SBA Service
info.app.name=@project.name@
info.app.description=@project.description@
info.app.version=@project.version@

#SERVER CONFIGURATION
server.port=10761
server.servlet.context-path=/

#MANAGEMENT CONFIGURATION
management.server.port=10769
management.server.servlet.context-path=/admin

##SPRING SECURITY CONFIGURATION
spring.security.user.name=admin
spring.security.user.password=adminadmin

#EUREKA DISCOVERY CONFIGURATION
eureka.instance.hostname=localhost
eureka.client.registry-fetch-interval-seconds=5
eureka.client.register-with-eureka=true
eureka.client.fetch-registry=true
eureka.client.service-url.defaultZone=http://admin:adminadmin@localhost:10761/eureka

#SPRING BOOT ADMIN CONFIGURATION
spring.boot.admin.discovery.enabled=true
spring.boot.admin.context-path=/spring-admin/

The above properties file show the Eureka and SBA configuration. The really important pieces are the fetch-registry, register-with-eureka, eureka.client.service-url.defaultZone, spring.boot.admin.discovery.enabled, and spring.boot.admin.context-path.

The fetch-registry props, allows the clients to be registered not only with Eureka but with SBA as well. The down side is that the service will attempt to fetch the registry before the service is completely up. So, you will seen an exception thrown on service startup.

The register-with-eureka, allow the service to register itself with Eureka and SBA.

eureka.client.service-url.defaultZone tells where to the service to register to. In this case, it's with itself. This same config needs to be applied to any client server wishing to register with Eureka and SBA.

spring.boot.admin.discovery.enabled allows cloud discovery to SBA. With out this set to true, on registry fetch SBA will not register any clients.

spring.boot.admin.context-path sets the context-path for SBA. This needed so you can avoid and url conflicts with Eureka.

In the main application class, make sure the following annotations are present

import de.codecentric.boot.admin.server.config.EnableAdminServer;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.cloud.client.discovery.EnableDiscoveryClient;
import org.springframework.cloud.netflix.eureka.server.EnableEurekaServer;

@EnableAdminServer
@EnableDiscoveryClient
@EnableEurekaServer
@SpringBootApplication
public class EurekaSbaApplication{

    public static void main(String[] args) {
        SpringApplication.run(EurekaSbaApplication.class, args);
    }
}

I also added a web security configuration class that disabled CSRF but I'm sure if it is really necessary.

UPDATE:

I forgot to mention that an exception thrown at times after a client has registered successfully. An IllegalStateException is thrown with the following message: "calling aysncError() is not valid for a request with Async state". I have not yet figure out why this happens, but it does not seem to break anything.

    2018-12-05 13:33:05.845 ERROR 19424 --- [io-10761-exec-9] o.a.catalina.connector.CoyoteAdapter     : Exception while processing an asynchronous request

java.lang.IllegalStateException: Calling [asyncError()] is not valid for a request with Async state [MUST_DISPATCH]
    at org.apache.coyote.AsyncStateMachine.asyncError(AsyncStateMachine.java:440) ~[tomcat-embed-core-9.0.13.jar:9.0.13]
    at org.apache.coyote.AbstractProcessor.action(AbstractProcessor.java:512) [tomcat-embed-core-9.0.13.jar:9.0.13]
    at org.apache.coyote.Request.action(Request.java:430) ~[tomcat-embed-core-9.0.13.jar:9.0.13]
    at org.apache.catalina.core.AsyncContextImpl.setErrorState(AsyncContextImpl.java:382) ~[tomcat-embed-core-9.0.13.jar:9.0.13]
    at org.apache.catalina.connector.CoyoteAdapter.asyncDispatch(CoyoteAdapter.java:239) ~[tomcat-embed-core-9.0.13.jar:9.0.13]
    at org.apache.coyote.AbstractProcessor.dispatch(AbstractProcessor.java:241) [tomcat-embed-core-9.0.13.jar:9.0.13]
    at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:53) [tomcat-embed-core-9.0.13.jar:9.0.13]
    at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:791) [tomcat-embed-core-9.0.13.jar:9.0.13]
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1417) [tomcat-embed-core-9.0.13.jar:9.0.13]
    at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) [tomcat-embed-core-9.0.13.jar:9.0.13]
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [na:1.8.0_172]
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [na:1.8.0_172]
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-9.0.13.jar:9.0.13]
    at java.lang.Thread.run(Thread.java:748) [na:1.8.0_172]

Answer 1

Can exclude Tomcat on your dependencies and add Reactor Netty:

 <dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-starter-netflix-eureka-server</artifactId>
    <exclusions>
        <!-- Exclude the Tomcat dependency -->
        <exclusion>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-tomcat</artifactId>
        </exclusion>
    </exclusions>
</dependency>

and add dependencies of Reactor Netty.

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-reactor-netty</artifactId>
    <version>{{version-needed}}</version>
</dependency>