Comment by lmm

You can have a system that puts out heartbeat timestamps and treat that as the single root upstream for all your inputs, or you can allow each source to timestamp its inputs and make a rule about how far your clocks are allowed to drift (which can be quite generous), or you can give every source an id and treat your timestamps as vectors and have a rule that any "combine events from source x and source y" must happen in a single canonical place.