Hazelcast is an open-source in-memory data grid (IMDG) that provides easy-to-use distributed data structures and caching capabilities. In a distributed system, Hazelcast allows you to handle client failover seamlessly. This blog post will guide you through the process of handling failover in a Hazelcast IMDG client application using Java.

What is Failover?

Failover is the process of transferring the workload from a failed component to a working one. In the context of Hazelcast, failover implies transferring the client’s tasks and data to another available member of the Hazelcast cluster if the currently connected member fails or becomes unavailable.

Configuring Hazelcast Client for Failover

To enable failover in a Hazelcast client application, you need to configure the client’s network settings properly. Here’s an example of how to configure your Hazelcast client in Java:

Config config = new Config();
config.setNetworkConfig(new NetworkConfig()
    .addAddress("localhost:5701", "localhost:5702", "localhost:5703")
    .setConnectionAttemptLimit(3)
    .setConnectionAttemptPeriod(500));

HazelcastInstance hazelcastInstance = HazelcastClient.newHazelcastClient(config);

In the above code, we configure the NetworkConfig to specify a list of addresses of Hazelcast members that the client can connect to. We set the connectionAttemptLimit to limit the number of connection attempts to the members in the list. We also set the connectionAttemptPeriod to specify the period (in milliseconds) between connection attempts.

Handling Failover in Hazelcast Client

Once the Hazelcast client is configured for failover, it will automatically handle failover scenarios. If the currently connected member fails, the client will attempt to reconnect to the other members in the specified list.

To ensure that your operations are not affected during a failover, you should handle exceptions that may occur when executing tasks or accessing data. Here’s an example of how to handle a HazelcastClientOfflineException when performing a Map operation:

try {
    IMap<String, String> map = hazelcastInstance.getMap("myMap");
    String value = map.get("key");
    // Perform operations on the retrieved value
} catch (HazelcastClientOfflineException e) {
    // Handle the exception and retry or log an error
}

In the code above, we catch the HazelcastClientOfflineException that may be thrown if the client is offline or disconnected from the cluster. You can choose to retry the operation or handle the exception based on your application’s requirements.

Conclusion

Handling failover is crucial for maintaining the availability of your Hazelcast IMDG client application. By properly configuring your client’s network settings and handling exceptions, you can ensure that your application seamlessly switches to available members in case of failures.

By following the techniques mentioned in this blog post, you can enhance the robustness and availability of your Hazelcast IMDG client application. Happy coding!

#Hazelcast #Failover