4.

Q: Create an interactive version (with Cancel messages) of 3PC.
No termination and recovery protocols are necessary.
You do not need to implement your solutions.


A: See Interactive3PC.pdf

A participant can send a cancel message even when it is in PRECOMMIT state. If 
this was not allowed, the system could be blocked waiting for all the 
akcnowledgements since the coordinator does not have a timeout for the 
participants' messages.

Notice that if we allow using CANCEL messages in precommit stage, then
we need to rewrite termination (and, in fact, we are no longer dealing
with a non-blocking protocol, as it is possible to move to both Commit
and Abort from Precommit.