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When a broker starts up, it registers its host ip in ZK. The high-level consumer later uses the registered host ip to establish the socket connection to the broker. By default, the registered ip is given by InetAddress.getLocalHost.getHostAddress. Typically, this should return the real ip of the host. However, in EC2, the returned ip is an internal one and can't be connected to from outside. The solution is to explicitly set the host ip to be registered in ZK by setting the "hostname" property in server.properties.

Should I choose multiple group ids or a single one for the consumers?

If all consumers use the same group id, messages in a topic are distributed among those consumers. In other words, each consumer will get a non-overlapping subset of the messages. Having more consumers in the same group increases the degree of parallelism and the overall throughput of consumption. See the next question for the choice of the number of consumer instances. On the other hand, if each consumer is in its own group, each consumer will get a full copy of all messages.

Why some of the consumers in a consumer group never receive any message?

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