Leader Election
===============

From control point of view, many algorithms that are called 
distributed are in fact centralized: there is a single co-ordinating 
process that performs certain functions on behalf of the others.
Such an algorithm , like any centralized algorithm , suffers from the 
great disadvantage that a failure of the co-ordinating process results in the 
failure of the algorithm as a whole.
This can be overcome by the remaining processes conducting a negotiation 
among themselves with the aim of electing one of their number to take over 
the role of co-ordinator.
The protocols associated with such negotiations are called elective 
algorithms.
These are based on the assumption that each process has a unique 
identifier , taking the form of a number.
If the convention is adopted that at any instance the co-ordinating 
process is the one with the largest identifier , then after a failure of this 
process the elective protocol consists in choosing the process with the 
largest of the remaining numbers.
The elective algorithm of this example applies to the case where the 
processes are linked in a unidirectional ring.
Each process Pi has a unique numerical identifier.
The principle is is simply one of of finding the maximum of a set of 
numbers asociated with the processes.
The processes can be arranged in any order around the ring.
Each process Pi "knows" the value of its own identifier i , which it 
transmits to its neighbour on the left , Pj say.  Pj then compares this 
with its own identifier j and transmits the greater to its left-hand 
neighbour: this is the "selective extinction" method.
The election starts with one process sending such a message and marking 
itself as taking part in the election - at this stage it will be the only 
one so markerd. The unmarked process receiving the message will act as 
just described and mark itself as participant. When a marked (i.e. - 
participating) process receiving its own identifier it knows that this 
must be the greatest and that therefore it is elected to play the special 
role.