Blog

import java.util.Map;

import com.hazelcast.core.Hazelcast;

import com.hazelcast.core.HazelcastInstance;

public class Server {

   public static void main(String... args){

      //initialize hazelcast server/instance

      HazelcastInstance hazelcast = Hazelcast.newHazelcastInstance();

      //create a simple map

      Map<String, String> vehicleOwners = hazelcast.getMap("vehicleOwnerMap");

      // add key-value to map

      vehicleOwners.put("John", "Honda-9235");

      // do not shutdown, let the server run

      //hazelcast.shutdown();

   }

}

Now, run the above class.

java -cp .\target\demo-0.0.1-SNAPSHOT.jar com.example.demo.Server

For setting up a client, we also need to add client jar.

<dependency>

   <groupId>com.hazelcast</groupId>

   <artifactId>hazelcast-client</artifactId>

   <version>3.12.12</version>

</dependency>

Let’s now create Client.java. Note that similar to Hazelcast members, clients can also be configured programmatically or via XML configuration (i.e., via -Dhazelcast.client.config or hazelcast-client.xml).

Example

Let’s use the default configuration which means our client would be able to connect to local instances.

import java.util.Map;

import com.hazelcast.client.HazelcastClient;

import com.hazelcast.core.HazelcastInstance;

public class Client {

   public static void main(String... args){

      //initialize hazelcast client

      HazelcastInstance hzClient = HazelcastClient.newHazelcastClient();

      //read from map

      Map<String, String> vehicleOwners = hzClient.getMap("vehicleOwnerMap");

      System.out.println(vehicleOwners.get("John"));

      System.out.println("Member of cluster: " +

      hzClient.getCluster().getMembers());

      // perform shutdown

      hzClient.getLifecycleService().shutdown();

   }

}

Now, run the above class.

java -cp .\target\demo-0.0.1-SNAPSHOT.jar com.example.demo.Client

Output

It will produce the following output −

Honda-9235

Member of cluster: [Member [localhost]:5701 - a47ec375-3105-42cd-96c7-fc5eb382e1b0]

As seen from the output −

• The cluster only contains 1 member which is from Server.java.
• The client is able to access the map which is stored inside the server.

Load Balancing

Hazelcast Client supports load balancing using various algorithms. Load balancing ensures that the load is shared across members and no single member of the cluster is overloaded. The default load balancing mechanism is set to round-robin. The same can be changed by using the load balancer tag in the config.

We can specify the type of load balancer using the load-balancer tag in the configuration. Here is a sample for choosing a strategy that randomly picks up a node.

<hazelcast-client xmlns="http://www.hazelcast.com/schema/client-config"

   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

   xsi:schemaLocation="http://www.hazelcast.com/schema/client-config

   http://www.hazelcast.com/schema/client-config/hazelcastclient-config-4.2.xsd">

      <load-balancer type="random"/>

</hazelcast-client>

Failover

In a distributed environment, members can fail arbitrarily. For supporting failover, it is recommended that an address to multiple members is provided. If the client gets access to any one member, that is sufficient for it to get addressed to other members. The parameters addressList can be specified in the client configuration.

For example, if we use the following configuration −

<hazelcast-client xmlns="http://www.hazelcast.com/schema/client-config"

   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

   xsi:schemaLocation="http://www.hazelcast.com/schema/client-config

   http://www.hazelcast.com/schema/client-config/hazelcastclient-config-4.2.xsd">

   <address-list>machine1, machine2</address-list>

</hazelcast-client>