Boy I'll bet the discussions over how to do this must have been really entertaining to watch.
On the one hand, the Cluster is a "single" thing, so why should each instance get a copy of the message from the Topic. Rather, each instance gets a distinct message, and the cluster processes message N times faster (with N being the number of instances in the cluster).
On the other, it's a bunch of loosely cooperating instances where something like a Topic would make complete sense to be used as a mechanism for sharing information across them.
Yea, I bet those were good times. Someone didn't walk away happy from those :-).
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