I believe that tracking the number of potentially-changed direct inputs per node would be sufficient, would have better performance and would require fewer work

I believe that tracking the number of potentially-changed direct inputs per node would be sufficient, would have better performance and would require fewer workarounds like durabilities. Further, it wouldn't turn O(N) updates into O(MN) ones (where M is the total size of your computation and N is the nodes affected by an update). However, it would require dependencies to be tracked in both directions, so it's probably not possible to engineer it into Salsa, which seems to only track dependencies from consumer to producer. I was going to blog about this design in detail, but never got around to it.