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MultiMap

Hazelcast MultiMap is a specialized map where you can store multiple values under a single key. Just like any other distributed data structure implementation in Hazelcast, MultiMap is distributed and thread-safe.

Hazelcast MultiMap is not an implementation of java.util.Map due to the difference in method signatures. It supports most features of Hazelcast Map except for indexing, predicates and MapLoader/MapStore. The entries are almost evenly distributed onto all cluster members. However, this distribution is based on the entry keys: if there are multiple entries having the same key but different values, such entries are stored on the same member, otherwise they are distributed among the members. When a new member joins the cluster, the same ownership logic used in the distributed map applies.

Getting a MultiMap and Putting an Entry

The following example creates a MultiMap and puts items into it:

        HazelcastInstance hazelcastInstance = Hazelcast.newHazelcastInstance();
        MultiMap<String , String > map = hazelcastInstance.getMultiMap( "map" );

        map.put( "a", "1" );
        map.put( "a", "2" );
        map.put( "b", "3" );
        System.out.println( "PutMember:Done" );

We use the getMultiMap method to create the MultiMap and then use the put method to put an entry into it.

Now let’s print the entries in this MultiMap using the following code:

        HazelcastInstance hazelcastInstance = Hazelcast.newHazelcastInstance();
        MultiMap<String, String> map = hazelcastInstance.getMultiMap("map");

        map.put("a", "1");
        map.put("a", "2");
        map.put("b", "3");
        System.out.printf("PutMember:Done");

        for (String key: map.keySet()){
            Collection<String> values = map.get(key);
            System.out.printf("%s -> %s\n", key, values);
        }

After you run ExampleMultiMap, run PrintMember. You will see the key a has two values, as shown below:

b → [3]

a → [2, 1]

Hazelcast MultiMap uses EntryListener to listen to events which occur when entries are added to, updated in or removed from the MultiMap. See the Listening for MultiMap Events section for information about how to create an entry listener class and register it.

Configuring MultiMap

When using MultiMap, the collection type of the values can be either Set or List. Configure the collection type with the valueCollectionType parameter. If you choose Set, duplicate and null values are not allowed in your collection and ordering is irrelevant. If you choose List, ordering is relevant and your collection can include duplicate but not null values.

You can also enable statistics for your MultiMap with the statisticsEnabled parameter. If you enable statisticsEnabled, statistics can be retrieved with getLocalMultiMapStats() method.

Currently, eviction is not supported for the MultiMap data structure.

The following are the example MultiMap configurations.

Declarative Configuration:

  • XML

  • YAML

<hazelcast>
    ...
    <multimap name="default">
        <backup-count>0</backup-count>
        <async-backup-count>1</async-backup-count>
        <value-collection-type>SET</value-collection-type>
        <entry-listeners>
            <entry-listener include-value="false" local="false" >com.hazelcast.examples.EntryListener</entry-listener>
        </entry-listeners>
        <split-brain-protection-ref>split-brain-protection-name</split-brain-protection-ref>
    </multimap>
    ...
</hazelcast>
hazelcast:
  multimap:
    default:
      backup-count: 0
      async-backup-count: 1
      value-collection-type: SET
      entry-listeners:
        - class-name: com.hazelcast.examples.EntryListener
          include-value: false
          local: false
      split-brain-protection-ref: split-brain-protection-name

Programmatic Configuration:

        MultiMapConfig mmConfig = new MultiMapConfig();
        mmConfig.setName( "default" )
                .setBackupCount( 0 ).setAsyncBackupCount( 1 )
                .setValueCollectionType( "SET" )
                .setSplitBrainProtectionName( "splitbrainprotectionname" );

The following are the configuration elements and their descriptions:

  • backup-count: Defines the number of synchronous backups. For example, if it is set to 1, backup of a partition will be placed on one other member. If it is 2, it will be placed on two other members.

  • async-backup-count: The number of asynchronous backups. Behavior is the same as that of the backup-count element.

  • statistics-enabled: Specifies whether the statistics gathering is enabled for your MultiMap. If set to false, you cannot collect statistics in your implementation (using getLocalMultiMapStats()) and also Hazelcast Management Center will not show them. Its default value is true.

  • value-collection-type: Type of the value collection. It can be SET or LIST.

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

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

Split-Brain Protection for MultiMap and TransactionalMultiMap

MultiMap & TransactionalMultiMap can be configured to check for a minimum number of available members before applying their 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 that support split-brain protection checks. The list is grouped by the protection types.

MultiMap:

  • WRITE, READ_WRITE:

    • clear

    • forceUnlock

    • lock

    • put

    • remove

    • tryLock

    • unlock

  • READ, READ_WRITE:

    • containsEntry

    • containsKey

    • containsValue

    • entrySet

    • get

    • isLocked

    • keySet

    • localKeySet

    • size

    • valueCount

    • values

TransactionalMultiMap:

  • WRITE, READ_WRITE:

    • put

    • remove

  • READ, READ_WRITE:

    • size

    • get

    • valueCount

Configuring Split-Brain Protection

Split-brain protection for MultiMap can be configured programmatically using the method setSplitBrainProtectionName(), or declaratively using the element split-brain-protection-ref. Following is an example declarative configuration:

  • XML

  • YAML

<hazelcast>
    ...
    <multimap name="default">
        <split-brain-protection-ref>splitbrainprotection-name</split-brain-protection-ref>
    </multimap>
    ...
</hazelcast>
hazelcast:
  multimap:
    default:
      split-brain-protection-ref: splitbrainprotection-name

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.