Partitions are what give Kafka parallelism and allow it to scale. Every message exists in exactly one partition.
Replicas are exact copies of partitions on different machines. They allow Kafka to be reliable and not lose messages if a machine dies.
So the answers are:
1. No, a message is sent to one partition.
2. Each partition is replicated separately, so it doesn't make sense to replicate from one partition to another.
3. Depends whether any parts of the cluster are currently in a failed state, see http://kafka.apache.org/documentation.html#replication
On 27 Feb 2014, at 06:17, Balasubramanian Jayaraman (Contingent) <[EMAIL PROTECTED]> wrote: