Github user kanakb commented on the pull request:
https://github.com/apache/helix/pull/30#issuecomment-118425221
1. Let's say we have a target resource with 2 partitions and 0 replicas,
with one partition assigned to node A, and one partition assigned to node B.
Job 0 runs on nodes A and B, it finishes on node A, and then Job 1 starts on
node A. Then imagine node B fails, and both partitions are now on node A. Job 1
is running on node A, but Job 0 did not finish for the partition that was
reassigned to node A. We have a dependency inversion, and that's why this is
unsafe.
2. If the job does not have a target resource, this change doesn't make
sense. An exception should be thrown if you attempt to submit an untargeted
workflow that has parallelism set.
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