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Hazelcast Sample

Clustered Spring Boot Admin Server deployment using Hazelcast for distributed event storage. This sample demonstrates high-availability setup where multiple Admin Server instances share state via Hazelcast.

Overview​

Location: spring-boot-admin-samples/spring-boot-admin-sample-hazelcast/

Features:

  • Distributed event store with Hazelcast
  • Multi-instance clustering
  • Shared notification state
  • High availability
  • TCP/IP cluster discovery
  • JMX monitoring enabled

Prerequisites​

  • Java 17+, Maven 3.6+

Running Multiple Instances​

Terminal 1 (Port 8080)​

cd spring-boot-admin-samples/spring-boot-admin-sample-hazelcast
mvn spring-boot:run

Terminal 2 (Port 8081)​

SERVER_PORT=8081 mvn spring-boot:run

Terminal 3 (Port 8082)​

SERVER_PORT=8082 mvn spring-boot:run

All instances automatically form a cluster and share:

  • Instance events
  • Notification state
  • Application registry

Dependencies​

<dependency>
    <groupId>de.codecentric</groupId>
    <artifactId>spring-boot-admin-starter-server</artifactId>
</dependency>
<dependency>
    <groupId>de.codecentric</groupId>
    <artifactId>spring-boot-admin-starter-client</artifactId>
</dependency>
<dependency>
    <groupId>com.hazelcast</groupId>
    <artifactId>hazelcast</artifactId>
</dependency>

Hazelcast Configuration​

@Bean
public Config hazelcastConfig() {
    // Event store map - holds all instance events
    MapConfig eventStoreMap = new MapConfig(DEFAULT_NAME_EVENT_STORE_MAP)
        .setInMemoryFormat(InMemoryFormat.OBJECT)
        .setBackupCount(1)  // 1 backup copy
        .setMergePolicyConfig(
            new MergePolicyConfig(PutIfAbsentMergePolicy.class.getName(), 100)
        );

    // Notification map - deduplicates notifications
    MapConfig sentNotificationsMap = new MapConfig(DEFAULT_NAME_SENT_NOTIFICATIONS_MAP)
        .setInMemoryFormat(InMemoryFormat.OBJECT)
        .setBackupCount(1)
        .setEvictionConfig(
            new EvictionConfig()
                .setEvictionPolicy(EvictionPolicy.LRU)
                .setMaxSizePolicy(MaxSizePolicy.PER_NODE)
        );

    Config config = new Config();
    config.addMapConfig(eventStoreMap);
    config.addMapConfig(sentNotificationsMap);
    config.setProperty("hazelcast.jmx", "true");

    // TCP/IP cluster discovery (local)
    config.getNetworkConfig().getJoin().getMulticastConfig().setEnabled(false);
    TcpIpConfig tcpIpConfig = config.getNetworkConfig()
        .getJoin()
        .getTcpIpConfig();
    tcpIpConfig.setEnabled(true);
    tcpIpConfig.setMembers(singletonList("127.0.0.1"));

    return config;
}

Configuration Details:

  • Event Store: Reliable storage with 1 backup
  • Sent Notifications: LRU eviction to prevent memory growth
  • Cluster: TCP/IP discovery on localhost
  • JMX: Enabled for monitoring

How Clustering Works​

Event Synchronization​

  1. Instance A receives status change event
  2. Event stored in Hazelcast distributed map
  3. Instances B and C immediately see the event
  4. All instances update their local state

Notification Deduplication​

  1. Instance A sends notification
  2. Records in Hazelcast sent-notifications map
  3. Instance B sees event, checks sent-notifications
  4. B skips sending (already sent by A)
  5. No duplicate notifications to users

Load Balancing​

Users can connect to any instance - they all show the same data.

Testing Clustering​

Verify Cluster Formation​

Check logs for:

Members {size:3, ver:3} [
    Member [127.0.0.1]:5701 - e1f2g3h4
    Member [127.0.0.1]:5702 - a5b6c7d8
    Member [127.0.0.1]:5703 - i9j0k1l2
]

Test Event Sharing​

  1. Register application on instance 8080
  2. Check instance 8081 - application appears
  3. Stop instance 8080
  4. Application still visible on 8081

Test High Availability​

  1. Start 3 instances
  2. Stop instance 8080
  3. Instances 8081 and 8082 continue operating
  4. All data preserved (1 backup)

Production Configuration​

Multicast Discovery​

config.getNetworkConfig()
    .getJoin()
    .getMulticastConfig()
    .setEnabled(true)
    .setMulticastGroup("224.2.2.3")
    .setMulticastPort(54327);

TCP/IP with Multiple Hosts​

tcpIpConfig.setMembers(Arrays.asList(
    "admin-1.company.com",
    "admin-2.company.com",
    "admin-3.company.com"
));

Kubernetes Discovery​

config.getNetworkConfig()
    .getJoin()
    .getKubernetesConfig()
    .setEnabled(true)
    .setProperty("namespace", "default")
    .setProperty("service-name", "spring-boot-admin");

Monitoring Hazelcast​

JMX​

Enable JMX and connect with JConsole:

jconsole

Look for com.hazelcast MBeans.

Hazelcast Management Center​

docker run -p 8080:8080 hazelcast/management-center

Connect to: http://localhost:8080

Key Takeaways​

✅ High Availability: No single point of failure
✅ Horizontal Scaling: Add instances dynamically
✅ Shared State: All instances synchronized
✅ Enterprise Features: Backup, eviction, merge policies

Next Steps​

See Also​