Frequently Asked Questions

Why 271 as the default partition count?

The partition count of 271, being a prime number, is a good choice because it will be distributed to the nodes almost evenly. For a small to medium sized cluster, the count of 271 gives an almost even partition distribution and optimal-sized partitions. As your cluster becomes bigger, you should make this count bigger to have evenly distributed partitions.

Is Hazelcast thread safe?

Yes. All Hazelcast data structures are thread safe.

How do nodes discover each other?

When a node is started in a cluster, it will dynamically and automatically be discovered. There are three types of discovery.

  • Multicast discovery: nodes in a cluster discover each other by multicast, by default.
  • Discovery by TCP/IP: the first node created in the cluster (leader) will form a list of IP addresses of other joining nodes and will send this list to these nodes so the nodes will know each other.
  • If your application is placed on Amazon EC2, Hazelcast has an automatic discovery mechanism. You will give your Amazon credentials and the joining node will be discovered automatically.

Once nodes are discovered, all the communication between them will be via TCP/IP.


Please refer to the Hazelcast Cluster Discovery section for detailed information.

What happens when a node goes down?

Once a node is gone (crashes), the following happens since data in each node has a backup in other nodes.

  • First, the backups in other nodes are restored.
  • Then, data from these restored backups are recovered.
  • And finally, backups for these recovered data are formed.

So eventually, no data is lost.

How do I test the connectivity?

If you notice that there is a problem with a node joining a cluster, you may want to perform a connectivity test between the node to be joined and a node from the cluster. You can use the iperf tool for this purpose. For example, you can execute the below command on one node (i.e. listening on port 5701).

iperf -s -p 5701

And you can execute the below command on the other node.

iperf -c <IP address> -d -p 5701

The output should include connection information, such as the IP addresses, transfer speed, and bandwidth. Otherwise, if the output says No route to host, it means a network connection problem exists.

How do I choose keys properly?

When you store a key and value in a distributed Map, Hazelcast serializes the key and value, and stores the byte array version of them in local ConcurrentHashMaps. These ConcurrentHashMaps use equals and hashCode methods of byte array version of your key. It does not take into account the actual equals and hashCode implementations of your objects. So it is important that you choose your keys in a proper way.

Implementing equals and hashCode is not enough, it is also important that the object is always serialized into the same byte array. All primitive types like String, Long, Integer, etc. are good candidates for keys to be used in Hazelcast. An unsorted Set is an example of a very bad candidate because Java Serialization may serialize the same unsorted set in two different byte arrays.

How do I reflect value modifications?

Hazelcast always return a clone copy of a value. Modifying the returned value does not change the actual value in the map (or multimap, list, set). You should put the modified value back to make changes visible to all nodes.

V value = map.get( key );
map.put( key, value );

Collections which return values of methods (such as IMap.keySet, IMap.values, IMap.entrySet, MultiMap.get, MultiMap.remove, IMap.keySet, IMap.values) contain cloned values. These collections are NOT backed up by related Hazelcast objects. Therefore, changes to them are NOT reflected in the originals, and vice-versa.

How do I test my Hazelcast cluster?

Hazelcast allows you to create more than one instance on the same JVM. Each member is called HazelcastInstance and each will have its own configuration, socket and threads, so you can treat them as totally separate instances.

This enables you to write and to run cluster unit tests on a single JVM. Because you can use this feature for creating separate members different applications running on the same JVM (imagine running multiple web applications on the same JVM), you can also use this feature for testing your Hazelcast cluster.

Let's say you want to test if two members have the same size of a map.

public void testTwoMemberMapSizes() {
  // start the first member
  HazelcastInstance h1 = Hazelcast.newHazelcastInstance();
  // get the map and put 1000 entries
  Map map1 = h1.getMap( "testmap" );
  for ( int i = 0; i < 1000; i++ ) {
    map1.put( i, "value" + i );
  // check the map size
  assertEquals( 1000, map1.size() );
  // start the second member
  HazelcastInstance h2 = Hazelcast.newHazelcastInstance();
  // get the same map from the second member
  Map map2 = h2.getMap( "testmap" );
  // check the size of map2
  assertEquals( 1000, map2.size() );
  // check the size of map1 again
  assertEquals( 1000, map1.size() );

