Partition Group Configuration

Hazelcast distributes key objects into partitions using a consistent hashing algorithm. Those partitions are assigned to nodes. An entry is stored in the node that owns the partition to which the entry's key is assigned. The total partition count is 271 by default; you can change it with the configuration property Please see the System Properties section.

Along with those partitions, there are also copies of the partitions as backups. Backup partitions can have multiple copies due to the backup count defined in configuration, such as first backup partition, second backup partition, etc. A node cannot hold more than one copy of a partition (ownership or backup). By default, Hazelcast distributes partitions and their backup copies randomly and equally among cluster nodes, assuming all nodes in the cluster are identical.

But what if some nodes share the same JVM or physical machine or chassis and you want backups of these nodes to be assigned to nodes in another machine or chassis? What if processing or memory capacities of some nodes are different and you do not want an equal number of partitions to be assigned to all nodes?

You can group nodes in the same JVM (or physical machine) or nodes located in the same chassis. Or you can group nodes to create identical capacity. We call these groups partition groups. Partitions are assigned to those partition groups instead of to single nodes. Backups of these partitions are located in another partition group.

When you enable partition grouping, Hazelcast presents three choices for you to configure partition groups.

  • You can group nodes automatically using the IP addresses of nodes, so nodes sharing the same network interface will be grouped together. All members on the same host (IP address or domain name) will be a single partition group. This helps to avoid data loss when a physical server crashes, because multiple replicas of the same partition are not stored on the same host. But if there are multiple network interfaces or domain names per physical machine, that will make this assumption invalid.
<partition-group enabled="true" group-type="HOST_AWARE" />
Config config = ...;
PartitionGroupConfig partitionGroupConfig = config.getPartitionGroupConfig();
partitionGroupConfig.setEnabled( true )
    .setGroupType( MemberGroupType.HOST_AWARE );
  • You can do custom grouping using Hazelcast's interface matching configuration. This way, you can add different and multiple interfaces to a group. You can also use wildcards in the interface addresses. For example, the users can create rack aware or data warehouse partition groups using custom partition grouping.
<partition-group enabled="true" group-type="CUSTOM">
Config config = ...;
PartitionGroupConfig partitionGroupConfig = config.getPartitionGroupConfig();
partitionGroupConfig.setEnabled( true )
    .setGroupType( MemberGroupType.CUSTOM );

MemberGroupConfig memberGroupConfig = new MemberGroupConfig();
memberGroupConfig.addInterface( "10.10.0.*" )
.addInterface( "10.10.3.*" ).addInterface("10.10.5.*" );

MemberGroupConfig memberGroupConfig2 = new MemberGroupConfig();
memberGroupConfig2.addInterface( "" )
.addInterface( "10.10.1.*").addInterface( "10.10.2.*" );

partitionGroupConfig.addMemberGroupConfig( memberGroupConfig );
partitionGroupConfig.addMemberGroupConfig( memberGroupConfig2 );
  • You can give every member its own group. Each member is a group of its own and primary and backup partitions are distributed randomly (not on the same physical member). This gives the least amount of protection and is the default configuration for a Hazelcast cluster.
<partition-group enabled="true" group-type="PER_MEMBER" />
Config config = ...;
PartitionGroupConfig partitionGroupConfig = config.getPartitionGroupConfig();
partitionGroupConfig.setEnabled( true )
    .setGroupType( MemberGroupType.PER_MEMBER );