help me write a irule about persist with uri

In the pool configuration, set "Action on Service Down" to "reselect", which will cause the BIG-IP to do another pool selection against available members.

I'm going to advise against having the traffic go back to the original server just for the sake of it. Sure, technically this can be achieved, but it's actually not that easy and requires you to first check if pool member A is active, and if so, unbind the connection to pool member B and then re-bind with pool member A. There's no real easy way to track that the pool member that was originally selected was member A without creating a table entry, and then checking it on each request.... all in all its technically possible but horribly inefficient and not really worth it.