Enable non-sink functions within a DAG to write to KVS

[cw75]

[vsreekanti]

This is already possible. The only thing we have to change to enable this is to have the client library instrument reads/writes to track causal dependencies. Everything else already works.

[cw75]

The client should also be able to specify which function in the DAG want to write the result to KVS and what's the name of output key.

[vsreekanti]

How is that any different from calling fluent.put(key, val)?

[cw75]

I think letting users embed these calls into functions is fine. Maybe we should just do this: by default the result of a func is only passed to downstream funcs, but if users want it to also write to kvs, then they need to explicitly write fluent.put(key, val) inside the func.

[vsreekanti]

Yes, that's how it works right now. That's what I meant when I said the only thing that we needed to change was adding support for causal dependencies.