Consider a distributed system with two sites,A and B.Consider whether site A can distinguish among the following
a.B goes down.
b.The link between A and B goes down.
c .B is extremely overloaded and its response time is 100 times longer than normal.
What implications does your answer have for recovery in distributed systems?
One technique would be for B to periodically send a I-am -up message to A indicating it is still alive.If A does not receive an i-am-up message,it can assume either B-or the network link- is down.
Note that an i-am-up message does not allow A to distinguish between each type of failure.One technique that allows A to better determine if the network is down is to send an Are-you-up message to B using an alternate route.If it receives a reply,it can determine that indeed the network link is down and that B is up.
If we assume that A knows B is up and is reachable (via the I-am-up mechanism) and that A has some value N which indicates a normal response time.A could monitor the response time from B and compare values to N,allowing A to determine if B is overloaded or not.The implications of both of these techniques are that A could choose another host—say C—in the system if B is either down,unreachable,or overloaded.