Join has multiple input channels and a single output channel. All messages received on any
input channel are sent to the output channel. There is no particular guarantee on the order
of messages on the output channel.
A multiplexer has multiple input inputs and a single output channel. Messages received on
the selected input channel are send on the output channel and messages on other inputs are
discarded. The channel can be selected via a separate input channel.
Join has multiple input channels and a single output channel. All messages received on any
input channel are sent to the output channel. There is no particular guarantee on the order
of messages on the output channel.