The driver auto-detects new Cassandra nodes added to the cluster through server side push notifications and through checking the system tables.

For each node, the address the driver will receive will correspond to the address set as rpc_address in the node's cassandra.yaml file. In most cases, this is the correct value. However, sometimes the addresses received through this mechanism will either not be reachable directly by the driver, or should not be the preferred address to use (for instance, it might be a private IP, but some clients may have to use a public IP, or go through a router).

This interface allows you to deal with such cases, by transforming the address sent by a Cassandra node to another address to be used by the driver for connection.

public class MyAddressTranslator implements AddressTranslator {
    public InetSocketAddress translate(InetSocketAddress address) {
        ... // your custom translation logic
    }
}

Cluster cluster = Cluster.builder()
    .addContactPoint("1.2.3.4")
    .withAddressTranslator(new MyAddressTranslator())
    .build();

Note: the contact points provided while creating the Cluster are not translated, only addresses retrieved from or sent by Cassandra nodes are.

EC2MultiRegionAddressTranslator is provided out of the box. It helps optimize network costs when your infrastructure (both Cassandra nodes and clients) is distributed across multiple Amazon EC2 regions:

• a client communicating with a Cassandra node in the same EC2 region should use the node's private IP (which is less expensive);
• a client communicating with a node in a different region should use the public IP.

To use this implementation, provide an instance when initializing the Cluster:

Cluster cluster = Cluster.builder()
    .addContactPoint("1.2.3.4")
    .withAddressTranslator(new EC2MultiRegionAddressTranslator())
    .build();

This class performs a reverse DNS lookup of the origin address, to find the domain name of the target instance. Then it performs a forward DNS lookup of the domain name; the EC2 DNS does the private/public switch automatically based on location.