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Hazelcast Set, also known as ISet, is a distributed collection that contains no duplicate elements and does not preserve the order of elements.

Hazelcast Set is a non-partitioned data structure; all the data that belongs to a set lives on one single partition in that member.

Hazelcast Set cannot be scaled beyond the capacity of a single machine. Since the whole set lives on a single partition, storing a large amount of data on a single set may cause memory pressure. Therefore, you should use multiple sets to store a large amount of data. This way, all the sets are spread across the cluster, sharing the load.

A backup of set is stored on a partition of another member in the cluster so that data is not lost in the event of a primary member failure. All set items are copied to the local member and iteration occurs locally.

Creating a Set

When you start a Hazelcast member with default configuration, it will have an empty set named default. See Start a Local Cluster in Docker for a quick cluster startup.

You can either use this default set or create your own using the set’s getter methods as shown in the below example. If you pass a name other than default as the set name in these methods, it creates a new set with the given name; otherwise, it will use this existing set.

In this example, we create a set called set, add items to it, and print the items.

  • Java

  • C++

  • C Sharp

  • Node.js

  • Python

  • Go

  1. Install the Java client library

  2. Add the following to your file:

    import com.hazelcast.client.HazelcastClient;
    import com.hazelcast.core.HazelcastInstance;
    import com.hazelcast.collection.ISet;
    import java.util.Collection;
    public class Client {
        public static void main(String[] args) throws Exception {
            HazelcastInstance client = HazelcastClient.newHazelcastClient();
            ISet<String> set = client.getSet("set"); (1)
            set.add("New York");
            System.out.println("Putting finished!");
  1. Install the latest C++ client library

  2. Add the following to your file:

    #include <hazelcast/client/hazelcast_client.h>
    int main() {
        auto hz = hazelcast::new_client().get();
        auto set = hz.get_set("set").get(); (1)
        set->add("New York").get();
        std::cout << "Putting finished!" << std::endl;
        std::cout << "Finished" << std::endl;
        return 0;
  1. Install the latest C Sharp client library

  2. Add the following to your file:

    using System;
    using System.Threading.Tasks;
    namespace Hazelcast.Examples.DistributedObjects
        public class SetExample
            public static async Task Main(string[] args)
                var options = new HazelcastOptionsBuilder()
                await using var client = await HazelcastClientFactory.StartNewClientAsync(options);
                await using var set = await client.GetSetAsync<string>("set"); (1)
                await set.AddAsync("Tokyo");
                await set.AddAsync("Paris");
                await set.AddAsync("London");
                await set.AddAsync("New York");
                Console.WriteLine("All: " + string.Join(", ", await set.GetAllAsync()));
                Console.WriteLine("Contains: " + await set.ContainsAsync("Paris"));
                Console.WriteLine("Count: " + await set.GetSizeAsync());
                await client.DestroyAsync(set);
  1. Install the Node.js client library.

    npm install hazelcast-client
  2. Add the following to your file:

    'use strict';
    const { Client } = require('hazelcast-client');
    (async () => {
        try {
            const client = await Client.newHazelcastClient();
            const set = await client.getSet('my-distributed-set'); (1)
            await set.add('Tokyo');
            await set.add('Paris');
            await set.add('London');
            await set.add('New York');
            const size = await set.size();
            console.log('Set size:', size);
            await client.shutdown();
        } catch (err) {
            console.error('Error occurred:', err);
  1. Install the Python client library.

    pip install hazelcast-python-client
  2. Add the following to your file:

    import hazelcast
    client = hazelcast.HazelcastClient()
    my_set = client.get_set("set") (1)
    my_set.add("New York")
    found = my_set.contains("Paris").result()
    print("Set contains Paris:", found)
    items = my_set.get_all().result()
    print("Size of set:", len(items))
    print("\nAll Items:")
    for item in items:
  1. Install the Go client library.

    go get github.com/hazelcast/hazelcast-go-client
  2. Add the following to your file:

    package main
    import (
    func main() {
    	setItems := 10
    	ctx := context.TODO()
    	client, err := hazelcast.StartNewClient(ctx)
    	if err != nil {
    	setName := fmt.Sprintf("sample-%d", rand.Int())
    	set, err := client.GetSet(ctx, setName) (1)
    	if err != nil {
    	for i := 0; i < setItems; i++ {
    		_, err = set.Add(ctx, fmt.Sprintf("Item %d", i))
    		if err != nil {
    	// Print contents of the set
    	items, err := set.GetAll(ctx)
    	if err != nil {
    	for _, val := range items {
    	// Shutdown client
1 Create the Set called set.
2 Add items to set.

Hazelcast Set uses item listeners to listen to events that occur when items are added to and removed from set. See the Listening for Item Events section for information about how to create an item listener class and register it.

Configuring Set

The following is an example set configuration.

  • XML

  • YAML

  • Java

    <set name="default">
      statistics-enabled: false
      backup-count: 1
      async-backup-count: 0
      max-size: 10
        - class-name: com.hazelcast.examples.ItemListener
      split-brain-protection-ref: splitbrainprotection-name
        Config config = new Config();
        CollectionConfig collectionSet = config.getSetConfig("MySet");

The following are the configuration elements and their descriptions:

  • backup-count: Count of synchronous backups. Set is a non-partitioned data structure, so all entries of a set reside in one partition. For example, if this parameter is 1, there is one backup of the set in one other member. If it is 2, two members will have the backup.

  • async-backup-count: Count of asynchronous backups. See Backup Types to learn more about synchronous and asynchronous backups.

  • statistics-enabled: Specifies whether the statistics gathering is enabled for your set. If set to false, you cannot collect statistics in your implementation. Its default value is true.

  • max-size: The maximum number of entries for this set. It can be any number between 0 and Integer.MAX_VALUE. Its default value is 0, meaning there is no capacity constraint.

  • item-listeners: Lets you add listeners (listener classes) for the list items. You can also set the attributes include-value to true if you want the item event to contain the item values. You can set local to true if you want to listen to the items on the local member.

  • split-brain-protection-ref: Name of the split-brain protection configuration that you want this set to use. See the Split-Brain Protection for ISet and TransactionalSet section.

Split-Brain Protection for ISet and TransactionalSet

ISet & TransactionalSet can be configured to check for a minimum number of available members before applying queue operations (see the Split-Brain Protection section). This is a check to avoid performing successful queue operations on all parts of a cluster during a network partition.

The following is a list of methods, grouped by the protection types, that support split-brain protection checks:



    • add

    • addAll

    • clear

    • remove

    • removeAll


    • contains

    • containsAll

    • isEmpty

    • iterator

    • size

    • toArray



    • add

    • remove


    • size

The configuration is done on the member side and the following is an example.

  • XML

  • YAML

  • Java

    <set name="default">
      split-brain-protection-ref: splitBrainProtectionRuleWithFourMembers
SplitBrainProtectionConfig splitBrainProtectionConfig = new SplitBrainProtectionConfig();

SetConfig setConfig = new SetConfig();

Config config = new Config();

The value of split-brain-protection-ref should be the split-brain protection configuration name which you configured under the split-brain-protection element as explained in the Split-Brain Protection section.