In the test above, everything happens in the same thread. When developing a multi-threaded test, you need to carefully handle coordination of the thread executions. it is highly recommended that you use CountDownLatch for thread coordination (you can certainly use other ways). Here is an example where we need to listen for messages and make sure that we got these messages.

public void testTopic() {
  // start two member cluster
  HazelcastInstance h1 = Hazelcast.newHazelcastInstance();
  HazelcastInstance h2 = Hazelcast.newHazelcastInstance();
  String topicName = "TestMessages";
  // get a topic from the first member and add a messageListener
  ITopic<String> topic1 = h1.getTopic( topicName );
  final CountDownLatch latch1 = new CountDownLatch( 1 );
  topic1.addMessageListener( new MessageListener() {
    public void onMessage( Object msg ) {
      assertEquals( "Test1", msg );
  // get a topic from the second member and add a messageListener
  ITopic<String> topic2 = h2.getTopic(topicName);
  final CountDownLatch latch2 = new CountDownLatch( 2 );
  topic2.addMessageListener( new MessageListener() {
    public void onMessage( Object msg ) {
      assertEquals( "Test1", msg );
  } );
  // publish the first message, both should receive this
  topic1.publish( "Test1" );
  // shutdown the first member
  // publish the second message, second member's topic should receive this
  topic2.publish( "Test1" );
  try {
    // assert that the first member's topic got the message
    assertTrue( latch1.await( 5, TimeUnit.SECONDS ) );
    // assert that the second members' topic got two messages
    assertTrue( latch2.await( 5, TimeUnit.SECONDS ) );
  } catch ( InterruptedException ignored ) {

You can start Hazelcast members with different configurations. Remember to call Hazelcast.shutdownAll() after each test case to make sure that there is no other running member left from the previous tests.

public void cleanup() throws Exception {

For more information please check our existing tests.

Does Hazelcast support hundreds of nodes?

Yes. Hazelcast performed a successful test on Amazon EC2 with 200 nodes.

Does Hazelcast support thousands of clients?

Yes. However, there are some points you should consider. The environment should be LAN with a high stability and the network speed should be 10 Gbps or higher. If the number of nodes is high, the client type should be selected as Dummy, not Smart Client. In the case of Smart Clients, since each client will open a connection to the nodes, these nodes should be powerful enough (for example, more cores) to handle hundreds or thousands of connections and client requests. Also, you should consider using near caches in clients to lower the network traffic. And you should use the Hazelcast releases with the NIO implementation (which starts with Hazelcast 3.2).

Also, you should configure the clients attentively. Please refer to the Java Client section section for configuration notes.

What is the difference between old LiteMember and new Smart Client?

LiteMember supports task execution (distributed executor service), smart client does not. Also, LiteMember is highly coupled with cluster, smart client is not.

How do you give support?

We have two support services: community and commercial support. Community support is provided through our Mail Group and StackOverflow web site. For information on support subscriptions, please see

Does Hazelcast persist?

No. However, Hazelcast provides MapStore and MapLoader interfaces. For example, when you implement the MapStore interface, Hazelcast calls your store and load methods whenever needed.

Can I use Hazelcast in a single server?

Yes. But please note that Hazelcast's main design focus is multi-node clusters to be used as a distribution platform.

How can I monitor Hazelcast?

Hazelcast Management Center is what you use to monitor and manage the nodes running Hazelcast. In addition to monitoring the overall state of a cluster, you can analyze and browse data structures in detail, you can update map configurations, and you can take thread dumps from nodes.

Moreover, JMX monitoring is also provided. Please see the Monitoring with JMX section for details.

How can I see debug level logs?

By changing the log level to "Debug". Below are sample lines for log4j logging framework. Please see the Logging Configuration section to learn how to set logging types.

First, set the logging type as follows.

String location = "log4j.configuration";
String logging = "hazelcast.logging.type";
System.setProperty( logging, "log4j" );
/**if you want to give a new location. **/
System.setProperty( location, "file:/path/" );

Then set the log level to "Debug" in the properties file. Below is example content.

# direct log messages to stdout #




log4j.appender.stdout.layout.ConversionPattern=%d{ABSOLUTE} %5p [%c{1}] - %m%n

The line is used to see debug logs for all Hazelcast operations. Below this line, you can select to see specific logs (cluster, partition, hibernate, etc.).

