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InetAddress.getAllByName(hostname)[0]

For some environments, where the broker hostnames are mapped by the DNS to multiple IPs, e.g. in clouds where the IPs point to the external load balancers, it is desirable that the clientclients, on failing to connect to one of the IPs, would try the other ones before giving up the connection.

Our use case is for multiple load balancers fronting the Kafka cluster. The Kafka advertised listeners advertise hostnames for which the DNS server hosts holds multiple A records, corresponding to the IP IPs of all LBs not just one .
If one LBs LB isn't available, but the client is able to use another IP for the same hostname (it connects to the 2nd LB for example) the service stays available.

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The NIO client will attempt a connection to one of the IPs. If the connection is refused or times out and the DNS had returned multiple IPs, then rather than failing the connection it will try to connect to one of the other IPs. 

If they all fail to connect then the behavior is remains like the current client when the only obtained IP had failed to connect:

• at bootstrap, move to the next hostname
• past bootstrap, retry the connection to the given node starting from the resolution of the hostname

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Note that the above Java API can return a set containing both IPv4 and IPv6 addresses. What The type is returned first is determined by the JVM - see  the Java System property  "java.net.preferIPv6Addresses". This KIP proposes to only use multiple IPs of the same type (IPv4/IPv6) as the first one, to avoid any change in the network stack while trying multiple IPs.

Compatibility, Deprecation, and Migration Plan

What impact (if any) will there be on existing users? 

By default the multiple IP resolution is disabled so there will be no impact

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. Even when enabled, no switch between IPv6 and IPv4 will be attempted as only one type of address will be used

Rejected Alternatives

• Making the client use all IPs by default as it may have impacted some users 
• Introducing a separate configuration entry, independent of the one introduced by KIP-235, so the DNS resolution enhancement suggested by this KIP is alternative and not conflicting with KIP-235