Over a million developers have joined DZone.

Spring Cloud Sidecar: Initializing the Nodes

This post explains how an application can go about querying the Sidecar registered nodes.

· Cloud Zone

Build fast, scale big with MongoDB Atlas, a hosted service for the leading NoSQL database on AWS. Try it now! Brought to you in partnership with MongoDB.

In my last blog post I had described how the Sidecar application can be used for registering Cassandra nodes with Eureka and more generally can be used for registering any non-JVM application with Eureka. 

Discovering Registered Nodes: Post Initialization

If the registered nodes are not required during the bean initialization phase then discovering the nodes is fairly straightforward along these lines:

public class SampleCommandLineRunner implements CommandLineRunner {

    private DiscoveryClient discoveryClient;

    public void postConstruct() {
//        System.out.println("Printing from postConstruct");
//        printDiscoveredNodes();

    public void run(String... strings) throws Exception {
        System.out.println("Printing from run method");

    public void printDiscoveredNodes() {
        System.out.println(" Printing Discovered Nodes ");

        for (ServiceInstance instance: discoveryClient.getInstances("samplecassandra.vip")) {
            System.out.println("Host: Port = " + instance.getHost() + ":" + instance.getPort());

These would print the nodes registered with a name of "samplecasssandra.vip" VIP.

Note that the nodes are being printed from the run method which gets called past the initialization of the Spring container. If however the nodes were attempted to be listed from one of the lifecycle stages, say the postConstruct method, then very likely an exception will be thrown (this behavior is seen with "Angel.SR3" release of Spring Cloud, but appears to work cleanly with "Brixton.*" versions)

Discovering Registered Nodes: During Initialization

Now if an application needs to discover the nodes during initialization the flow is a little more complicated, for a potential issue look at this ticket.

The DiscoveryClient is initialized very late in the Spring Lifecycle and if DiscoveryClient is used in any post-processing activity of a bean it is likely to give an exception.

As an example, say the Cassandra nodes registered using Sidecar is now used by an application to initialize Cassandra connectivity, a way to do it would be to create a wrapper around Cassandra connectivity this way:

import org.springframework.data.cassandra.core.CassandraTemplate;

public class CassandraTemplateWrapper extends CassandraTemplate {

    public void afterPropertiesSet() {


Here CassandraTemplate is being overridden to prevent the check in afterPropertiesSet method that a Cassandra session exists, as a session will be established much later in the start-up cycle.

A Cassandra session can be injected into this custom CassandraTemplate lazily in a bean that implements SmartLifecyle along these lines:

package mvctest.cassandra;

import com.datastax.driver.core.Cluster;
import com.datastax.driver.core.Session;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.FactoryBean;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.SmartLifecycle;
import org.springframework.core.Ordered;
import org.springframework.data.cassandra.core.CassandraTemplate;
import org.springframework.stereotype.Component;

public class EurekaCassandraTemplateFactoryBean implements SmartLifecycle, FactoryBean<CassandraTemplate>, Ordered {


    public boolean isAutoStartup() {
        return true;

    public void start() {
        LOGGER.info("About to start Discovery client lookup of Cassandra Cluster!!!");
        final Cluster cluster = this.eurekaClusterBuilder.build();
        Session session = cluster.connect(this.cassandraProperties.getKeyspace());
        LOGGER.info("Completed Discovery client lookup of Cassandra Cluster!!!");
        running = true;

    public boolean isRunning() {
        return this.running;

    public int getPhase() {
        return Integer.MAX_VALUE;

    public int getOrder() {
        return 1;

This way the Cassandra session can be created very late in the cycle. Somewhat rough, but the approach works.

If you are interested in exploring this sample further I have this code available in my github repo here.

Now it's easier than ever to get started with MongoDB, the database that allows startups and enterprises alike to rapidly build planet-scale apps. Introducing MongoDB Atlas, the official hosted service for the database on AWS. Try it now! Brought to you in partnership with MongoDB.


Published at DZone with permission of Biju Kunjummen, DZone MVB. See the original article here.

Opinions expressed by DZone contributors are their own.

The best of DZone straight to your inbox.

Please provide a valid email address.

Thanks for subscribing!

Awesome! Check your inbox to verify your email so you can start receiving the latest in tech news and resources.

{{ parent.title || parent.header.title}}

{{ parent.tldr }}

{{ parent.urlSource.name }}