What is the difference between client-server and embedded topologies?

In the embedded topology, nodes include both the data and application. This type of topology is the most useful if your application focuses on high performance computing and many task executions. Since application is close to data, this topology supports data locality.

In the client-server topology, you create a cluster of nodes and scale the cluster independently. Your applications are hosted on the clients, and the clients communicate with the nodes in the cluster to reach data.

Client-server topology fits better if there are multiple applications sharing the same data or if application deployment is significantly greater than the cluster size (for example, 500 application servers vs. 10 node cluster).

How do I know it is safe to kill the second node?


PartitionService partitionService = hazelcastInstance.getPartitionService().isClusterSafe()
if (partitionService().isClusterSafe()) {
  hazelcastInstance.shutdown(); // or terminate

Or declaratively:

PartitionService partitionService = hazelcastInstance.getPartitionService().isClusterSafe()
if (partitionService().isLocalMemberSafe()) {
  hazelcastInstance.shutdown(); // or terminate


Please refer to the Cluster-Member Safety Check section for more information.

When do I need Native Memory solutions?

Native Memory solutions can be preferred:

  • when the amount of data per node is large enough to create significant garbage collection pauses.
  • when your application requires predictable latency.

Is there any disadvantage of using near-cache?

The only disadvantage when using Near Cache is that it may cause stale reads.

Is Hazelcast secure?

Hazelcast supports symmetric encryption, secure sockets layer (SSL), and Java Authentication and Authorization Service (JAAS). Please see the Security chapter for more information.

How can I set socket options?

Hazelcast allows you to set some socket options such as SO_KEEPALIVE, SO_SNDBUF, and SO_RCVBUF using Hazelcast configuration properties. Please see hazelcast.socket.* properties explained in the System Properties section.

I periodically see client disconnections during idle time?

In Hazelcast, socket connections are created with the SO_KEEPALIVE option enabled by default. In most operating systems, default keep-alive time is 2 hours. If you have a firewall between clients and servers which is configured to reset idle connections/sessions, make sure that the firewall's idle timeout is greater than the TCP keep-alive defined in the OS.

For additional information please see:

How to get rid of "java.lang.OutOfMemoryError: unable to create new native thread"?

If you encounter an error of java.lang.OutOfMemoryError: unable to create new native thread, it may be caused by exceeding the available file descriptors on your operating system, especially if it is Linux. This exception is usually thrown on a running node, after a period of time when the thread count exhausts the file descriptor availability.

The JVM on Linux consumes a file descriptor for each thread created. The default number of file descriptors available in Linux is usually 1024. If you have many JVMs running on a single machine, it is possible to exceed this default number.

You can view the limit using the following command.

# ulimit -a

At the operating system level, Linux users can control the amount of resources (and in particular, file descriptors) used via one of the following options.

1 - Editing the limits.conf file:

# vi /etc/security/limits.conf

testuser soft nofile 4096<br>
testuser hard nofile 10240<br>

2 - Or using the ulimit command:

# ulimit -Hn


The default number of process per users is 1024. Adding the following to your $HOME/.profile could solve the issue:

# ulimit -u 4096

Does repartitioning wait for Entry Processor?

Repartitioning is the process of redistributing the partition ownerships. Hazelcast performs the repartitioning in the cases where a node leaves the cluster or joins the cluster. If a repartitioning will happen while an entry processor is active in a node processing on an entry object, the repartitioning waits for the entry processor to complete its job.

Why do Hazelcast instances on different machines not see each other?

Assume you have two instances on two different machines and you develop a configuration as shown below.

Config config = new Config();
NetworkConfig network = config.getNetworkConfig();

JoinConfig join = network.getJoin();

When you create the Hazelcast instance, you have to pass the configuration to the instance. If you create the instances without passing the configuration, each instance starts but cannot see each other. Therefore, a correct way to create the instance is the following:

HazelcastInstance instance = Hazelcast.newHazelcastInstance(config);

The following is an incorrect way:

HazelcastInstance instance = Hazelcast.newHazelcastInstance();

What Does "Replica: 1 has no owner" Mean?

When you start more nodes after the first one is started, you will see replica: 1 has no owner entry in the newly started node's log. There is no need to worry about it since it refers to a transitory state. It only means the replica partition is not ready/assigned yet and eventually it will be.