In the context of a group communication service, provide example message exchanges that illustrate the difference between causal and total ordering.

What will be an ideal response?

As a reminder, causal ordering takes into account the causal relationships between messages, that is if a message happens before another message, this causal relationship must be preserved in the delivery of the associated message to all processes in a group. In total ordering, every process must see all messages in the same order. Clearly, total ordering is a stronger property than causal ordering in that causal ordering only imposes a delivery constraint on messages where a happens before relationship exists. Looking at this the other way round, causal ordering says nothing about the delivery order of messages that are deemed independent of one another.
. If total ordering is employed, every user of the bulletin board would see every posting in exactly the same order. If causal ordering is employed, and we assume there is a causal ordering between messages related to the same thread, but there is no such relationship across threads, then users would see the ordering preserved within a thread, that is the conversation would be presented in the same order. Other messages would be interleaved though in different ways across all users.
As a second example, consider a multiplayer online game where there are two styles of message, one that updates the state of the game and others where players communicate with one another. To maintain the consistency of the game state, it is important to maintain a causal ordering across updates. It may also be important to maintain ordering of the other messages sent between players or players may see a reply to a message before the initial message. Clearly these causal relationships need to be preserved. A total ordering may actually be required here as well if the game developers feel it is important to relate messages precisely to the current state of the game.