[gpderetta]

proc1 sends a message to proc2 via bus1, then sends a signal to proc3 via a shared memory channel by incrementing a counter in shared memory; proc3 sees the message and signals proc4 via bus2;

There is no partial ordering between proc1->proc2 and proc3->proc4 as the memory channels are invisible to bus1, but Proc2 and proc4 can detect the total ordering violation by using another counter in shared memory.

[tomegun]

Right, we only enforce the _order_ of message delivery, not the _time_ of deliver. I.e., consider the sequence of messages received on each peer, these sequences each respect a global, total order on all messages. But there are no timestamps on them, and no way to compare sequences from separate peers.