This is really interesting, although I still can't get my head around the fact that core.async.flow topologies are immutable. I feel like most problems can't be solved with fixed topologies.

I guess one could in theory swap flows the same way values are swapped, but I wonder if this is the way this library is supposed to be used. I also wonder what happens to non-empty channel buffers in this case.

I am curious to hear other opinions.

Flow is intended for processes with long-running stable topologies. Rich has been thinking about options to "patch" the running topology but it is quite tricky due to the concurrency issues and I'm not sure that will ever be added.

Even though the flow topology is fixed, it's perfectly acceptable for a flow component to use other variable resources and act merely as a coordinator. So you could for example have a process that send data out to an external dynamic thread pool and gets callbacks via a channel.