A sends a message to "x = 2" to B and C.
B receives this message and sends "x = 3" to C.
Now, C receives two messages one from A and one from B. It could receive them in any order. However, there is no way for it to tell whether there is a causal order or not.
With a clock embedded in the messages, one can state that "x=2" happened before "x=3". (or more strictly, "x=3" def did not happen before "x=2").
Extending this, if C and D receive copies of the above messages from A and B, we want C and D to both process them in the same (deterministic) order.
A sends a message to "x = 2" to B and C. B receives this message and sends "x = 3" to C.
Now, C receives two messages one from A and one from B. It could receive them in any order. However, there is no way for it to tell whether there is a causal order or not.
With a clock embedded in the messages, one can state that "x=2" happened before "x=3". (or more strictly, "x=3" def did not happen before "x=2").
Extending this, if C and D receive copies of the above messages from A and B, we want C and D to both process them in the same (deterministic) order.