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Discovering Members by Multicast

With the multicast auto-discovery mechanism, Hazelcast allows cluster members to find each other using multicast communication. The cluster members do not need to know the concrete addresses of the other members, they just multicast to all the other members for listening. It depends on your environment if multicast is possible or allowed.

To set your Hazelcast to multicast auto-discovery, set the following configuration elements. Please refer to the multicast element section for the full description of the multicast discovery configuration elements.

  • Set the enabled attribute of the multicast element to "true".
  • Set multicast-group, multicast-port, multicast-time-to-live, etc. to your multicast values.
  • Set the enabled attribute of both tcp-ip and aws elements to "false".

The following is an example declarative configuration.

<hazelcast>
   ...
  <properties>
    <property name="hazelcast.discovery.enabled">true</property>
  </properties>
   ...
  <network>
    ...
        <join>
            <multicast enabled="true">
                <multicast-group>224.2.2.3</multicast-group>
                <multicast-port>54327</multicast-port>
                <multicast-time-to-live>32</multicast-time-to-live>
                <multicast-timeout-seconds>2</multicast-timeout-seconds>
                <trusted-interfaces>
                   <interface>192.168.1.102</interface>
                </trusted-interfaces>   
            </multicast>
            <tcp-ip enabled="false">
            </tcp-ip>
            <aws enabled="false">
            </aws>
        </join>
  <network>

Pay attention to the multicast-timeout-seconds element. multicast-timeout-seconds specifies the time in seconds that a node should wait for a valid multicast response from another node running in the network before declaring itself as the leader node (the first node joined to the cluster) and creating its own cluster. This only applies to the startup of nodes where no leader has been assigned yet. If you specify a high value to multicast-timeout-seconds, such as 60 seconds, it means that until a leader is selected, each node will wait 60 seconds before moving on. Be careful when providing a high value. Also be careful not to set the value too low, or the nodes might give up too early and create their own cluster.